In this article we will discuss about Culex:- 1. Habit and Habitat of Culex 2. External Features of Culex 3. Life History.
Habit and Habitat of Culex:
Culex pipiens is found in temperate regions all over the world, and Culex fatigans throughout the tropics and sub-tropics. Culex lives in houses, in cities and farms, and is abundant also in rural areas. They are most abundant during spring, but hibernate during un-favourable climatic conditions, the adults hide in hollows of trees, caves, crevices, barns, etc.
The life span of male mosquitoes is seldom more than three weeks, they die after fertilizing the females. The females live from four weeks to several months, but they die when all their eggs are laid. Culex has several generations in a year.
External Features of Culex:
Shape, Size and Colouration:
The body of Culex is small, soft, and covered with small scales. Culex measures about 3 to 4 mm in length. The body colour is grey-black.
Division of Body:
The body is divisible into head, thorax and abdomen.
1. Head:
Head is globular and highly mobile on a slender neck. There are two very large black compound eyes, there are no ocelli. The top of the head has an epicranium below which is a clypeus which is thick and projects in front.
There are two filiform antennae, each with 15 joints, the basal segment is the scape which is concealed by a very large globular second segment, the pedicel containing a Johnson’s organ which is auditory in function, the other 13 joints form a flagellum having many bristles lying in rings.
The bristles are longer and much more numerous on the antennae of males giving them a bushy appearance. In the female the antennae have rings of few, short bristles, thus, sexes can be distinguished readily by the antennae. The head bears two maxillary palps and a proboscis.
The maxillary palps are stiff and have many bristles, the palps in the female are short and three-jointed, but in the male they are as long, or even longer than the proboscis, they are five-jointed.
Mouth Parts:
The proboscis is a straight, long tube formed by a fleshy ventral labium which has a deep groove on its upper side, in this groove is a long pointed and ventrally-grooved labrum epipharynx. At the distal end of the labium is a pair of small tactile labella which are reduced labial palps.
The groove of the labium also contains five needle-like stylets in a female Culex, they are two mandibles, two maxillae, and a hypo pharynx. The mandibles are finer than the maxillae, but both have saw-like edges on their tips. The hypo pharynx is also needle-like and has a fine salivary duct running through it and opening at the tip, through this duct saliva is poured to prevent coagulation of blood of the victim.
In the male the labrum-epipharynx and the labium are the same as in the female, but the mandibles and maxillae are very short and functionless and the hypo pharynx is fused with the labium.
Feeding:
The normal food of both sexes are nectar of flowers and juices of plants, but the female has its mouth parts modified for obtaining additional meals of blood of vertebrates. A female mosquito sits on a vertebrate and presses its labella against the skin, they act as a guide for the piercing mandibles and maxillae which are sunk into the flesh; the en-sheathing labium bends back to allow the needles to go in.
The labrum-epipharynx and hypo pharynx together act as a tube forming a food canal through which blood is sucked up from the wound; the suction is caused by the pharynx by which blood comes into the mouth.
Thus, the mouth parts are for piercing and sucking. Mosquitoes have three oesophageal food reservoirs in addition to the stomach, the reservoirs are used for storage of food, such as plant juices, but not for blood which passes directly to the stomach.
2. Thorax:
Thorax is arched, it has mesothorax which is very large and its tergum has three sclerites, a scutum, a trilobed scutellum and a post-scutellum. Prothorax and metathorax are very small. On the thorax there are two pairs of spiracles. From the mesothorax arise a pair of membranous functional wings which are long and narrow.
The nervures of wings are beset with scales, and the posterior margin of the wings is fringed with bristle and scales. The wings of metathorax are reduced to form a pair of small halteres, each of which has a swollen base orscabellum, a narrow stem or pedicel, and a distal swollen knob or capitellum.
Halteres vibrate 300 times per second during flight, they probably act as balancers, but their function is doubtful, however, if halteres are removed flight becomes difficult or even impossible.
From the thorax arise three pairs of legs which are very long and slender, they are fragile and have the usual parts of an insect leg, but the coxae are short and tarsi long with five joints ending in a pair of simple claws, below each claw is a pad-like pulvilus. The legs also have many scales and bristles.
3. Abdomen:
Abdomen consists of 10 segments of which the first is vestigial and fused to the metathorox; the second to the eighth are clearly seen, each has a pair of spiracles; the ninth and tenth segments are partly telescoped into the eighth. In the female the 10th segment is blunt and bears a pair of cerci, between them is a small post-genital plate which is part of the tenth sternum.
In the male the 9th and 10th segments are complex, they undergo a torsion of 180° as soon as the mosquitoes are born, so that the terga and anus become ventral and the sterna dorsal. The ninth segment in ring-like with a bilobed ventral tergum, it bears a pair of large claspers, each with a broad basal coxite followed by a narrow style which ends in a claw.
The 10th segment has a bilobed dorsal sternum from which project two processes with curved and toothed tips, the male intromittent organ or aedeagus projects posteriorly, it is formed by fusion of gonapophyses of ninth segment. The ejaculatory duct opens into the aedeagus. During copulation the male holds the female by its claspers and the aedeagus is inserted into the vagina.
Life History of Culex:
(i) Eggs:
After mating the female lays eggs on still water, the eggs may be laid on ponds, or pools, or rain-filled containers.
The eggs are cigar-shaped, tapering at one end. The eggs are laid at night and one female may lay up to 300 eggs. The eggs are laid side by side standing erect, and glued together by the legs to form boat-shaped rafts which float on water. The eggs hatch in 1 to 3 days and a larva emerges from the lower end of each egg.
(ii) Larva:
The larvae are called wrigglers because of their wriggling movements, they are microscopic on hatching. The larva leads an active life, it swims about, feeds and grows, and the larval life lasts from 3 to 14 days according to temperature. During this period it moults four times and grows larger after each moulting.
The larva has a large chitinous head which is flattened dorsoventrally, it has compound eyes developing, and closely behind each is a larval ocellus, it has a labrum, small toothed mandibles, a pair of maxillae with feeding bristles lying internally, labial plates, and a pair of jointed antennae.
It has a mouth over which is a pair of rotary feeding brushes, formed of stiff bristles, the brushes cause a current of water by which small particles of food are wafted into the mouth.
Food consists of algae and small organic particles, the larva feeds on these below the surface of water. Thorax is globular, its segments are fused together. On the head, thorax and abdomen are paired bristles, some of them forming bushy tufts, especially on the thorax. Abdomen is slender and has nine segments, on the first seven abdominal segments are tufts of bristles.
The eighth segment has a chitinous and tubular respiratory siphon, at the tip of the siphon are two spiracles leading into tracheae. Around the spiracles are five leaf-like lobes which can close over the spiracles to prevent water from entering. The respiratory system is metapneustic in which only the last pair of abdominal spiracle is open.
The larva though aquatic breathes air through the siphon and comes to the surface to take in air. When resting the larva pierces the surface film of water by its siphon which projects just above the surface and draws in air, and it hangs by the siphon with its head downwards, but it is inclined at an angle.
The siphon on its ventral side has two tufts of bristles, and two rows of flat spines called pecten. On the eighth segment is a patch of small scales in one or two rows forming a comb. In some species of Culex the comb has scales in several rows. The ninth segment of the abdomen is slender and covered by a chitinous dorsal plate.
At the end of the ninth segment is an anus surrounded by four leaf-like tracheal gills which differ from true gills in having tracheae instead of blood vessels. The ninth segment has a tuft of dorsal bristles at its tip, and ventrally a bushy tuft of bristles called ventral brush. The larva sinks in water being heavier, and it rises by wriggling movements of the abdomen. After the fourth moult the larva changes into a pupa.
(iii) Pupa:
The pupa is comma- shaped and is called a tumbler. It has a large cephalothorax formed by the head and thorax.
On the mid-dorsal side of the cephalothorax is a pair of tubular respiratory trumpets which are broader at the distal end, they communicate with an anterior pair of thoracic spiracles. By means of the trumpets the pupa hangs from the surface film of water and takes in air through their distal ends which project slightly above water.
Inside the cephalothorax may be seen cases containing compound eyes, one pair of ocelli, antennae, wings, and legs of the adult. Behind the cephalothorax is a ventrally flexed abdomen formed of nine segments of which the first is very small but segments 2 or 9 are distinct and movable. On the abdomen are tufts of bristles.
The last segment bears a pair of chitinous leaf-like paddles by which the pupa swims. The pupa is a resting stage, during this period it does not feed, but the pupae of mosquitoes are peculiar in not being quiescent, they are active and can swim about. Unlike the larva the pupa is lighter than water and requires a muscular effort to sink down.
The pupal period lasts from two to seven days depending upon the temperature. During this time remarkable changes occur in the pupa while the adult insect called imago is being formed. When the imago is completed the skin of the pupa splits mid- dorsally along the back between the trumpets and the imago emerges, first the head, then body and appendages are extricated.
The imago rests for some time on the pupal skin, it stretches and dries its wings, then flies off. It can start laying eggs in a week’s time and, thus, repeat the life history.
(iv) Metamorphosis:
The young one that hatches from an egg is quite different from the adult insect in structure and mode of life, it is known as a larva. The larva feeds, moves, moults, and grows, then it passes into a quiescent stage, the pupa which is different from both the larva and the imago. Finally the adult is formed in the pupa.
This form of development is termed complete metamorphosis or holometabolus metamorphosis. It occurs in higher insects as seen in the mosquito. Growth and moulting up to the end of the larval period are controlled by the juvenile hormone of corpora allata.
The pupa though quiescent undergoes very great internal changes in order to form the imago. Most of the larval organs in the pupa, except the central nervous system and developing genital organs, are broken down, the process of breaking down and disintegration of larval organs is called histolysis.
The process of histolysis is brought about largely by blood corpuscles called phagocytes which feed upon the tissues of disintegrating organs, and their products of digestion pass into the blood to form new tissues.
For the formation of organs of the imago groups of formative cells are set aside in the larva, they are called imaginal buds or histoblasts. The imaginal buds are found all over the body of the larva, close to its internal organs or in invaginations of the epidermis.
The imaginal buds are the rudiments of future organs. The process of formation of new adult organs inside a pupa is known as histogenesis. Imaginal buds are dormant, they are stimulated by a hormone of prothoracic endocrine glands, these glands become active only during metamorphosis secreting a pupation hormone which causes imaginal buds to develop.
By this process the imago develops inside the pupa; when development is completed the pupal covering splits and a perfectly formed imago emerges. The final moulting into an adult is also controlled by the hormone of prothoracic glands, it occurs only after the juvenile hormone of corpora allata is not being produced.
Thus, in holometabolus metamorphosis the stages in the life cycle are, egg → larva → pupa → imago and the adult wings develop from inside from imaginal buds and are not visible externally. In hemimetabolous or heterometabolous metamorphosis, as seen in a cockroach the stages are, egg → nymph → imago, and the adult wings develop externally from the integument.