In this article we will discuss about Arthropods:- 1. Taxonomic History of Arthropods 2. Definition of Arthropods 3. Habit and Habitat 4. Classification 5. Phylogeny.
Taxonomic History of Arthropods:
The earliest record of the study of arthropods is available from the work of Aristotle (384-322 B.C.), who coined the term Malacostraca to include crabs and the related forms.
The present trend of studying Arthropoda began with the work of Linnaeus (1707-1778), who created a group Insecta aptera to include Crustaceans, Myriapods and Spiders. The name Crustacea and Myriapoda were first introduced by Cuvier (1769-1832) and Latreille (1825).
Lamarck (1744-1829) in his classification, included spiders, mites, myriapods and silver fishes under Arachnida and grouped prawns, lobsters, crabs and water fleas within Crustacea. It was Cuvier who first suggested to include these animals and annelids under one large group, Articulata. Von Siebold (1845) later separated the annelids and the rest were included under Arthropoda.
Definition of Arthropods:
Bilaterally symmetrical and metamerically segmented animals; body covered with jointed chitinous exoskeleton, moulted periodically and with jointed segmental appendages; coelom highly reduced and haemocoelomic body cavity.
Habit and Habitat of Arthropods:
The arthropods are seen from 30,000 feet below to 20,000 feet above the sea level. These bilaterally symmetrical, jointed-leg invertebrates may be marine, fresh-water, terrestrial, subterranean and aerial. Some arthropods like barnacles are sedentary.
Innumerable crustaceans which live as planktons move passively in the current of water. But well-developed structures are present in many arthropods for moving effectively by swimming, crawling and flying.
Some arthropods live within burrows, some are efficient diggers and many others build well-designed nests. Certain arthropods like honey-bees, ants and termites are polymorphic and lead a complicated social life. All the food habits—herbivorous, carnivorous and omnivorous are seen among arthropods and various food-getting devices are met within this group.
Large numbers of arthropods live as parasites, and structural changes occur in them to adjust with the peculiar mode of life. Many arthropods are well- known for their habit of migration. Some of them can produce sound and nearly all are equipped with efficient sense organs.
Some forms exhibit a phenomenon—suspended animation, to overcome unfavourable conditions. Sexual reproduction is often accompanied by courtship dances. The members may either be oviparous or viviparous or ovoviviparous and some forms exhibit parental care. Parthenogenesis is quite common in arthropods.
Characteristic Features of of Arthropods:
1. Body is bilaterally symmetrical and is metamerically segmented, coelomates.
2. Anterior segments are specialized to form a distinct head and tagmatization (body region) is highly developed (e.g., head, thorax and abdomen).
3. Body is covered by jointed hard chitinous exoskeleton (usually composed of carbohydrate and protein) with sclerotised plates moulted periodically.
4. Presence of paired jointed segmental appendages.
5. Presence of musculature with distinct striped muscles.
6. Body cavity or true coelom is much reduced and acts as haemocoel (blood cavity).
7. Circulatory system is open type (e.g., blood vessels open within haemocoel) with a dorsally placed tubular heart containing paired lateral ostia.
8. Haemocyanin is the usual respiratory pigment.
9. Nephridia are represented by the paired saccular excretory organs (e.g., coxal glands, antennal or maxillary glands) of many arthropods. The Malpighian tubules act as excretory organs, found in many terrestrial arthropods.
10. Central nervous system includes a dorsally placed anterior brain and ganglionated double ventral nerve cord.
11. Presence of compound eyes in many groups (e.g., many crustaceans and most insects), in which each eye is composed of several visual units (e.g., ommatidia).
12. Dorsal coelomic gonads.
13. Sexes are usually separate (= gonochoristic); some hermaphrodite.
14. Parthenogenesis is seen in some members of Insecta, Branchiopoda and Copepoda.
15. Fertilization internal in terrestrial species but external in aquatic species.
16. Eggs are centrolecithal.
17. Cleavage commonly superficial; a few spirals (e.g., Barnacles, copepods, cladocerans).
18. Development may be direct or indirect (e.g., via larval stages).
19. Absence of ciliated larvae.
Classification of Arthropods:
Classification in Outline:
In the studies of the middle part of 20th century, considerable rearrangement has been made in the scheme of arthropod classification.
The classification given below is based primarily on the studies of Vandel, A (In Traite de Zoologie, Tome VI, ed, P. Grassae pp. 79-158, 1949) and Snodgrass, R. E. (Arthropoda, in ‘McGraw-Hill Encyclopedia of Science and Technology’ Vol. I, McGraw- Hill, New York, 1960).
Phylum Arthropoda:
I. Subphylum Trilobitomorpha or Trilobita:
(i) Class Trilobita, e.g., Agnostus, Trinucleus.
II. Subphylum Chelicerata or Arachnomorpha:
(i) Class Merostomata
1. Subclass Xiphosurida e.g., Limulus, Tachypleus.
2. Subclass Eurypterida e.g., Eurypterus.
(ii) Class Arachnida
Order Scorpionida e.g., Palamnaeus, Buthus.
Order Uropygi e.g., Mastigoproctus, Trithyreus.
Order Amblypygi e.g., Sarax, Myodalis, Tarantula.
Order Palpigradi e.g., Eukoenenia, Prokoenenia.
Order Araneida e.g., Araneus, Argiope, Lycosa, Latrodectus.
Order Ricinulei e.g., Cryptocellus, Ricinoides.
Order Pseudoscorpionida e.g., Garypus, Faella.
Order Solifugae e.g., Galeodes, Eremobates.
Order Opiliones e.g., Trogulus, Mitobates.
Order Acardia e.g., Trombicula, Argas, Boophilus, Dermacentor.
(iii) Class Pycnogonida e.g., Nymphon, Pycnogonum.
III. Subphylum Mandibulata:
(i) Class Crustacea
1. Subclass Cephalocarida
Order Cephalocarida e.g., Hutchinsoniella, Lightiella.
2. Subclass Branchiopoda
Order Anostraca e.g., Artemia, Branchipus.
Order Notostraca e.g., Triops, Lepidurus.
Order Diplostraca e.g., Leptodora, Daphnia.
3. Subclass Ostracoda
Order Mydocopa e.g., Philomedes, Cypridina.
Order Cladocopa e.g., Polycope
Order Platycopa e.g., Cytherella.
Order Podocopa e.g., Cypris, Darwinula.
4. Subclass Mystacocarida e.g., Derocheilocaris
5. Subclass Copepoda
Order Calanoida e.g., Calanus, Diaptomus
Order Harpacticoida e.g., Attheyella, Harpacticus.
Order Cyclopoida e.g., Cyclops, Ergasitus
Order Notodelphyoida e.g., Natodelphys, Doropygus.
Order Monstrilloida e.g., Monstrilla.
Order Caligoida e.g., Caligus, Eudactylina.
Order Lernaeopodoida e.g., Brachiella, Lernaea.
6. Subclass Branchiura
Order Branchiura e.g., Argulus, bolops.
7. Subclass Cirripedia
Order Thoracica e.g., Balanus, Lepas.
Order Acrothoracica e.g., Cryptophialus, Trypetesa.
Order Apoda e.g., Proteolepas
Order Rhizocephala e.g., Sacculina, Peltogaster.
Order Ascothoracica e.g., Synagoga, Dendrogaster.
8. Subclass Malacostraca.
1. Super order Phyllocarida
Order Nebaliacea e.g., Nebaliopsis, Nebalia.
2. Super order Hoplocarida
Order Stomatopoda e.g., Squilla, Coronida.
3. Super order Syncarida
Order Anaspidacea e.g., Anaspides, Paranaspides.
Order Bathynellacea e.g., Bathynella.
4. Super order Peracarida
Order Mysidacea e.g., Mysis, Neomysis.
Order Cumacea e.g., Cumopsis, Diastylis.
Order Tanaidacea e.g., Tanais, Neotanais
Order Isopoda e.g., Oniscus, Ligia.
Order Amphipoda e.g., Gammarus, Caprella.
5. Super order Eucarida
Order Euphausiacea e.g., Euphausia, Nematoscelis.
Order Decapoda e.g., Palaemon, Homarus, Palinurus, Scyllarus, Hippa, Eupagurus, Cancer.
(ii) Class Chilopoda
Order Scutigeromorpha e.g., Scutigera.
Order Lithobiomorpha e.g., Lithobius.
Order Scolopendromorpha e.g., Scolopendra
Order Geophilomorpha e.g., Geophilus.
(iii) Class Symphyla e.g., Scutigerella, Scolopendrella.
(iv) Class Pauropoda e.g., Pauropus.
(v) Class Diplopoda e.g., Scutigerella, Scolopendrella.
(a) Subclass Pselaphognatha
Order Pselaphognathae e.g., Polyxenus.
(b) Subclass Chilognatha
Order Platydesmida e.g., Platydesmus.
Order Polyzoniida e.g., Polyzonium.
Order Polydesmida e.g., Polydesmus.
Order Chordeumida e.g., Chordeuma.
Order Julida e.g., julus
Order Spirobolida e.g., Spirobolus
Order Spirostreptida e.g., Thyropygus.
(vi) Class Insecta or Hexapoda
(a) Subclass Apterygota
1. Super order Entognatha
Order Protura e.g., Eosentomon, Acerentomen.
Order Collembola e.g., Isotoma, Neanura.
Order Diplura e.g., Campodea, Heterojapyx.
2. Super order Gctognatha
Order Thysanura e.g., Lepisma, Machilis.
(b) Subclass Pterygota
1. Section Paleoptera
Order Ephemeroptera e.g., Ephemera, Hexagenia.
Order Odonata e.g., Aeschna, Libellula, Ischnura.
2. Section Polyneoptera
Order Dictyoptera e.g., Periplaneta, Mantis.
Order Isoptera e.g., Termes, Odontotermes.
Order Zoraptera e.g., Zorotypus.
Order Plecoptera e.g., Perla, Isoperla.
Order Notoptera e.g., Crylloblatta.
Order Cheleutoplera e.g., Carausius, Phyllium.
Order Orthoptera e.g., Hieroglyphus, Tryxalis, Locusta Schistocerca, Gryllotalpa.
Order Embioptera e.g., Embia.
Order Dermaptera e.g., Forficula.
3. Section Oligoneoptera
Order Coleoptera e.g., Photinus, Calandra, Adalia, Dineutus.
Order Megaloptera e.g., SialiSi Corydalis.
Order Raphidioptera e.g., Raphidia.
Order Planipennia e.g., Mantispa, Myrmeleon.
Order Mecoptera e.g., Panorpa.
Order Trichoptera e.g., Rhyacophilia.
Order Lepidoptera e.g., Parides, Papilio, Bombyx.
Order Diptera e.g., Anopheles, Musca.
Order Siphonaptera e.g., Pulex, Ctenocephalus.
Order Hymenoptera e.g., Apis, Vespa, Formica.
Order Strepsiptera e.g., Stylops.
4. Section Paraneoptera
Order Psocoptera e.g. Psocus.
Order Mallophaga e.g., Menopon.
Order Anoplura e.g., Pediculus.
Order Thysanoptera e.g., Heliothrips.
Order Homoptera e.g., Cicada, Aphis, Tachardia.
Order Heteroptera e.g., Cimex, Anasa, Triatoma.]
Indicates extinct group.
The classification, as given in the text book of Parker and Haswell (1972, 7th ed.), shows that the phylum Arthropoda is divided into seven subphyla.
The subphyla are:
Subphylum I. Onychophora
Subphylum II. Tardigrada
Subphylum III. Pentastomida
Subphylum IV. Trilobitomorpha
Subphylum V. Chelicerata
Subphylum VI. Pycnogonida
Subphylum VII. Mandibulata
The classification of Arthropoda of Ruppert and Barnes (1994) as given partly in this textbook (4th ed.), shows that the phylum is divided into four subphyla.
They have upgraded the class Crustacea into subphylum rank and the subclasses under Crustacea have also been upgraded into class rank. Most recent zoologists are in favour to retain Crustacea as a class and Remipedia, Cephalocarida, Branchiopoda, Ostracoda, etc., keep them as subclasses rank.
These subphyla include the following classes:
Remarks:
Ruppert and Barnes (1994) have placed pentastomids as a separate class Pentastomida, related to Branchiurans and Copepods but Anderson (1998) has placed these parasitic worms in a separate phylum Pentastomida.
Classification with Characters:
1. Subphylum Trilobita (or Trilobitomorpha) [Gk. tri = three, lobos = lobe, morphe = shape = three-lobed form]
Characters:
1. Extinct marine arthropods.
2. Body more or less oval and flattened from above downwards.
3. Body is divided into three regions:
(i) The anterior head or cephalon,
(ii) the middle region trunk or thorax and
(iii) a posterior pygidium.
4. Each region of the body is divided into 3 lobes by two longitudinal furrows, hence the animals derive their name Trilobites or three-lobed form.
5. Size varies from 10 mm to 60 cm.
6. Head and pygidium were covered by an un-jointed calcareous exoskeleton, called carapace.
7. Presence of a pair of compound eyes, found laterally on the anterior part of the body.
8. A pair of many-jointed antennae represents the pre-oral appendage.
9. Post-oral appendages are uniform, biramous and unspecialized. The innermost branch of each appendage was without long setae and was probably adapted for walking and the outermost branch had long filaments used for swimming or filtering food materials. The two branches are sometimes called endopodite and exopodite also.
10. Each leg has 8 segments.
11. The anal opening was on the last segment of the pygidium.
The subphylum includes 4000 species which are grouped under 5 classes and the class Trilobita includes the largest number of species.
Examples:
Agrestus, Ampyx, Mesonocis, Holmia, Trinucleus, etc.
2. Subphylum Chelicerata or Arachnomorpha [Gk. chele = claw]
Characters:
1. Heterogenous group of arthropods, in all of which pre-oral antennules or first antennae are absent (nonantennate).
2. Body is divided into two parts— cephalothorax (or prosoma) and abdomen (or opisthosoma) with no distinct head.
3. Cephalothorax (or prosoma) possesses five postoral segments, each with a pair of appendages.
4. First pair of appendages on the first postoral segment is called chelicerae and are feeding appendages. The chelicerae become pre-oral in position. First pair of appendages is not antennae but chelicerae, used in feeding.
5. Chelicerates have no jaws (mandibles); hence may be called amandibulates.
6. Each chelicera is jointed and bears a terminal chela.
7. Abdomen (opisthosoma) consists of 12-13 segments and a telson (telson and many abdominal segments are absent in certain forms).
8. Second abdominal segment bears genital aperture which remains covered by a modified abdominal appendage, called operculum.
9. Compound eyes in most cases degenerated.
10. Median simple eyes present.
11. Development usually direct.
12. Primarily marine arthropods, although most living forms are terrestrial.
It includes 3 classes:
(i) Merostomata
(ii) Arachnida and
(iii) Pycnogonida.
Class 1. Merostomata [Gk. meros = the thigh; stomatos, genitive of stoma = mouth]:
Characters:
1. Marine forms with fairly developed compound eyes, present laterally.
2. Head and thorax are fused into a single unit—the prosoma or cephalothorax covered by a single sheet of exoskeleton, the carapace.
3. First pair of appendages on the prosoma is the Chelicerae followed by 5 pairs of appendages, the walking legs.
4. Prominent caudal spine, called telson, present at the end of the body, used as a lever in pushing and balancing during locomotion.
5. Respiratory organs are gills (book- gills), which are borne on the plate-like appendages of the mesosoma.
6. Adults crawl on earth with the face downwards, but young can swim actively.
The class is divided into two subclasses:
(i) Xiphosura and
(ii) Eurypterida.
Subclass 1. Xiphosura [Gk. Xiphos = sword, oura = tail], Horse-shoe crab. Many fossil forms and 4 living species.
Features:
1. Cephalothorax (prosoma) is covered by a broad, smooth, and horse-shoe shaped carapace which is convex above and bears 2 pairs of eyes, one compound and lateral, and the other pair simple and median in position.
2. Caudal spine is elongated, slender and pointed.
3. Dorsal ridge is visible in the abdomen.
4. Abdomen (opisthosoma) bears 5 pairs of book-gills.
5. Excretion is performed by a four-lobed coxal gland.
6. Development with a larval stage, called trilobite larva.
7. These marine, bottom dwellers are commonly called horse-shoe crabs.
Examples:
Limulus, Tachypleus, Carcinoscorpius.
Subclass 2. Eurypterida or Gigantostraca (Water scorpions)
Features:
1. All are extinct forms.
2. Scorpion-like appearance.
3. Body is compressed dorsoventrally and protected by a chitinous exoskeleton.
4. Short prosoma is covered by dorsal carapace and consists of 6 segments fused together.
5. Trunk or abdomen is followed behind prosoma and consists of 12 free segments.
6. At the end of the trunk or abdomen there is a post and tail-plate or spine which may be triangular (e.g., Eurypterus) or divided into two lobes (e.g., Pterygotus).
7. Mouth ventrally placed of the prosoma.
8. Five pairs of prosomal appendages except 1st pair are 6-8 joints each and are associated with locomotion.
Examples:
Euryptarus (Ordovician- Permian), Pterygotus (Ordovician), Slimonia (Silurian).
Class 3. Arachnida [Gk. arachne = spider] Approx. 74,000 species.
Characters:
1. Body divided into two regions— cephalothorax (Prosoma) and abdomen.
2. Eyes usually simple.
3. Compound eyes when present are degenerated.
4. Two pairs of jointed cephalic append- ages-chelicerae and pedipalpi present. The first pair of cephalic appendages, known as chelicerae, which are preoral and the 2nd pair, the pedipalps, are postoral and serve partly as jaws.
5. Four pairs of thoracic legs present.
6. Abdominal segments often reduced and abdominal appendages not associated with locomotion.
7. Antennae absent.
8. In terrestrial forms, the respiratory organs are book-lungs or tracheae or both in some species.
9. Excretory organs are malpighian tubules or coxal glands or both.
10. Sexes separate.
11. Eggs yolky and centrolecithal.
12. Development not accompanied by metamorphosis.
13. Carnivorous and mostly terrestrial.
The class Arachnida is divided into 10 orders:
These are:
(i) Scorpionida,
(ii) Uropygi,
(iii) Amblypygi,
(iv) Palpigradi,
(v) Araneida,
(vi) Ricinulei,
(vii) Pseudoscorpionida,
(viii) Solifugae,
(ix) Opiliones and
(x) Acarida.
Order 1. Scorpionida:
1. These are terrestrial arachnids found under legs, stones or in rock crevices of tropical and subtropical regions.
2. Body is divided into prosoma (cephalothorax) and opisthosoma (abdomen).
3. Prosoma unsegmented and covered by carapace bearing a pair of chelicerae, a pair of pedipalpi and four pairs of walking legs.
4. Abdomen divisible into two parts— broad pre-abdomen (mesosoma) consisting of 7 segments and narrow post- abdomen (metasoma) consisting of 5 segment with a caudal spine formed by the modification of telson.
5. Chelicerae are small 3-segmented but pedipalpi are large and 6-segmented.
6. Second segment of the pre-abdomen bears two comb-shaped pectines.
7. Four pairs of respiratory organs, called book lungs.
8. Viviparous.
This order includes scorpions.
Examples:
Palamnaeus, Mesobuthus, Buthus, Scorpio, etc.
Order 2. Uropygi:
1. These are commonly called whip scorpions.
2. Size ranges from 2-65 mm in length.
3. Telson is present as a long whip-like flagellum at the posterior end.
4. Poison glands are absent but an acidic substance from the anus is sprayed.
5. Female provided with a brood sac to carry eggs.
6. Upper lip of the rostrum and the bases of the pedipalps form a peculiar ball and socket joint around the mouth to act as a filtering apparatus.
7. Short and stout pedipalp.
8. Second leg is antenna-like and elongated.
9. Abdomen bears 12 segments.
10. Book lungs placed on the second and third abdominal segments,
11. Nocturnal and carnivorous.
Examples:
Mastigoproctus, Trithyreus.
Order 3. Amblypygi:
1. Commonly known as tailless whip scorpions.
2. Size varies from 4-45 mm in length.
3. Body flattened dorsoventrally.
4. Telson almost absent.
5. Abdomen consists of 11 segments.
6. Chelicerae are 2-jointed and hook-like.
7. Pedipalps consist of seven segments and are stout and raptorial.
8. A prominent movable claw is present at the distal end of each pedipalp.
9. Book lungs open on the ventral side of second and third abdominal segments.
10. Females carry brood-sacs during breeding season.
Example:
Tarantula.
Order 4. Palpigradi:
1. Commonly called microwhip scorpions.
2. Size ranges from 0.5-3 mm in length.
3. In the cephalothorax last two segments are not united.
4. Telson with a many-jointed flagellum.
5. Chelicerae are 3-segmented and chelated.
6. Pedipalpi leg-like.
7. Both simple and compound eyes are absent.
8. Second pair of legs act as antennae.
9. Respiration is cutaneous.
10. In addition, paired eversible sacs in the abdominal segments also act as respiratory organs.
Example:
Eukoenenia.
Order 5. Araneida:
1. Commonly called spiders.
2. Cephalothorax un-segmented.
3. Abdomen in general is also un-segmented, soft and round.
4. Four pairs of eyes are present.
5. Chelicerae are complex and poison glands open through them.
6. Pedipalpi simple and six-jointed.
7. Book lungs are associated with trachea for respiration.
8. Spinning glands usually present and appendages of fourth and fifth abdominal segment form spinnerets.
Examples:
Argiope, Aranea (orb-web spider), Latrodectus, Lycosa (wolf-spider), etc.
Order 6. Ricinulei:
1. Commonly called Ricinuleids.
2. Sizes usually less than 10 mm in length.
3. Body short and compact.
4. Cephalothorax is drawn anteriorly into a many-jointed movable projection, called cucullus.
5. Eyes are absent.
6. Abdomen superficially looks like four segmented but truly it consists of nine segments.
7. Both the chelicerae and pedipalpi are chelated.
8. Tracheae are the respiratory organs.
Examples:
Cryptocellus, Ricinoides.
Order 7. Pseudoscorpionida:
1. These are commonly called pseudoscorpions.
2. Size is never more than 8 mm in length.
3. Body oval and flattened dorsoventrally.
4. Abdomen broad and consists of eleven segments.
5. Abdomen is not separated into pre- and post-abdomen and does not bear the caudal sting.
6. Simple eyes are absent.
7. Chelicerae are small and contain silk glands.
8. Pedipalpi is scorpion-like and contains poison glands.
9. Respiratory organs are tracheae.
10. Malpighian tubules are absent.
11. Oviparous.
12. These are found under the bark of trees.
Examples:
Chelifer, Neobisium.
Order 8. Solifugae:
1. Commonly known as wind scorpions.
2. Size varies form 1-7 cm in length.
3. These are swift runners.
4. Body is divided in 3-regions—head, thorax and abdomen.
5. Abdomen oval and contains ten segments.
6. Chelicerae large and prominent.
7. Pedipalpi elongated and resembles the legs.
8. Poison glands are absent.
9. Respiratory organs are well-developed tracheae.
10. All are nocturnal, ferocious and predators.
Examples:
Galeodes, Eremobates.
Order 9. Opiliones:
1. Commonly called harvestmen.
2. Size varies from 1-22 mm in length.
3. Cephalothorax un-segmented.
4. Abdomen consists of 10 segments and is not distinctly separated from the cephalothorax.
5. Telson absent.
6. Compound eyes are lacking.
7. Pedipalpi is leg-like.
8. Walking legs are usually very long and slender.
9. Respiration takes place through tracheae.
10. No poison or spinning glands.
11. Males possess copulatory organ and females are provided with ovipositor.
Examples:
Caddo, Phalangium, Trogulus, Mitobates.
Order 10. Acarida:
1. Commonly known as ticks and mites.
2. Number of forms is microscopic in sizes.
3. Body without any external division.
4. Chelicerae and pedipalpi are usually small and associated with the mouth- parts which are adapted for—biting, piercing or sucking.
5. Chelicerae are usually composed of 2 to 3 segments but may have up to 6 segments.
6. Chelicerae may be pincer-like, fanglike or lance-like.
7. Pedipalpi also may be variously modified.
8. Legs are provided with claws.
9. Respiratory organs are tracheae or in many, respiration is cutaneous.
10. Most of them are parasites on man and other animals.
11. Some are notorious pests of agricultural products.
Examples:
Tetranychus, Demodex, Hoplophorella, Annectacarus, Sarcoptes.
Class 3. Pycnogonida (= Pantopoda):
Approx. 1,000 species; about 16 Indian species.
Characters:
1. Partially sedentaric marine chelicerates, commonly called sea spiders.
2. Young’s are parasitic on different soft bodied invertebrates.
3. Externally segmented body.
4. Reproductive openings present on the leg segments and not abdominal.
5. Chelicerae short and pedipalpi segmented.
6. Head with proboscis.
7. Third pair of appendages in the male carries the eggs and is called the ovigers.
8. Trunk of 3-6 segments with long walking legs.
9. Opisthosoma much reduced with a terminal anus.
10. Protonymphon larva in the life cycle of most pycnogonids.
Examples:
Nymphon, Pycnogonum, Colossendeis.
3. Subphylum Crustacea:
[L. crusta – a hard shell] Copepods, shrimps, prawns, barnacles, lobsters crabs. Approx. 45000 species; about 3000 Indian species.
Characters:
1. Body divisible into 3 regions—head, thorax and abdomen.
2. Two pairs of antennae are a distinguishing feature among crustaceans.
3. Other cephalic appendages are a pair of mandibles and two pairs of maxillae.
4. Thoracic and abdominal appendages are usually 8 pairs and 6 pairs, respectively, variable in lower crustacea.
5. Appendages typically biramous except of antennules.
6. Carapace covers all or part of the body.
7. Head bears a pair of compound eyes on movable jointed stalk.
8. Respiration takes place either by gills or by the general surface of the body when the exoskeleton is thin or by some of the limbs.
9. Vascular system consists of a contractile heart, arteries and haemocoelomic spaces.
10. Excretory organs are the modification of coelomoducts may be either antennal glands (green glands) or shell glands (maxillary glands found in the second pair of maxillae).
11. Brain formed by the fusion of first four embryonic ganglia and is connected with ventral nerve cord by oesophageal connectives.
12. Sexes separate.
13. Distinct sexual dimorphism present.
14. Eggs usually centrolecithal, i.e., yolk present in the central part of the egg, or may be telolecithal, i.e., yolk occupies one-half of the egg, or alecithal, i.e., without yolk.
15. Development includes a larval form, the nauplius, bearing a single median eye and 3 pairs of appendages.
16. Mainly aquatic, mostly marine, many freshwater and some have invaded into terrestrial condition.
The subphylum crustacea is divided into 11 classes:
(i) Remipedia,
(ii) Cephalocarida,
(iii) Branchiopoda,
(iv) Ostracoda,
(v) Copepoda
(vi) Mystacocarida,
(vii) Branchiura,
(viii) Pentastomida,
(ix) Tantulocaride,
(x) Cirripedia and
(xi) Malacostraca.
Remarks:
Many zoologists, such as Ruppert and Barnes (1994) use the category subphyla:
(i) Crustacea for typical bi-ramous appendages (e.g., copepods, barnacles, shrimps, lobsters and crabs) and
(ii) Uniramia for uniramous appendages (e.g., centipedes, millipedes and insects) instead of subphylum Mandibulata.
Pechenik (2000) and many zoologists agreed to use Mandibulata containing the classes— Crustacea, Myriapoda and Insecta.
He used Myriapoda (Gk. many feet) containing 4 orders, such as:
(i) Order Chilopoda (e.g., centipedes),
(ii) Order Diplopoda (e.g., millipedes),
(iii) Order Symphyla (e.g., symphylans) and
(iv) Order Pauropoda (e.g., pauropods).
The class Myriapoda is characterised by many segmented trunk, with each bearing uniramous legs, a pair of antennae, compound eyes absent and malpighian tubules for excretion.
Class 1. Remipedia:
This group was first recognised in 1983 with twelve known species.
Features:
1. Small and worm-like bodies, range up to 30 mm in length.
2. Head covered by a head-shield, followed by a trunk of 20-30 similar segments.
3. Each segment of the body bears a pair of lateral biramous appendages.
4. Telson with caudal rami.
5. Hermaphrodite.
They are the inhabitants of tropical marine caves.
Examples:
Lasionectes, Speleonectes.
Remarks:
Morphological data of this group suggest to be the closest of all the animals to the ancestral crustacean body but molecular data remain ambiguous.
Class 2. Cephalocarida [Cephalocarids; Approx. 9 species]:
The members of this group are considered to be most primitive among living crustaceans and the first member was discovered in Long Island Sound in 1955. The all species are marine and have collected in the soft sediments of the bottom up to the depths of over 1,500 m.
Features:
1. Small-sized animals exceeding 3.7 mm in length.
2. Horse-shoe shaped head followed by an elongated trunk.
3. First 8 trunk segments bear biramous appendages which are identical in appearance.
4. The appendages are tripartite.
5. Exopodites of these appendages are four-jointed and leaf-like and bear lateral pseudoepipodite.
6. Endopodites are segmented, cylindrical and ambulatory in function.
7. Movements of the limbs produce water current for locomotion and also for collecting food.
8. Eyes are buried in the head.
9. Hermaphrodite.
Examples:
Hutchinsoniella, Lightiella.
Class 3. Branchiopoda (Gk. branchiona fin); Approx. 800 species:
Features:
1. Mostly freshwater species, a few are marine.
2. Trunk appendages are uniform and leaf-like.
3. Presence of one pair un-jointed or jointed caudal styles.
4. Carapace either absent or shield-like or bivalve.
5. First antennae and maxillae are small and in some cases absent.
6. Mandibular palp either rudimentary or absent.
It has 3 living orders:
(i) Anostraca
(ii) Notostraca and
(iii) Diplostracs.
Order (i) Anostraca:
1. It includes forms like fairy shrimps and brine shrimps.
2. Carapace is absent.
3. They possess stalked eyes.
4. Antennae redudced and triangular in females.
5. In males, the antennae are stout copulatory structures.
6. Trunk is elongated and first 11 segments bear alike legs.
7. Caudal styles un-jointed.
Examples:
Branchipus, Artemia.
Order (ii) Notostraca:
1. Commonly called Tadpole shrimp.
2. Carapace is large and shield-shaped.
3. Possesses 35-71 pairs of legs.
4. Eyes are sessile and placed close together.
5. Antennae rudimentary.
6. Caudal styles filamentous and many- jointed.
7. Parthenogenesis is frequent.
Examples:
Triops, Lepidurus.
Order (iii) Diplostraca:
1. It includes forms like clam-shrimps (Estheria) and water fleas (Daphnia, Leptodora).
2. Carapace laterally compressed, bivalved and enclosing trunk.
3. Biramous and large antennae are used during swimming.
4. Caudal styles clawed and un-jointed.
Examples:
Daphnia, Sida, Leptodora.
Class 4. Ostracoda (Gk. ostrakodes – testaceous resembling a shell); Approx. 7000 species:
Features:
1. Small crustaceans and commonly called seed-shrimps.
2. Mostly marine or freshwater, a few in terrestrial habitats.
3. Body enclosed in a hinged bivalved carapace.
4. Trunk appendages never more than 2 pairs.
5. Mandible with a palp.
6. Both pairs of antennae modified for swimming.
7. Respiration usually cutaneous.
8. Eyes may or may not be present.
9. Males are rare and the second antennae of the males serve as clasping organs.
The class Ostracoda includes 4 orders:
(i) Mydocopa
(ii) Cladocopa
(iii) Platycopa and
(iv) Podocopa.
Order (i) Mydocopa:
1. Carapace bears an aperture through which the antennae protrude.
2. Antenna is operated by powerful muscles.
3. Limbs are four pairs.
4. Heart with paired ostia.
5. Sessile compound eyes.
Examples:
Philomedes, Cypridina.
Order (ii) Cladocopa:
1. The aperture for the protrusion of antennae is absent.
2. Each second antenna bears two branches or rami.
Example:
Polycope.
Order (iii) Platycopa:
1. Antennal aperture absent.
2. Rami of second antennae broad.
3. Limbs three pairs.
Example:
Cytherella.
Order (iv) Podocopa:
1. Antennal aperture on the carapace absent.
2. Endopodite of the second antenna well developed than exopodite and carries a claw.
3. Mandibular palp composed of four articles.
4. Four pairs of limbs are posteriorly placed.
Examples:
Cypris, Darwinula.
Class 5. Copepoda (Gk. kope = handle) Approx. 8,500 known species:
Features:
1. Mostly small crustaceans.
2. Body with well-marked segments.
3. Trunk composed of a thorax bearing 5 pairs of biramous appendages used for swimming.
4. Abdomen without appendages.
5. Presence of a pair of caudal styles.
6. Head-shield present but no carapace.
7. Single median nauplius eye present but paired compound eyes absent.
8. Well-developed antennae may or may not be used for swimming.
9. Seventh segment of the body bears the reproductive apertures.
It includes 7 orders:
(i) Calanoida
(ii) Harpacticoida
(iii) Cyclopoida
(iv) Notodelphyoida
(v) Monstrilloida
(vi) Caligoida and
(vii) Lernaeopodoida.
Order (i) Calanoida:
1. Free-living and size is fairly large.
2. Posterior part of the trunk is distinctly separated from the anterior part and between genital and pregenital segments.
3. First antennae of female have 23-25 segments.
4. The second antenna is biramous.
5. The female possesses a single median egg-sac.
Examples:
Canthocalanus, Eucalanus, Calanus, Paracalanus, Acrocalanus, Centropages, Phyllodioptomus, Heliodioptomus, Allodioptomus, Temora, Candacia, Calanopia, Acardia, Tortanus, Diaptomus.
Order (ii) Harpacticoida:
1. Free-living.
2. Trunk not constricted in the middle.
3. First antenna in the female is short and consists of 5-9 segments.
4. Second antenna is biramous.
Example:
Attheyella.
Order (iii) Cyclopoida:
1. Free-living.
2. Posterior part of the trunk is separated from the anterior part and it includes pregenital segments.
3. First antenna of female has more than 17 articles.
4. Second antenna is uniramous.
5. Female carries a pair of egg-sacs.
6. Some members are parasitic.
Examples:
Cyclops, Ergasilus, Eucyclops.
Order (iv) Notodelphyoida:
1. Only in males the posterior half of the trunk bears pregenital segments.
2. Both sexes live as commensal within a tunicate, Ascidia.
Example:
Notodelphys.
Order (v) Monstrilloida:
1. Free-swimming and completely marine.
2. Adults without mouth parts, antennae and alimentary canal.
3. Larva starts as a free-swimming nauplius and parasitises a mollusc or annelid.
4. It draws nourishment by its antennae from the host.
5. Adult, when full grown, becomes again free-swimming and perform only reproduction.
Example:
Monstrilla.
Order (vi) Caligoida:
1. Lives as ectoparasite on fishes.
2. Posterior part of trunk carries two pregenital segments.
3. The antennae are modified to act as adhesive organs.
Examples:
Caligus, Eudactylina.
Order (vii) Lernaeopodoida:
1. Adults remain as ectoparasites up to the attainment of sexual maturity.
2. The sexually matured forms become free-swimming.
3. After copulation, female again becomes parasitic on fishes.
4. It changes into a worm-like form and carries the eggs in a brood chamber.
5. Larvae start as free-swimming nauplius but soon infect a new host.
Examples:
Lernaea, Lernaeocera.
Class 6. Mystacocarida —Approx. 8 known species:
This subclass was created after the discovery of several crustaceans in the year in 1943.
Features:
1. Marine and interstitial.
2. Length of the body always within 1 mm.
3. Cylindrical bodies with distinct cephalic appendages.
4. Trunk consists of 5 segments each with a pair of appendages.
5. Nauplius eye persists and the compound eyes absent.
6. Two caudal styles work as pincers.
Examples:
Derocheilocaris.
Class 7. Branchiura —Approx. 130 known species:
Features:
1. Fish ectoparasites.
2. Dorsoventrally flattened body with suctorial mouth.
3. Broad shield-like carapace covers the cephalothorax.
4. Small, un-segmented and bilobed abdomen.
5. Sessile compound eyes present.
6. Flagella present in the appendages of some body segments.
7. 5 pairs thoracic appendages.
8. Fifth body segment bears the genital apertures.
9. Males have two testes but females possess a single ovary.
10. Commonly called fish lice.
It includes a single order, having the same name and includes a single family.
Examples:
Argulus, Dolops.
Class 8. Pentastomida—Approx. 100 known species:
Features:
1. All the members are parasitic and live mainly in the lungs and nasal passages of reptiles, but some species parasitize amphibians, birds and mammals including dogs and man.
2. Worm-like body ranges 2 to 13 cm long, of which the females are 10 cm in length.
3. Larva possesses 2-3 pairs of un-jointed Legs.
4. Adults are legless but possess only 4 pairs of anterior chitinous hooks, used for clinging to the host tissues.
5. Body covered by a non-chitinous cuticle and exhibits annular markings over the abdomen in the adult.
6. Exoskeleton moulted periodically.
7. Muscles are striated and metamerically arranged.
8. Most of the systems, such as digestive, excretory and reproductive are modified to adapt the endoparasitic life.
9. Completion of the life history requires intermediate host.
10. They are gonochoristic, i.e., the sexes are separate.
11. Fertilization internal.
12. Cleavage spiral.
13. Pentastomids are popularly known as Tongue worms or sometimes referred to as “five mouths”.
Remarks:
The taxonomic status of Pentastomids has long been uncertain. Previously this group was treated as a separate phylum. But recently the sperm ultrastructure and analyses of DNA sequences coding for 18s ribosomal RNA indicate the similarities with crustaceans and suggest that Pentastomids are closely related to marine crustaceans, specially with branchiurans and copepods.
Examples:
Raillietiela, Cephalobaena, Linguatula, Armillifer.
Class 9. Tantulocarida—Approx. 5 known species:
Features:
1. Ectoparasite on other deep water crustaceans.
2. Deep water crustaceans.
3. Appendages absent in trunk segments.
Examples:
Basipoplella.
Class 10. Cirripedia (L. cirrus = curled, pedis = foot). Approx. 1000 known species:
Features:
1. All are marine.
2. Adults are sedenteric.
3. Body poorly segmented.
4. Six pairs biramous filamentous appendages present.
5. Abdomen almost absent, with only a pair of caudal style.
6. Body enclosed within a bivalve carapace with calcareous plates on it.
7. Adults without eyes and antennae.
8. Usually hermaphrodite.
9. Young passes through nauplius and cypris stage.
10. Commonly called barnacles.
This subclass includes 5 orders:
(i) Thoracica
(ii) Acrothoracica
(iii) Apoda
(iv) Rhizocephala and
(v) Ascothoracica
Order (i) Thoracica:
1. Adults are permanently fixed by its preoral region on substrates at inter- tidal level.
2. Presence of six pairs of appendages in the trunk called cirri.
3. Abdomen without segments.
4. Some forms have a stalk, others are sessile.
Examples:
Examples are Lepas (Goose barnacles), Balanus (Acorn barnacles).
Order (ii) Acrothoracica:
1. Sessile forms and are fixed on the shell of molluscs.
2. Mantle devoid of calcareous plates.
3. Presence of only four pairs of appendages in the trunk.
4. Male lives as parasite within female.
Example:
Alcippe.
Order (iii) Apoda:
1. Completely parasitic forms.
2. Small distinctly segmented body.
3. Trunk appendages absent.
Example:
Proteolepas.
Order (iv) Rhizocephala:
1. Completely parasitic.
2. Mantle remains, but the calcareous shells are absent.
3. Body absolutely degenerated in adults due to the loss of alimentary canal and appendages.
4. No trace of segmentation.
Examples:
Sacculina, Peltogaster.
Order (v) Ascothoracica:
1. These are endoparasites on Anthozoa and Echinodermata.
2. Mantle bilobed or sac-like but plates are absent.
3. Oral appendages are modified for piercing and sucking.
4. Presence of 6 pairs of appendages in the trunk.
Examples:
Symagoga, Laura, Dendrogaster.
Remarks:
Recent trend of the crustacean classification shows that the subclasses Mystacocarida, Copepoda, Branchiura, Tantulocarida and Cirripedia are included under the class Maxillopoda for the characteristic features—6 thoracic and 5 abdominal segments and the first pair of trunk appendages are maxillipeds.
Class 11. Malacostraca (Gk. malakos = soft + ostracon = a shell). Over 20,000 species:
Features:
1. Body consists of 20-21 segments.
2. Thoracic and abdominal appendages distinct from one another.
3. Carapace covers the head and at least some thoracic segments.
4. Mandible with a palp.
5. Presence of compound eyes on stalk.
6. Antennule with two-many-jointed flagella.
7. Male and female gonopores on the bases of 8th and 6th thoracic appendages.
The class includes five super orders:
(i) Phyllocarida
(ii) Hoplocarida
(iii) Syncarida
(iv) Peracarida and
(v) Eucarida.
Super order (i). Phyllocarida (Gk. phyllon = a leaf, L. caridis, genitive of caris = a shrimp):
Features:
1. A movable rostrum present at the anterior end of cephalothorax.
2. Carapace large, bivalved and encloses both the cephalothoracic and abdominal segments.
3. Thoracic legs are all alike and folia- ceous.
4. Abdomen has 7 segments.
5. Well-developed appendages in the first four abdominal segments.
6. Telson with a movable caudal furca.
It includes only one order Nebaliacea.
Order Nebaliacea:
Presence of seven abdominal segments. Telson with a pair of caudal styles. A prominent carapace covers almost the entire length of the body. Thoracic appendages are leaf-like and serve as respiratory surface. Abdominal appendages are biramous. The common example is Nebalia.
Super order (ii). Hoplocarida (Gk. hoplon = a weapon):
Features:
1. All are marine.
2. Carapace flat, shield-shaped and encloses the second thoracic segment.
3. Two movable segments lie anterior to the carapace.
4. Anterior one bears stalked eye and the posterior one carries the antennules.
5. Each antennule has three rami.
6. Antenna smaller than antennule.
7. Second pair of thoracic appendages is raptorial and bear a blade-like edge at its distal end.
8. Biramous abdominal appendages.
It includes only a single order Stomatopoda.
Order Stomatopoda:
Last four thoracic segments are free from the carapace. Two movable segments are present in front of the head. Gills are carried by the abdominal appendages. Gastric glands extend up to the telson. Heart extends up to abdomen and has thirteen pairs of ostia. The well-known examples are Scjuilla, Pseudoscjuilla and Coronida.
Super order (iii). Syncarida (Gk. syn = together):
Features:
1. Carapace absent.
2. Elongated and tube-like heart.
3. Only the first pair of thoracic appendages modified as maxillipeds, rest are alike.
4. Biramous thoracic appendages.
5. Gills present on the thoracic appendages excepting the last one.
6. Last pair of abdominal appendages, called uropods, is fan shaped.
It includes two orders:
(i) Anaspidacea and
(ii) Bathynellacea.
Order (i) Anaspidacea:
1. First thoradc segment is fused with head.
2. They are inhabitants in the freshwater of ponds, streams of Australia, New Zealand and South America.
Examples:
Anaspides, Allanaspides.
Order (ii) Bathynellacea:
1. They are the smaller than the members of Anaspidacea.
2. First thoracic segment is not fused with the head.
3. They are living in freshwater sediments and are world-wide in distribution.
Example:
Bathynella.
Super order (iv). Peracarida (Gk. pira = a pouch):
Features:
1. Carapace may or may not be present. When present, carapace never covers last four thoracic segments.
2. Coxopodites of thoracic appendages bear a brood-pouch in females.
3. Presence of a tube-like, elongated heart.
4. Development direct.
This super order includes 5 orders:
(i) Mysidacea
(ii) Cumacea
(iii) Tanaidacea
(iv) Isopoda and
(v) Amphipoda.
Order (i) Mysidacea:
1. First antennae are biramous.
2. Second antenna is with scale-like squama.
3. Possesses filter feeding mechanism.
4. First pair of thoracic appendages is modified as maxillipeds.
5. Carapace is the chief respiratory surface.
6. Broad tail fin is formed by flat uropods and telson.
7. Heart is elongated but extends up to thorax and has two pairs of ostia. The common Examples are Mysis, Hemimysis.
Order (ii) Cumacea:
1. Cephalothorax is posteriorly narrow.
2. Carapace is drawn out anteriorly to form rostrum and ventrally to form gill-chamber.
3. Sessile eyes are usually fused to form a single eye.
4. Antennae are usually un-segmented.
5. Second antennae are without exopodites and well developed in males than in females.
6. Abdomen is slender and segmented.
7. Abdominal appendages are absent in the female.
8. Uropods are rod-shaped and thus fan- shaped tail fin is absent.
The well-known examples are Cumopsis, Diastylis and Pseudocuma.
Order (iii) Tanaidacea:
1. Carapace covers first two thoracic segments.
2. Eyes, when present, are mounted on immovable stalks.
3. Second thoracic appendages are large and chelated.
4. A small squama may be present with the second antenna.
5. Uropods are slender.
The examples are Tanais, Apseudes and Neotanais.
Order (iv) Isopoda:
1. It includes aquatic, terrestrial and parasitic forms.
2. Carapace is absent, only first thoracic segment is fused into head.
3. Eyes are either without stalk or they are carried on small immovable processes.
4. Body is dorsoventrally flattened.
5. Antennule is small and rudimentary.
6. First pair of thoracic appendages is modified as maxillipeds, while the others are alike.
The well-known examples are Anthura, wood borer (Limnoria), the ectoparasite of the fish, the Cymothoidae; parasites on the gills of shrimps and crabs (Bopyrus), wood lice (Ligia, Liriopsis, Oniscus and Tylos, Adinda, Agnara, etc.).
Oniscus is an example of terrestrial crustacea). The wood boring isopods, Limnoria lignorum, make deep funnels in wooden harbour for sheltering their teeming animals. They can also damage the submarine cables.
Order (v) Amphipoda:
1. Body is flattened laterally.
2. Carapace is absent.
3. Antennules are well developed and biramous.
4. Second and third pairs of thoracic appendages are prehensile structures, called gnathopods.
5. Some thoracic appendages bear gills at their bases.
6. Abdominal appendages are of two distinct morphological forms.
The examples are Gammarus, Caprella.
Super order (v). Eucarida (Gk. eu = true, L. caridis, genitive of caris = a shrimp):
Features:
1. Carapace covers head and all the thoracic segments.
2. Mandible without sharp blade.
3. Stalked eyes.
4. Small bag-like heart, placed on the dorsal side of the thorax.
Eucarida has two orders:
(i) Euphausiacea and
(ii) Decapoda.
Order (i) Euphausiacea:
1. Thoracic appendages do not form maxillipeds and are all alike.
2. Single gill is present at the base of each thoracic appendage.
3. Pleopods are flattened.
4. In the males first two pairs are modified for copulation.
5. Elongated telson bears a movable large spine. Uropods are elongated.
The examples are krill (Euphausia, Nematoscelis, etc.), and all the members are marine, pelagic, shrimp-like animals, and have a world-wide distribution. In the Southern Ocean they constitute a major food source of Baleen whales.
Order (ii) Decapoda (Gk. deka = ten, podos = foot):
1. Three maxillipeds are formed by the modification of first three thoracic appendages.
2. Three sets of gills are present which differ in their arrangements.
This order is divided into 2 suborders:
(i) Dendrobranchiata and
(ii) Pleocyemata.
Suborder Dendrobranchiata:
Gills are dendrobranchiate type. The body is almost laterally compressed. The examples are penaeid shrimps (e.g., Penaeus, Funchalia, Parapenaeus, Metapenaeopsis, Metapenaeus), sergestid shrimp (Sergestes, Lucifer, Acetes).
The penaeid shrimps (fam. Penaeidae) are characterised by the well-developed and toothed rosturm, carapace without postorbital spine, 3rd and 4th pairs of pleopods biramous and telson sharply pointed with or without spines. They are found in sandy, mud estuaries, back water and near shore areas. The penaeids are the most valuable commercial shrimps exploited in many parts of the world.
A list of some Indian penaeid shrimps is given below (Table 18.13).
The other families of commercial shrimps are Solenoceridae, Aristeidae, and Sicyoniidae. The sergestid shrimps (Fam. Sergestidae) are characterised by small size, short rostrum and last 2 pairs of legs are shorter or absent. The examples are Sergestes, Lucifer, etc.
Suborder Pleocyemata:
Gills are phyllobranchiate or trichobranchiate type. It includes 6 infraorders—Stenopodidea, Caridea, Astacidea, Palinura, Anomura and Brachura.
The three Caridean families:
Pandalidae, Cragnonidae and Palaemonidae are more commercially important and are found in seas and brackish waters. The Caridea includes snapping shrimps (Fam. Alpheidae, e.g., Alpheus), sand shrimps (Fam. Crangonidae, e.g., Crangon), Cock shrimps (Fam. Hippolytidae, e.g., Hippolysmata), Pandalid shrimps (Fam. Pandalidae, e.g., Helerocarpus), Palaemonid shrimps or Prawn (e.g., Palaemon, Macrobrachium, Leander, Brachycarpus, etc.).
In India the members of Palaemonidae are highly commercially important and characterised by the carapace cylindrical with laterally compressed rostrum, 1st pair of legs shorter and more slender than the 2nd pair, telson elongated with 2 pairs of dorsal and 2 or 3 pairs of posterior spines, and exopods absent on the legs. They are found in the freshwater ponds, canals, brackish water and seas.
The freshwater prawn, Macrobrachium rosenbergii, sometimes indicated as Palaemon carcinus auc non (Linnaeus, 1758), is found mainly in freshwater ponds, canals, rivers and also in brackish water. They are called the Golda chingri in Bengal.
Other important commercial species in India are Macrobrachium malcomsonii and M. rude. The monsoon river prawn, M. malcomsonii is found in rivers and estuarine areas of both coasts of India and Bangladesh. The species is extensively fished in the Chilika Lake, Orissa (India).
The Cray fish, Lobsters and Lobsterettes belong to the infraorder Astacidea. It is characterised by the well-developed abdomen, first 3 pairs of walking legs with pincers and especially its first pair usually enlarged. The examples of lobsters are Acanthacaris, Metanephrops and Homarus, etc. and freshwater crayfish are Astacus and Cambarus.
The prickly deep sea lobster, Acanthacaris tenuimana (Fam. Nephropidae) is a deep sea lobster, found in west coast and Andaman lobster; Metanephrops andamanicus (Fam. Nephropidae) live between 200 m and 750 m and has some commercial importance. Homarus is an American lobster.
The Infraorder Palinura is characterised by carapace more or less cylindrical, abdomen somewhat flattened and legs may be chelate or sub-chelate. The spiny lobsters (Fam. Palinuridae) are characterised by without median rostrum but carapace with spines. The first pair of limbs is greatly enlarged and first four walking legs are without chelate.
The examples are Palinurus, Panulirus, Puerulus, etc. which is found in rocky and gravel bottoms. In India Panulirus ornatus and Panulirus polyphagus are the more or less common spiny lobsters, found in some pockets of west coast and east coast. They are commercially valuable species.
The Slipper lobsters are included under the family Scyllaridae which are characterized by the carapace usually granular, sometimes with spines and without rostrum. The flattened body and all walking limbs are without chelate. Examples are Thenus, Ibacus, Scyllarus, etc.
The flathead lobsters, Thenus orientalis is found in east and west coast of India and are the inhabitants of sandy or muddy sea bottoms. The locust lobster, Scyllarus orientalis is found in the south west coast of the Arabian Sea and inhabitants of sandy or muddy bottoms.
The Infraorder Anomura is characterized by depressed carapace and abdomen more or less reduced. Examples are hermit crabs, Diogenes, Pagurus, Eupagurus, Petrochirus, Dardanus, robber crab or coconut crab (Birgus), mole crabs, Emerita, Hippa, etc.
The coconut crab, Birgus latro is distributed in the Indo-Pacific Islands and can climb to the top of the tallest coconut trees to pluck the coconuts. They can un-husk the outer fibrous layer of the coconut and eat the inner endocarp part (white part) by crushing the hard shell with the help of chela.
Lithodes, Paralithodes—commonly called king crabs because of their size among the largest crustaceans, are found in the North Pacific. The mole crabs are found in the wave- swept sandy beaches of the tropical seas.
The hermit crabs generally take refuge in the empty molluscan gastropode shells.
The Infraorder Brachyura is characterized by the broad carapace, abdomen greatly reduced and permanently flexed beneath the body. The eyes are retracted into the cavities.
Examples are spider crabs (Maja, Arcania, Macrocheira), cancer crabs (Cancer sp.), swimming crabs (Portunus sp. Carcinus sp., Scylla sp.), pea crabs (Pinnotheres sp.), mud crabs (Pilumnus sp., Neptunus sp., Charybdes sp., Varuna sp.), freshwater crabs (Paratelphusa sp., Potamon sp.), ghost crabs (Ocypode sp.), fiddler crabs (Uca), grapsid crabs (Sesarma sp., Crapsus sp.).
The giant spider crab (Macrocheira kaemferi) is found in the coasts of Japan and the size may reach as much as 4 m between the tips of its legs.
Scylla serrata is found in the brackish water region in India. This crab is mainly found in the deltaic regions of Bengal and is called ‘Nona Kakra’ and consumed by the local people. They are in great demand in the markets of kolkata and other towns, and mainly supplied from the Canning areas (near Kolkata). Other brackish food crabs are Portunus sanguinolentus, Portunus pelagicus and Varuna literata.
The crab Scylla is identified by smooth carapace, hand smooth and inflated, body regions are not well differentiated and anterolateral borders are marked by 9 large teeth. The food crab, Portunus is characterized by the divided carapace, hand prismatic and costate, and antero-lateral borders with 9 large teeth.
The freshwater crab, Paratelphusa is identified by the absence of swimming legs or pleopods and broad abdomen. Paratelphusa mansomana, Paratelphusa guerini and Paratelphusa spinigera, etc. are found in the freshwater habitat in different parts of India. Paratelphusa is consumed as a food and as a cure for common cold.
The bushy crab, Pilumnus is characterized by moderately convex carapace and lateral border with sharp teeth. Pilumnus hirsutus, P. virensis, P. cursor, P. caerulescens and P. longicornis are found in Andamen and Nicobar Islands, and P. vespertilio is found in Ross Island (Andaman) and Krusadai Island (Tamil Nadu). Graspid crabs, Separma sp. live in burrows in the depth of 2 m.
The ghost crabs Ocypode ceratophthalma live in the burrows above the highly tide mark on the upper sandy beaches, frequently found in the shore of Digha, Sankarpur (W. Bengal), Talsari (Orissa) and other places in India. They are called ‘Red crab’ (Lai Kakra) in W. Bengal.
They are extremely agile and sometimes they are present by the hundred. The brilliantly coloured ‘Sally light foot crab’ Grapsus grapsus is seen over the black rocks along the shore in the Galapogos. They are also found in the Pacific coast of the American mainland.
4. Subphylum Uniramia:
Features:
1. Un-branched appendages,
2. Mandibles un-jointed and without palp.
3. Presence of a single pair of antennae.
4. Gas exchange takes place with the help of tracheal system.
5. Excretory organs are Malpighian tubules.
The subphylum is divided into 5 classes:
These are:
1. Chilopoda,
2. Symphyla,
3. Pauropoda,
4. Diplopoda and
5. Insecta or Hexapoda.
Class 1. Chilopoda [Gk. Cheilos = a lip]:
Approx. 3000 known species; about 100 Indian species.
Features:
1. Body usually dorsoventrally flattened.
2. First pair of trunk appendages modified as maxillipeds and work as poison claws.
3. Most of the trunk segments bear a single pair of uniramous walking legs.
4. Number of legs varies from 15 to more than 100 pairs but no form possesses even number of pairs.
5. Head bears a pair of antennae, a pair of mandibles and two pairs of maxillae.
6. Segment in front of telson is called genital segment.
7. Usually genital segment bears a pair of gonopods, help in reproduction.
8. Respiration takes place by trachea.
9. Excretion by a pair of Malpighian tubules.
10. Primarily carnivorous.
11. Nocturnal and stay in humid areas.
12. Generally called centipedes or hundred-leggers.
13. Terrestrial, surface dwellers or some burrowers.
Geographical distribution:
They are distributed throughout the world both in tropical and temperate regions.
Four well-known orders:
(i) Scutigeromorpha,
(ii) Lithobiomorpha,
(iii) Scolopendromorpha and
(iv) Geophilomorpha are placed within this class.
Order (i). Scutigeromorpha:
1. Legs are 15 pairs and very long.
2. Compound eyes are present.
3. Antennae are very long and originate from the posterior region of the anterior border of head.
4. Dorsal side of the head is arch-shaped.
5. Almost all the trunk segments bear a median spiracle, on their dorsal side. The common example is Scutigera.
Order (ii). Lithobiomorpha:
1. Legs are 15 pairs but very short.
2. Antennae arise from the anterior border of the head.
3. Head and trunk both are flattened on the dorsal side.
4. Spiracles are laterally placed. The example is Lithobius.
Order (iii). Scolopendromorpha:
Strongly built body carries 21-23 pairs of legs; only anterior part of the trunk bears lateral spiracles.
On the dorsal side of the trunk long plates alternate with shorter ones. The example is Scolcrpendra.
Order (iv). Geophilomorpha:
1. Number of legs varies from 35-181.
2. Body is narrow, worm-like and the legs are small.
3. Eyes are absent.
4. Spiracles are placed laterally. The example is Geophilus.
Class 2. Symphyla (Symphylans):
Approx. 160 known species. About 4 Indian species.
Features:
1. Mouth parts are directed forward.
2. Trunk composed of 12 legs bearing segments, covered by 15-24 terga.
3. Second maxillae are united to form the labium, similar to the insect.
4. Eyes are lacking.
5. The penultimate segment bears a pair of sensory bristles and a pair of spinnerets.
6. Genital openings are located on the fourth trunk segment.
7. Telson absent.
8. Spiracles present only in the head and trachea extends posteriorly only up to first three anterior trunk segments.
9. It includes herbivorous and omnivorous forms.
10. They are terrestrial, live in soil or leaf litter, found throughout the world.
Examples:
Scolopendrilla, Scutigerella.
Class 3. Pauropoda (Pauropods):
Approx. 500 known species; probably no known Indian species.
Features:
1. Length rarely exceeds 1 mm.
2. Head has 5 segments.
3. The floor of the preoral chamber is formed by the fused pair of maxillae, called the gnathochilarium.
4. Antennae branched.
5. Trunk contains 12 segments.
6. Heart and tracheae (except in some primitive species) absent.
7. Eyes absent.
8. Legs are present in segments second to tenth.
9. Gonopores on 3rd trunk segment.
10. Saprophytic, mainly found in forest litter.
They are distributed both in tropical and temperate regions.
Examples:
Pauropus.
Class 4. Diplopoda (Millipedes) [Gk. diplos = double]:
Approx. 10,000 described species.
Features:
1. Elongated and segmented forms.
2. Trunk with a large number of leg- bearing segments.
3. First trunk segment (collum) is legless and next three segments with a single pair of legs in each segment and the rest doubled segments (diplosegments) bear 2 pairs of legs in each segment.
4. Antennae 7 segmented.
5. Maxillae are united to form gnathochilarium.
6. Tracheae are mostly un-branched tubes.
7. Gonads unpaired but reproductive ducts are paired.
8. Gonopores on the second pair of legs.
9. Usually vegetarian and found usually beneath leaves, logs, bark and stones.
10. They are commonly called Millipedes or thousandleggers
11. They are terrestrial and are mainly distributed in the tropics.
It includes two subclasses— Pselaphognatha (= Pencillata) and Chilognatha.
Subclass 1. Pselaphognatha (Pencillata):
Features:
1. The size of the body is very small.
2. Body is soft due to the absence of hard exoskeleton.
3. Gonopods are absent in males.
4. Head contains trichobothria.
5. Integument is often armed with lateral setae, hairs or bristles.
It includes a single order Pselaphognathae and the example is Polyxenus.
Subclass 2. Chilognatha:
Features:
1. The integument is provided with hard exoskeleton.
2. Head has no trichobothria.
3. Setae are not clustered.
4. Gonopods are present.
5. Presence of gnathochilarium.
The subclass includes seven orders:
1. Platydesmida,
2. Polyzoniida,
3. Polydesmida,
4. Chordeumida,
5. Julida,
6. Spirobolida and
7. Spirostreptida.
Order 1. Platydesmida:
1. All parts of the gnathochilarium are present.
2. A prominent groove is present in the middle line of the dorsal surface. The example is Platydesmus.
Order 2. Polyzoniida:
1. Only a triangular plate is present in the gnathochilarium, other parts are obscure.
2. No median groove is present on the dorsal side.
The example is Polyzonium.
Order 3. Polydesmida:
1. The segments vary from 18-22.
2. Lines of fusion between the exoskeletal plates are indistinct.
3. The dorsal plate, tergum, projects laterally as parnota.
4. Eyes are absent.
5. Only the first pair of legs in the seventh segment are modified as gonopods.
The example is Polydesmus.
Order 4. Chordeumida:
1. Number of segments is always more than thirty.
2. Ventral plates are separated by sutures.
3. Eyes are distinct.
4. Both the pairs of legs in the seventh segment act as gonopods.
5. Last segment bears 1-3 pairs of spinnerets.
The example is Chordeuma.
Order 5. Julida:
1. Spinnerets are not present on the last abdominal segment.
2. Both pairs of legs of the seventh segment are modified as gonopods; in some cases one pair may be absent. The example is Julus.
Order 6. Spirobolida:
1. Gnathochilarium is free from men- tum.
2. Only one pair of legs is present in the fifth segment.
The example is Spirobolus.
Order 7. Spirostreptida:
1. Possesses two pairs of legs in the fifth segment.
2. Second or posterior gonopods are almost absent.
3. A small tail is usually present. The example is Thyropygus.
Remarks:
The arthropods belonging to Chilopoda, Symphyla, Pauropoda and Diplopoda were formerly included under the class Myriapoda. The word ‘Myriapoda’ is still used to refer these animals, but is now out of taxonomic usage.
Class 5. Insecta or Hexapoda [L. in = into, sectus – cleft, cut or L. insecti = an insect; Gk. hexa = six, podos, genitive of pous = a foot]:
Approx. 10,00,000 known species.
Features:
1. Size varies from 250 pm—25 cm in length.
2. Body consists of three distinct tagmata (regions)—head, thorax and abdomen.
3. Head is formed by the fusion of six segments and its appendages are a single pair of antennae, a pair of mandibles and two pairs of maxillae.
4. In adults, the thorax includes 3 segments—prothorax, mesothorax and metathorax and each segment bears one pair of walking legs. Hence, called Hexapoda for the three pairs of legs.
5. In winged insects, the mesothorax and metathorax bear a pair of wings in each segment.
6. A pair of compound eyes present.
7. Paried appendages absent in the adult abdomen.
8. Respiratory organs are in the form of tracheae which extensively developed.
9. Chief excretory organs are the Malpighian tubules closely associated with alimentary caual.
10. Development usually pass through complicated metamorphis but in some cases it may be direct. It has two subclasses Apterygota and Pterygota.
Subclass Apterygota (Gk. a = without, pterygotos = winged):
Features:
1. Wings absent.
2. Presence of terminal cerci.
3. Development direct.
Two superorder—Entognatha and Ectognatha belong to this subclass.
Super order Entognatha (Gk. entos = within; gnathos = Jaw).
Labium being united with the cranium on the lateral side completely covers the mandibles and maxillae.
It includes 3 orders—Protura, Collembola and Diplura.
Order 1. Protura (Gk. proto = first, uro = tail):
1. Abdomen has twelve segments in the adult.
2. Rudimentary appendages are present on the first three abdominal segments.
3. Compound eyes and antennae are not present.
The examples are Acerentomon and Eosentomon.
Order 2. Collembola (Gk. kolla = glue, ballo = put):
1. The members are commonly called the springtails.
2. Abdomen never possesses more than six segments.
3. Eyes, Malpighian tubules and usually the tracheae are absent.
4. Last segment carries appendages for jumping.
The examples are Podura, Orchesella, Bourletiella, Isotoma and Neanura.
Order 3. Diplura [Gk. diplos – double, oura = tail]:
1. Abdomen consists of eleven segments.
2. Terminal segment of the abdomen bears cerci or forceps.
3. Malpighian tubules are usually absent. The examples are Campodea, Heterojapyx.
Super Order Ectognatha [Gk. ecto – outside):
Mandibles and maxillae are not covered by the lateral fusion of labium and cranium.
Order Thysanura (Gk. thysanos = tassel):
1. Abdomen consists of eleven segments.
2. Rudimentary appendages may occur in some abdominal segments.
3. The last segment or anal segment has two or three many-jointed anal cerci.
4. Malpighian tubules and compound eyes are usually present.
The example are Lepisma (silver fish), Machilis.
Subclass Pterygota (Gk. pterygotos = winged):
Features:
1. Adults possess wings which may be secondarily lost.
2. Excepting cerci, other appendages are absent in the abdomen.
3. Malpighian tubules are present.
4. Metamorphosis may be complete or incomplete.
This large subclass is subdivided into four sections—Paleoptera, Polyneoptera, Oligoneoptera and Paraneoptera.
Section Paleoptera:
1. At the time of rest, the wings cannot be placed parallel to the abdomen.
2. Surface of the wing is thickened only in correlation with veins.
3. Wings originate as external buds.
4. Malpighian tubules are many.
Two living orders—Ephemeroptera and Odonata are included under this section.
Order 1. Ephemeroptera:
1. The members of this order are called the mayflies.
2. Adults are aerial but larvae are aquatic.
3. Mouth parts are degenerated in adults.
4. Wings are not of same size, the hind wings are degenerated.
5. Wings appear in the last immature stage and is followed by ecdysis.
6. This is the only winged insect where ecdysis occurs after the appearance of wings.
7. Terminal part of the abdomen bears two elongated cerci and a median filament.
The examples are Ephemera, Hexagenia.
Order 2. Odonata (Gk. Odous = tooth) Dragon flies and Damsel flies:
1. All the insects are of large size.
2. Two pairs of almost equal wings.
3. At rest the wings are either held unfolded or extended laterally.
4. Rudimentary antennae are present.
5. Mouth parts are adapted for biting.
6. The eyes are very big and conspicuous.
7. Larvae are fully aquatic.
The well-known examples are Anax, Aeschna, Ischnura and Lestes.
Section Polyneoptera:
1. Wings are provided with rich supplies of veins.
2. At the time of rest, the wings are always kept folded over the abdomen.
3. Numerous Malpighian tubules are present.
This section includes nine orders which are given below:
Order 1. Dictyoptera:
1. Cockroaches and Preying mantids are the representatives of this order.
2. These insects usually run.
3. Mouth parts are of primitive condition and used for biting.
4. Cerci are jointed.
5. Tarsi composed of five segments.
6. Eggs remain within a capsule called ootheca.
The examples are periplaneta, Mantis.
Order 2. Isoptera (Gk. isos = equal, pteron = wing):
1. The order is exemplified by the white ants or termites.
2. They exhibit polymorphism.
3. Females have much enlarged abdomen.
4. In winged forms, the wings are of same sizes and can be separated at will.
5. Each wing has a longitudinal venation and chitinised network in between.
6. Mouth parts are adapted for biting.
The examples are Cryptotermes, Ameritermes, Kalotermes, Neotermes, Glyptotermes, etc. These genera are wood termites and some of them are serious pests of rubber and tea plantations. A list of the species of the Glyptotermes (wood termites) from India is given below.
The imago of Glyptotermes is identified by head-capsule guadrate or subcircular, posterior magin round, fontanelle absent, eyes broadly oval, antennae with 11-17 segments, wings smoky brown, abdomen long, epicranial sutures present and cerci 2-jointed and short.
Order 3. Zoraptera:
1. Size is extremely small.
2. They exhibit polymorphism.
3. Males are usually without wings.
4. Tarsi have two joints. The example is Zorotypus.
Order 4. Plecoptera:
1. The order includes the stone flies.
2. Antenna is long.
3. Cerci are distinct.
4. Larva is aquatic.
5. In course of development a terrestrial stage appears which contains, only wing buds.
6. Winged forms develop from this stage and become aerial.
The examples are Perla, Isoperla.
Order 5. Notoptera:
1. Wings are absent.
2. Larva resembles the adult in all structural details.
The example is Grylloblatta.
Order 6. Cheleutoptera:
1. Some are wingless, whereas others may have wings.
2. All the members exhibit structural features to mimic either leaves or branches of the tree.
3. Winged forms exhibit gradual appearance of various structures from larva to the adult.
4. But in wingless forms the larva resembles the adult in all structural details.
5. The eggs resemble the structures of seeds.
The examples are Carausius (stick-in- sect), Phyllium (leaf-insect).
Order 7. Orthoptera (Gk. orthos = straight, pteron = wing):
1. Grasshoppers, locusts and different crickets are the representatives of this order.
2. Structure of head resembles that of cockroach.
3. Legs of the metathoracic segment are adapted for jumping.
4. A well-developed ovipositor is present.
5. Mouth parts are of biting type.
The examples are Hieroglyphus, Tryxalis, Locusta, Schistocerca, Gryllotalpa (mole cricket).
Order 8. Embioptera:
1. The members of this order are the web-spinners.
2. Size is small.
3. Tarsi are three-jointed.
4. Possess silk glands to form silken tunnels for living.
The example is Embia
Order 9. Dermaptera (Gk. derma = skin):
1. Ear-wings are the members of this order.
2. Anterior wings are short.
3. Posterior wings are papery and have radially arranged veins on its surface.
4. Posterior wings may be folded both transversely and longitudinally.
5. Mouth parts are adapted for biting.
6. Anal cerci are like forceps.
The example is Forficula.
Section Oligoneoptera:
1. Metamorphosis is complete.
2. Jugal area of the wing contains only one vein.
3. Limited number of Malpighian tubules is present.
4. Wings always develop from inner wing buds.
5. Mouth parts are adapted either for biting or for sucking.
It includes following eleven orders:
Order 1. Coleoptera (Gk. Koleos = sheath):
1. The members of this order include the beetles.
2. Prothorax is freely movable.
3. Posterior pair of wing is membranous.
4. Anterior pair of wing is stiff and covers the folded posterior wing during rest.
5. Well-developed jaws are built up for biting and chewing.
6. Metamorphosis is complete.
7. Larvae may be maggot-like or cater- pillar-like.
8. No special covering is present around pupa.
The examples are Photinus, Calandra, Adalia and Dineutus.
Order 2. Megaloptera:
1. Mouth parts are adapted for biting.
2. At the margin of the wing, the longitudinal veins exhibit sign of bifurcation.
3. Metamorphosis is complete and the larvae are aquatic.
The examples are Sialis (Alder-flies), Corydalis (Dobson-flies).
Order 3. Raphidioptera:
1. The snake-flies are the members of this order.
2. Prothorax is long.
3. Head is narrow.
4. Metamorphosis is complete.
5. Larvae and pupae are all terrestrial.
The example is Raphidia.
Order 4. Planipennia:
1. Mouth parts adapted for biting in the adult but sucking in the larva.
2. In the late larval stage, Malpighian tubules are modified to secrete silk which is used in the formation of a cocoon.
The examples are Montispa, Myrmeleon (Antlion).
5. Mecoptera (Gk. mekos = a length):
1. The members of the order are the Scorpion-flies.
2. Abdomen in male is curved upwards.
3. Head bears a beak-like prolongation and mouth parts are present at the tip of this beak.
4. Wings are membranous and all alike. The example is Panorpa.
Order 6. Trichoptera (Gk. thrix = hair):
1. Caddis-flies are the common representatives of the order.
2. Wings are hairy, membranous and are of dissimilar sizes.
3. At rest the wings remain as a roof-like peak.
4. Mandibles are generally absent.
5. Mouth parts are specialised for licking.
6. Larvae are aquatic and produce silken case within which they reside.
The examples are Rhyacophilia, Mayatrichia.
Order 7. Lepidoptera (Gk. lepis = scale):
1. All the butterflies and moths belong to this order.
2. Broad and well-developed wings are enclosed by scales which are modified hairs.
3. Wings are generally oriented with varied specks of colours.
4. Maxillae in adults are modified into a spirally-coiled sucking tube.
5. Remaining mouth parts excepting the labial palps are lacking.
6. First two divisions of the thorax are fused.
7. Metamorphosis is complete.
8. Larvae possess three thoracic feet and in some cases several abdominal legs may be present.
9. Mouth parts of larvae are modified for biting.
10. Pupa is always covered with a case. The examples are Pieris, Samia, Venesa, Teinopalpus, Papilio, Parides and Bombyx.
Order 8. Diptera (Gk. dis = two):
1. Flies and mosquitoes belong to this order.
2. Metathoracic wings are modified as the halteres, which act as balancers.
3. Mesothoracic wings are welldeveloped but with sparse venation.
4. Mouth parts may be adapted for piercing and sucking or only sucking.
5. Metamorphosis is complete.
The examples are Anopheles, Culex Musca, etc.
Order 9. Siphonaptera (Gk. siphon = sucker, a = without; pteron = wing):
1. External parasites on warm-blooded animals.
2. Wings are absent in adults.
3. Coxae of the legs are exceedingly large.
The examples are the fleas represented by the genera, Pulex, Ctenocephalus, etc.
Order 10. Hymenoptera (Gk. hymen = membrane):
1. Wings are membranous.
2. Two pairs of wings remain interlocked by hooks on the anterior border of hind-wing.
3. The appendages around mouth are arranged for biting, licking and sucking.
4. All the thoracic segments are united and the first abdominal segment is fused with it.
5. Polymorphism occurs in certain forms.
6. Metamorphosis is complete.
7. Development is complicated and larvae are helpless.
8. Pupa is covered by a cocoon.
The well-known examples are Apis (Honey bee), Vespa (Wasp), and Formica (Ant).
Order 11. Strepsiptera:
1. Wingless and degenerated females are endoparasites but the males are free- living and winged.
2. Anterior wings in males work as halters.
3. At rest the hind wings of male remain folded like fan.
4. Metamorphosis is complete. The example is Stylops.
Section Paraneoptera:
1. Wings develop from external wing buds.
2. Wings are usually poorly developed.
3. Very limited Malpighian tubules are present.
4. Metamorphosis varies from partially complete to fully complete condition.
5. Mouth parts are adapted for either biting or sucking.
All the forms are either parasites or pests and are included within the orders— Psocoptera, Mallophaga, Anoplura, Thysanoptera, Homoptera and Heteroptera.
Order 1. Psocoptera:
1. Book-lice are the members of this order.
2. Size is extremely small.
3. Wings may or may not occur.
4. When present, the anterior pair is larger and both are membranous.
5. Cerci are absent.
6. Mouth parts are built up for biting. The example is Psocus.
Order 2. Mallophaga (Gk. mallos = wool):
1. This order includes the bird-lice.
2. Body is dorsoventrally flattened.
3. Antennae are short.
4. Wings are absent.
5. Mouth parts are degenerated but adapted for biting.
6. Young’s resemble the adults in structural details.
The example is Menopon.
Order 3. Anoplura (Gk. anoplos = unarmed):
1. The order includes the sucking lice.
2. All are ectoparasites of mammals.
3. Mouth parts are adapted for piercing and sucking.
4. Wings are absent.
5. Body is dorsoventrally flattened. The example is Pediculus.
Order 4. Thysanoptera (Gk. thysanos = fringe):
1. All thrips are the examples of this order.
2. All are plant pests.
3. Mouth parts are adapted for sucking.
4. Wings may or may not be present.
5. When present the wings are slender and with elongated setae at the margin. The example is Heliothrips.
Order 5. Homoptera (Gk. homos = same):
1. Mouth parts are adapted for sucking.
2. Pronotum is rudimentary.
3. Wings are membranous and during rest are held in roof-like fashion.
The examples are Aphis, Cicada and Tachardia (Lac insect).
Order 6. Heteroptera (Gk. hetero = dissimilar):
1. Mouth parts are sucking.
2. Pronotum is large.
3. Wings, at the time of rest, lie one over the other.
4. Mesothoracic wings are thick and its lower half is pigmented.
The examples are Cimex (Bed bug), Anasa, Leptocorisa (Rice bug), etc.
Phylogeny of Arthropods:
Whether the arthropods are monophyletic or polyphyletic has long been a debated issue. The monophyletic theory is proposed by Snodgrass (1938), Sharov (1966) and they suggest that all arthropods have evolved from a single annelid-like ancestral stock whose limbs are lobopod-like.
This lobopod- annelid gave rise to Tetracephalosomita and the most primitive arthropods— Trilobitomorpha evolved from the Tetracephalosomita, and Chelicerata and Mandibulata originated from Tetracephalosomita. Onychophora evolved from lobopod annelid-like ancestor and represent an early lateral branch of the evolutionary line (Fig. 18.120).
The monophyletic theory is based on the segmentation between the annelids and arthropods, and the prostomium and pygidium of the annelids correspond to the acron and telson of arthropods.
A number of anatomical features, such as chitinous cuticle, haemocoel, dorsal blood vessel, segmental jointed appendages and centrolecithal eggs constitute the basis for the view that the arthropods are monophyletic (Anderson, 1998).
The polyphyletic theory is proposed by Tiegs and Manton (1958).
They suggested that there were two ancestors of arthropods:
(i) Lobopod-annelids and
(ii) Proto- annelids (Fig. 18.121).
(i) The lobopod-annelid is considered to be the ancestor of Onychophora which was originated in the Pre-Cambrian period about 525 million years ago. The group Uniramia which includes chilopodes, diplopodes and insects is thought that they have evolved from the base of onychophores.
(ii) The protoannelid ancestor is considered as the primitive annelids from which the most primitive extinct arthropod group- Trilobitomorpha arose, from which two extant groups—Crustacea and Chelicerata evolved by two separate evolutionary lines. Manton (1973), Ruppert and Barnes (1994), support the polyphyletic theory.
They suggest that 3 different living groups-Chelicerata (e.g., horse-shoe crabs, spiders, scorpions, etc.), Crustacea (e.g., copepods, barnacles, prawns, crabs, etc.) and Uniramia have evolved separately, possibly from the annelid ancestors. This view is based on the locomotory structures and embryonic development of the different major groups.