In this article we will discuss about Arthropods:- 1. Integumentary System of Arthropods 2. Muscular System of Arthropods 3. Body Cavity 4. Digestive System 5. Circulatory System 6. Nervous System 7. Reproductive System 8. Life Cycle.
Integumentary System of Arthropods:
In all arthropods, the integument consists of:
(i) An innermost extremely thin stellate cell layer, called basement membrane,
(ii) A monolayer of closely packed hexagonal cells, hypodermis (epidermis) and
(iii) Outer non- cellular layer, cuticle.
The cuticle is secreted by the hypodermis and excepting the regions of joints it is many-layered. The cuticle also lines the inner wall of foregut, hindgut, trachea and genital atrium.
The cuticle consists of two layers—outer epicuticle and inner procuticle. The cuticle is extremely thin and usually does not contain chitin (exceptions are a few Centipeds and Pycnogonids).
Specially in insects, it has been seen to contain wax, lipids, proteins and steroids. The wax and lipids make it impermeable to water. The procuticle contains chitin, a special kind of polysaccharide and is divisible into two layers—exocuticle and endocuticle.
The outer exocuticle is a tough layer and the inner endocuticle is many-layered and flexible. In Crustacea and Diplopods, various calcareous substances are seen to be deposited in the exocuticle. The different colouration of the pigments is due to the presence of pigment cells in the hypodermis.
The outer part of the integument may have various striations and markings. Its out-pushings may form spiny structures and in-pushings give rise to apodemes for the attachment of muscles.
Muscular System of Arthropods:
In arthropods, the muscles are striated. In the thorax are longitudinal muscles are present as a pair of dorsal and a pair of ventral bundles. Each joint is provided with two sets of muscles, one of which is antagonistic to the others.
Some of these somatic muscles can work at astounding speed, e.g., wing muscles of insects, muscles operating the stridulating organs in various arthropods. The splanchnic muscles are present around the gut, heart, aorta, diaphragm, etc. These muscles are arranged either as layers of longitudinal and circular muscles or as myofibrilar network.
Body Cavity of Arthropods:
In Arthropoda, true coelom appears as pouches in the embryonic stage In course of development its walls are used up in the formation of organs and the space becomes continuous with the blastocoel. It is called mixocoel and as blood flows through it, this is also referred to as haemocoel.
In Crustacea true coelom is restricted to the space of ophthalmic artery, within excretory and reproductive parts. Almost similar condition is found in Onychophores, where true coelom is restricted only around the excretory and reproductive parts. But in Myriapods and Insects, the coelomic spaces, are retained only in reproductive parts.
Digestive System of Arthropods:
The digestive system is concerned with nutrition. The process primarily involves three phases—ingestion, digestion and egestion. As arthropods live in varied habitats, they carry out these phases in different ways. Each group has developed the structures perfectly suited to its particular way of life.
The digestive system includes:
(1) Alimentary canal and
(2) Digestive glands.
The digestive system is absent in certain adult insects, e.g., Mayflies and much modified in a few parasitic crustaceans like Sacculina.
(1) Alimentary Canal of Arthropods:
In general the alimentary canal is divisible into three parts:
(a) Foregut,
(b) Midgut, and
(c) Hindgut.
The structure varies in different arthropods (Fig. 18.126), but in all the fore and hind guts are lined internally by cuticle.
Crustacea:
In parasitic Crustaceans, specially in endoparasites the alimentary canal shows marked degeneration. But in free-living forms it extends along the entire length of the body. In most Crustaceans, the foregut includes mouth, gullet and oesophagus. But in Malacostraca, the next part, stomach is also included within foregut.
The mouth in general is ventrally placed and some distance away from the anterior end. The gullet is vertical and the stomach is more or less sac-like. In most Crustaceans, the midgut is straight and dorsally placed.
The intestine is coiled in Cladocera. Near the posterior end of the midgut in Amphipoda, single or paired caeca are present. The hindgut contains a bulb-like rectum which opens to the exterior through a posterior terminal aperture, called anus.
Myriapods:
Length of the foregut varies in different Chilopods. In Diplopods, a pre- oral cavity is present in front of the mouth and it leads to a pharynx of varied length. The lining of midgut in the same group contains both secretory and absorptive cells. The hindgut is considerably long and in some (Julidae) is subdivided into three parts.
The last part is usually sac-like and eversible. In the Pauropoda, the foregut is contractile and the midgut begins from third segment of the trunk. The hindgut is divisible into a tubular part and a sac-like part. In Symphyla the hindgut is divisible into four parts.
Insects:
Mouth is placed at the ventral and terminal end of the head. Mouth is bounded by mouth parts which differ according to the food habit of the insect. The other parts of the foregut include a well- developed pharynx, bag-like crop and a muscular gizzard or proventriculus. The gizzard is prominent in Coleoptera and ants. In honey-bee, it acts as a honey stomach.
The midgut varies widely in insects. The anterior end of the midgut in most insects gives rise to diverticulum or caecum. The caecum is absent in Lepidoptera and Collembola. In some Diptera the midgut is long, coiled and divisible into an anterior digestive part and a posterior absorptive part. In Heteroptera, the anterior part is sac-like and known as stomach, while the much coiled posterior part is called intestine.
In Homoptera, the foregut and hindgut join with each other and the midgut is set aside as a loop. In Coccidae the last part of the foregut and the first part of the midgut remain inside the hindgut.
In most insects, a constriction separates the midgut from hindgut and the latter is divisible into a slender anterior part and broad posterior part. In some Coleoptera and termites the hindgut is sac-like and contains cellulose-splitting bacteria.
Chelicerates:
The position of mouth varies. In Xiphosurids, the slit-like mouth lies in between the second and fifth gnathobases. A pre-oral cavity is present in front of the mouth of Scorpionids and Uropygi. In Palpigradi, mouth is present on the segment which bears pedipalp. In Ricinulei, the pre- oral cavity is covered anteriorly by a flap-like projection of carapace, called cucullus.
A projected rostrum in Solifugae bears the mouth at its tip. Usually the pharynx and in some cases (spider) the stomach is suctorial. The midgut sends paired and much branched diverticula in all chelicerates, where both digestion and absorption take place. Only unbranched diverticula are seen in Opiliones, Ricinulei and Acari.
In Xiphosurids the diverticula fill up the prosoma but in Arachnids it is restricted only to the abdomen (excepting scorpion). In Scorpionids, the hindgut is the straight continuation of the midgut and is called rectum. In Xiphosura, the short tubular rectum has folded walls. In spider and Pseudoscorpionids, dorsal diverticula are given out from the rectum.
(2) Digestive Glands of Arthropods:
In Crustacea, the most important digestive gland is hepatopancreas. It contains two kinds of cells—hepatic and pancreatic. The gland is formed by numerous finger-like tubules.
In prawns and crabs, the hepatopancreas is placed within the cephalothoracic cavity. But in Amphipoda and Isopoda it extends within the abdomen. In Stomatopoda, these digestive glands are arranged in ten metameric pairs. Salivary glands are known in certain forms.
Among the Myriapods, four pairs of salivary glands are seen in Chilopoda and Diplopoda. But in Pauropoda the number are reduced to two pairs and in Symphyla there are only one pair of large salivery glands. In addition to the salivary glands, the cells present in the lining of midgut are also responsible for producing digestive juices.
In Insecta, three sets of glands—labial glands, maxillary glands and mandibular glands are often referred together as salivary glands, which secrete digestive juices. Labial glands are slender, elongated tubes in muscoid flies and paired sac-like structures with lining of secretory cells in mosquito. Labial glands of larval Lepidoptera work as silk glands.
The maxillary glands are functional in the adult Protura and Collembola. Mandibular glands are present in most Apterygota and in Dictyoptera, Isoptera, Trichoptera and Hymenoptera. Of these three kinds of salivary glands, usually only one kind is functional and other two are degenerated. But it may be that two or all the three sets continue to be functional.
In honey-bee the labial glands work as wax glands, and the mandibular glands produce secretion to soften the pupal case. True salivary glands originating from the pharyngeal system open within the pharynx. The saliva, in addition to its enzymes, often contains anticoagulant. The digestive juices are also produced from the lining of the midgut.
In Chelicerates, the salivary function is carried by a pair of rostral glands and a pair of maxillary glands. The diverticula of the midgut produce digestive juices.
Mechanism of Digestive System:
The Crustaceans exhibit an evolutionary trend in the food-getting devices. Primitive crustaceans (Cephalocarida) use their indentical appendages for locomotion and also for filtering food particles from the surrounding water. But in advanced groups the appendages are differentiated to capture food.
The best example is prawn, where maxillae and maxillipeds, while producing water current for respiration, assist the food to enter into the mouth. Mandibles cut the food into pieces. Some of the walking legs being chelated can grab the food. The stomach is also modified for crushing the food and also to digest it.
The Myriapods have powerful mandibles for capturing and cutting the food. In this group, the hindgut exhibits special structural changes for preventing any loss of water. The organisation of mouth parts in Insects speaks about the advancement of this group over others regarding food procurement.
The intestine and diverticula perform the breakdown and absorption of food efficiently to meet the excess demand of energy. Among Chelicerates only the Xiphosurids are capable of ingesting solid food. The Arachnids have devices by which the prey is pre-digested either by injecting enzymes or by taking it in a special pre-oral cavity. The partly digested liquid food is sucked inside the alimentary canal.
Circulatory System of Arthropods:
The circulatory system is primarily concerned with the distribution of metabolic substances and respiratory gases. In all Arthropods, excepting Insects, the circulatory system performs this dual role.
In Insects, the circulatory system is free from the burden of carrying respiratory gases. This system includes, blood, blood vessels and a pumping organ, the heart. The circulatory system in Arthropods shows a trend of transition from primitiveness to specialisation.
Blood:
In Crustaceans, the blood contains a fluid part, plasma, and a few colourless amoeboid cells. A copper-containing pigment, haemocyanin, is present in the plasma of Decapods to render a blue colour to the blood. In Crustaceans like Triops and Cypris, the colouring pigment is haemoglobin which makes the colour of their blood red.
Among the Myriapods, the Chilopods have colourless blood and in Lithobius the blood is violet. In Insects, the blood is usually colourless. In leaf-eating Insects, the blood is green and in some other forms it may be brownish or yellowish. In Chironomous larvae, the blood is red. In Arachnids, the presence of haemoglobin in blood plasma has given red colour to the blood.
Blood Vessels:
In general, the blood flows through coelomic spaces, called haemocoel, but in addition there are vessels with definite walls in Crustacea and others. Such vessels are called the arteries. In Arachnida, there are both arteries and veins with definite walls.
Hearts:
Certain Arthropods, like Cirripeds, Ostracods, Copepods and Pauropods do not possess any heart. When present, the heart is always dorsal. The primitive heart is tubular and extends along the entire length of the body. In other forms various types of shortening, thickening and compartmentalisation are noted. Among the Crustaceans, the primitive type of elongated tubular heart is noted in Branchiopoda and Anostraca.
In each segment it communicates through a pair of ostia. More or less similar condition is seen in Leptostraca, Stomatopoda, Isopoda and Amphipoda. The heart is short and sac-like in Cladocera and Decapoda and it has only a few pairs of ostia. In Myriapoda, the elongated tubular heart is internally divided into several chambers.
In Scolopendra, it is enclosed within a cardiac diaphragm. The wall of the diaphragm is attached with the body wall and special sets of muscle, called alary muscle, remain associated with it. In Insecta, the tubular heart is generally confined to the abdomen but in many instances it extends up to the thorax. The heart of cockroach is composed of thirteen chambers.
The inner chambers of the heart are interconnected by openings which are guarded by valves. The heart opens into the pericardial cavity by several ostia and the wall of the heart remains attached with the body wall by alary muscles.
In Arachnida, the length of heart and the number of chambers vary. For example, the heart of scorpion has seven chambers and the spider possesses only three. In Xiphosurids, the heart has eight chambers.
Nervous System of Arthropods:
The nervous system includes:
(a) Central nervous system,
(b) Peripheral nervous system and
(c) Sense organs.
Some higher Crustaceans and Insects possess a sympathetic nerve cord which begins from the central nervous system and extends along the wall of the alimentary canal. In spite of tremendous diversity, the nervous system in Arthropoda is built up on a basic plan which includes a pair of ganglia per metamere.
These ganglia are interconnected by nerve cords and peripheral nerves are given out to the particular segment. This basic plan has been modified in various Arthropods, according to the modification of their body and in many instances fusion of the ganglia have taken place (Fig. 18.130).
(a) Central and Peripheral Nervous Systems:
Within Crustacea, the Branchiopods exhibit primitive type of nervous system which resembles that of nervous system which resembles that of Annelids. Here brain is formed by the fusion of two pairs of ganglia representing the segments bearing eyes and antennules respectively.
These two pairs are known as protocerebrum and deuterocerebrum. From brain arises paired circum oesophageal connectives which come in contact with paired ventral nerve cords.
The nerve cords run posteriorly and carry a pair of ganglia in each segment. The anterior-most pair of ganglia is known as sub-oesophageal ganglia. But in other Crustaceans the ganglia of the antennal segment also fuse with the brain and form its third lobe, called tritocerebrum.
In some Crustacea, paired ganglia of each segment are fused; in others the ventral nerve cord is reduced. In the case of prawn, the ganglia of the thoracic segments are fused to form a single ganglionic mass and in crab (Carcinus) entire ventral nerve cord is fused.
Its segmental ganglia are united to form a large ventral ganglionated mass. Similar reduction is also seen in Cirripeds. Such reduction in Cirripeds is possibly due to their parasitic existence.
In Myriapods, the ventral nerve cord exhibits its double nature and this is distinct specially among the Chilopods. Three ganglia on the ventral nerve cord corresponding to the first three trunk segments unite to form a sub- or infra- oesophageal mass.
In Insects, the usual pattern is that, after suboesophageal mass, each segment of the thorax and abdomen has a pair of ganglia on the ventral nerve cord. But in Nepa and Acanthia, the first thoracic ganglia are fused with the suboesophageal.
In Gyrinus, all the abdominal ganglia are fused and in Lachnosterna the abdominal ganglia are fused with the ganglia of meso- and metathorax. The best examples of condensation are seen in Diptera. In Sarcophaga, all the thoracic and abdominal ganglia are fused together. Such condensation includes even the suboesophageal ganglia in the parasitic Diptera, Pupipara.
Among the Chelicerates the nervous system shows different grades of fusion, in Xiphosurids, suboesophageal ganglia remain fused with the ganglia belonging to the segments second to eighth. The ventral nerve cords bear four ganglia and the last ganglia are formed by the fusion of ganglia belonging to the last three segments.
In Scorpionids, the thoracic ganglia are fused with the suboesophageal ganglionic mass but most of the abdominal ganglia are distinct. In Araneida, all the ganglia are fused into a mass, which is piereced by the oesophagus.
(b) Sense Organs:
The possession of well- developed sense organs has played important role in the success of arthropods. Starting from setae and bristles there are extremely specialised sense organs like compound eyes (Fig. 18.131) to act as ports of entry of different stimuli.
Crustacean Sense Organs:
The important sense organs are eyes, statocysts and olfactory setae. In addition, there are various sensory hairs and bristles.
(i) Eye:
The eye may be single or paired. In Ostracoda, single median eye is placed on the anterior end. Similar median eyes on the dorsal side are seen in free-swimming Copepods. The eyes are absent in parasitic Copepods and degenerated in Cirripedia and Rhizocephala. The paired eyes are generally mounted on stalks but may be sessile as in Cumacia, Tanaidacea and Amphipoda.
In many Branchiopods, the eyes remain within a fold of epidermis. The eye stalk is usually jointed but in many Branchiopods the stalk is unjointed. In Cladocera, the paired eyes are fused. The unpaired median eye is known as nauplius eye and is simple in design. But the compound eye contains numerous visual elements, called ommatidia (sing, ommatidium). The structure of an ommatidium is same as in prawn.
(ii) Statocyst:
The balancing organs or statocysts are present in the antennule. In Mysidacea, the balancing organs are present in the uropods. In the land- living Crustaceans (including true crabs and hermit crabs), the statocysts are also responsible for receiving vibrations.
(iii) Olfactory setae:
These setae work for receiving smell and are distributed over the antennae.
Myriapod Sense Organs:
The sense organs are generally seen as sensory hairs, eyes and other specialised organs.
(a) Sensory hairs:
These hairs either remain scattered all over the body or are found, in specialised groups, in certain regions of the body. In Pauropoda, five pairs of tactile hairs are arranged along the sides of the tergal plates of second to sixth segments.
In between eyes and antennae of Diplopods, a small pit contains projectile hairs. Its gnathochilarium (formed by the fusion of antennae, maxillae and mandibles) carries tufts of sensory hairs. In Diplopoda, sensory hairs and sensory spines are seen in different parts of the body.
(b) Eye:
Eyes are absent in Symphyla (excepting a few cases, where only one pair is present). An eye-like surface is visible in Pauropoda, but true eye is not present. Several simple eyes are clumped together in Diplopoda. In others the eyes, when present, are usually simple. In Scutigera, a kind of pseudocom-pound eye is present.
(c) Specialised sensory organs:
(i) Globulus:
It is present on the ventral branch of Pauropoda.
(ii) Organs of Tomosvary:
In Diplopoda, the head bears a pair of small projections. It is dressed externally with fine hairs and inside the projection, the hypodermal cells are provided with nerves to receive sounds.
(iii) Maxillary organs:
The inner side of the base of the first maxilla possesses a pit with profuse lining of setae.
(iv) Parapodia:
In Symphyla, last ten pairs of legs have wart-like stumpy processes, called parapodia. Each parapodium contains a sac having sensory function.
Insect sense organs:
Following sense organs are usually seen in insects:
(i) Eyes,
(ii) Olfactory organs,
(iii) Organs for touch and taste,
(iv) Auditory organs and
(v) Chordotonal organs.
(i) Eyes:
The eyes of insects may be compound or simple. The compound eyes are without stalks and are spread on the marginal part of the head. Many subterranean insects are completely blind. The number of simple eyes varies widely. These are absent in Dermaptera and in some Hemiptera. When present, the simple eyes are grouped along the sides of the head.
(ii) Organs of smell:
The tip of each antenna bears hairs which are embedded in pits and act as the sense organ for smell.
(iii) Organs for touch and taste:
Numerous -hairs with nerve connections protrude throughout the surface of the body for getting stimuli in the form of touch. Specially the hairs around palp serve dual functions of touch and taste. The hairs present around mouth parts are probably for the function of taste.
(iv) Organs for receiving sound:
Some insects (male Culicids) have specialised hairs over the surface of the antennae for detecting sound. A special organ, called tympanum, is seen in several insects (in the anterior segments of Acridiidae, in anterior legs of Locustidae). This organ has a fluid-filled vesicle inside to act as membranous labyrinth. The organ has connections with nerves supplied from third thoracic ganglion.
(v) Chordotonal organs:
These specialised sense organs are located in different parts of the body specially in legs and abdomen. Each organ consists of a pack of sensory cells and accessory structures, called scolopales.
These may remain tightly stretched and attached on both the ends of hypodermis or only one end remains attached. They perform various functions which include regulation of leg movement (Proprioceptive organ) and also reception of sound waves.
(v) Chelicerate sense organs:
Following sense organs are seen in different Chelicerates— eyes, sensory setae, trichobothria, slit sense organs, frontal organs and pectines.
(i) Eye:
In Eurypteridae, the cephalothorax bears a pair of large eyes and a pair of small eyes. In Limulus, similar disposition of eyes is marked but the median eyes are compound in nature. Scorpions have a pair of large centrally placed eyes on the cephalothorax and several pairs of small eyes along the anterolateral margin.
The median eyes are intermediate between simple and compound eyes. Each eye has a single cuticular lens like that of a simple eye but retinal cells are disposed like compound eyes.
In the Pedipalpida, two median eyes are large and six small eyes are placed along the margin. The arrangements of 6-8 simple eyes in spiders vary widely and serve as the criteria in classification. The Solifugae and Phalangida possess a pair of eyes on the head and the Acarids are without eyes.
(ii) Sensory setae:
All over the cuticle, specially on the masticating processes of the thoracic limbs, numerous hair-like structures are present. These setae are provided with the branches of nerves and are sensory in function.
(iii) Trichobothria:
These flask-shaped sense organs with a mobile seta are arranged on each chela in different planes. These are responsible for determining air current.
(iv) Slit sense organs:
These slit-like sense organs are distributed all over the body, specially over the appendages. Each minute slit is covered by a membrane and leads to a crevice. Inner end of the crevice leads to a membrane-lined tube which is internally supplied by numerous nerve fibres. These sense organs co-ordinate joint movement and work as vibration receptors.
(v) Pectines:
These specialised sense organs are found in Scorpion. Males with the help of these sense organs detect suitable surface for depositing spermatophores and the females use these for collecting spermatophores.
(vi) Frontal organs:
These specialised sense organs in the form of hairy areas are seen in Xiphosurids. These organs act as photoreceptors in larva but their function is not known in adult.
Reproductive System of Arthropods:
The secret of success of the Arthropods as a phylum lies in its prolific rate of multiplication. This is done by efficient reproductive organs and effective reproductive behaviour. Majority of the arthropods are unisexual. A good number of hermaphrodites are seen in all the classes excepting’ Arachnida.
Each reproductive duct is a modified coelomoduct and reproductive gland or gonad opens into it. The position of the reproductive organ varies as also the sites of reproductive openings. Majority of the arthropods are oviparous and it is curious enough that in most of them there are certain devices for internal fertilization. Some forms of viviparity are also seen, but only in Onychophores, true viviparous condition is found.
Modification of different classes:
Crustacea:
Free-living Crustaceans are generally unisexual. But in Cirripeds, parasitic Isopods and in a few other species, hermaphroditism is encountered. Such hermaphrodites are called protandrous because in them male reproductive organs appear first and then the female organs. Though males often possess well-developed appendages, yet generally they are smaller in size than females.
In some forms of parasitic Crustaceans, males are extremely minute and cling to the body of the females. Such males are called complemental males. In many Crustaceans, males may have modified appendages to act as clasping or intromittent organs. The intromittent organs (structures for transferring sperms to the female body) may also be formed by the modification of the protrusible terminal part of the vasa deferentia.
The reproductive organs in both the sexes are hollow and are usually united either completely or incompletely above the alimentary canal.
In all the Crustaceans (excepting Cirripedia, Malacostraca and some Cladocera) both the sexes have reproductive openings in the same segment. The genital apertures in most Crustaceans are placed near the posterior end of the thorax. Both in Cirripeds and in some Cladocera the male apertures are terminal and the female opening in Cirripeds is at the first thoracic segment.
The sperm cells may be of varied forms—Polyphemus (Cladocera)—amoeboid; Copepods—sausage-shaped; Decapods, Euphausids and Stomatopods—spherical and with rigid radial processes; Isopods and Amphipods—thread-like; Ostracods— sperms are many times larger than the body of the individual.
Round eggs may have varied concentrations of yolk. Usually the female carries the eggs after fertilization through some devices but in some cases the eggs may remain uncared.
Myriapoda:
The reproductive organs are unpaired. In Centipeds, genital aperture is placed near the posterior-most end of the body. But in Millipeds the apertures are not far away from the head. Some Myriapods protect their eggs up to certain period after lying.
Insects:
All insects are unisexual excepting Icerya purchasi, which is hermaphrodite and practises self-fertilization.
The testes are generally small, paired and sometimes follicular. Number of follicles varies from one in Diptera to many in Orthoptera. A duct, called the vas efferens, connects the follicles. Thread-like sperms are usually packed as spermatophores.
The ovary is made up of ovarioles. The number of ovarioles is usually six to eight but in female Termite it is 1500, in queen bee several hundreds, but in Tsetse fly only one.
Usually two oviducts unite to form a single duct but in Ephemidae and Lepisma two oviducts open separately.
The genital openings are placed in the ninth and tenth abdominal segments and have in many cases copulatory structures.
The egg is usually ladden with yolk, excepting parasitic Hymenoptera and has a covering of vitelline membrane and rigid chorion. Several openings are present on the chorion for sperm entrance.
Copulation usually takes place long before fertilization and sperms are kept within the spermathecae of the female body. Fertilization occurs at the time of egg laying and thus is under voluntary control of the female. For the laying of eggs many insects are provided with special structures which are generally used for digging.
The number of eggs laid varies in different insects. Termites, ants and bees lay a few thousands of egg at a time. But in Tsetse fly, one egg is released every 9-10 days. Here fertilization and major part of the development take place within a special chamber, called ‘uterus’. Many insects lay their eggs directly within the body of some other insects.
Chelicerate:
Excepting spiders and certain mites, no sexual dimorphism is noted in Arachnida. In Scorpion both the processes, fertilization and development are internal and some form of courtship is noted. The eggs are small and are without yolk. Similar viviparous developments are also noted in Pedipalpi and mites.
In Limulus, fertilization is external and occurs on land.
In Araneida, in both the sexes the openings are present on the middle line of the epigastric furrow. A special structure, called epigynum is associated with the genital opening of the female. The epigynum is simplest in Pirata and modified for egg laying in Aranaea angulate.
Male Araneids have pedipalps modified to act as intromittent organs. In females a single oviduct connects both the ovaries. In both the sexes of Chelicerates, single genital opening serves as the outlet, except in Limulus where it is paired.
Life History of Arthropods:
Though sexual reproduction is a prerequisite for the initiation of development in Arthropods yet instances of parthenogenesis are plenty. Among the Crustaceans, Branchiopoda and Ostracoda usually develop parthenogenetically. In Triops, sexual reproduction is restricted only at a particular time of the year and during the rest of the time development takes place parthenogenetically.
Among the Insects parthenogenesis is common in aphids and certain members of Hymenoptera. The common black wasp usually develops parthenogenetically, because male varieties are extremely rare in nature. The larvae of a Dipteran insect, Master can produce eggs which develop parthenogenetically.
Usually one embryo develops from one egg but in parasitic Hymenoptera belonging to the family Chlacididae, one egg splits into several hundred bits, each of which forms a complete embryo. This phenomenon of polyembryony is extremely interesting from the point of view of embryology, because it results in the production of identical twins (here, of course, identical hundreds) having similar genetic make-up.
A general survey of life history in the different groups of Arthropods reveals the existence of three categories of development:
(1) Direct development:
From the egg hatches out an individual, which resembles the adult in all respects except the size.
(2) Incomplete metamorphosis:
The young resembles the adult but many adult structures are lacking. Such structures appear later in course of further development.
(3) Complete metamorphosis:
The young which comes out of the egg has no semblance with the adult and it lives an independent and completely different sort of life. From this condition it passes usually into a stationary phase and from it emerges the adult. The details of these three categories of development will be discussed separately in the different classes of Arthropoda.
Crustacea:
The metamorphosis is usually complete in Crustacea. The young one which comes out of the egg is called a larva. It usually passes through an independent life and subsequently transforms into an adult. In certain Crustaceans, e.g., Palaemon, Argulus, larva does not come out of the egg.
Thus transformation occurs internally and a young resembling the adult is hatched out. Within the class Crustacea, variety of larval forms (Fig. 18.132 and 18.133) are seen and in many groups one type of larva transforms into another type and finally becomes the adult.
