BIOLOGY OF THE MOSQUITO

Mosquitoes are more commonly called gnats and are the insects which are such a nuisance on summer evenings because of their biting habits. Several different kinds of this pest occur in Britain, and one is the mosquito which carries and spreads the malaria parasite in Italy and the East. This mosquito, Anopheles maculipcnnis, does not transmit the disease in our country to-day, but did so formerly as the widespread prevalence of ague in past times tells us. A mosquito is a small two-winged insect, easily recognizable by the long proboscis which projects from its head and by its long slender legs. It has a small head with large eyes, a short thorax and a long narrow abdomen. The common British species is

Culex pipiens, a small brown insect with dark legs and whitish transverse bands on the abdomen. Another common gnat is Theobaldia annulata, which can be distinguished from Culex by the dark spots on its wings and the white rings on its legs. Anopheles also has dark spotted wings, but differs from the other two mosquitoes in the position it takes up when it is resting. When at rest the abdomen of Anopheles points upwards, whereas in the case of Culex and Theobaldia the abdomen is held parallel to the surface on which the insect is standing. The male mosquito feeds on the juices of flowers and fruits, and it is only the female which sucks blood. Apparently blood is not indispensable as a food for mosquitoes, for their distribution extends to the Arctic regions where there is little chance of obtaining a meal of blood. Under such circumstances the females also feed on plant juices.

The Head and Mouth-parts of a Mosquito. A pair of large compound eyes cover most of the head of a mosquito, which also bears a pair of antenna?. The feelers provide an easy means of distinguishing the male from the female, for those of the male have a bushy appearance whereas those of the female are like slender threads. The proboscis of the female is a tube containing instruments which have a piercing and sucking function, but the male uses his proboscis for sucking only. The structure of the mouth-parts of the female is as follows :—

The tube is formed by the labium, which has two long lobes at the end and has a slit-like opening running along the upper side.

Within the groove of the labium is another tube with a slit along the lower side, and which is termed the labrum-epipharynx.

The slit in the labrum-epipharynx is closed by an underlying, flat, pointed lancet, the hypopharynx, down the middle of which runs a salivary duct.

Within the tube formed by the labium are four slender, pointed lancets.

Projecting from the head at the base of the proboscis are a pair of jointed rods, the maxillary palps. These lie outside the proboscis, and are long in the male mosquitoes and in the female Anopheles, but short in the female Culex.

What Happens when a Female Mosquito Bites

When the female mosquito bites, the bilobed end of the labium is placed on the skin of the victim and the enclosed stylets are pushed in. As the labium tube does not enter the skin it is bent into a loop. The mandibles and maxilla; are merely piercing instruments, but besides piercing, the labrum-epipharynx and the hypopharynx form a tube up which the blood is sucked. The saliva which passes down the hypopharynx prevents the blood of the victim from clotting and so blocking up the sucking tube. The irritation caused by a mosquito bite is mainly caused by some substance which passes into the wound from the mouth of the insect. The blood poisoning which sometimes results from a gnat bite is generally due to germs introduced by the scratching of the inflamed part of the skin. When one is bitten by a mosquito in Britain the offender is usually Theobaldia, for Culex pipiens does not often bite human beings.

The Life History of a Mosquito

The Egg

The early stages of the life of a mosquito take place in water, and the eggs of Culex and Theobaldia are laid in water butts, ditches and small pools which are near houses. In May the female lays 200 to 300 eggs, which are small and cigar-shaped. The eggs of Culex and Thcobaldia are stuck together to form a small raft which floats on the surface of the water, but those of Anopheles are laid separately.

The Larva

The larva emerges from the egg in two or three days, and enters the water by breaking off ’ a cap at the lower end of the egg. It is a small wriggling creature something like a caterpillar, and shows division into head, thorax and abdomen. The head bears paired eyes, feelers and jaws and also two small brush-like structures, which by their movement cause small organisms and other food particles to be drawn into the mouth. The thorax is not segmented, but the abdomen consists of nine ring-like segments. The Culex larva breathes through a tube termed the siphon, situated on the 8th segment, and which opens at the end by five small Haps or valves. When breathing the larva pushes the siphon through the surface film of the water, and then opens the five flaps so that it hangs head downwards from the surface film. In this position the larva can feed and breathe at the same time, but if it is disturbed the flaps are closed and it sinks from the surface by its own weight. To regain the surface the larva swims upward by wriggling movements of its body. The 9th segment of the abdomen bears a tuft of bristles and also four small plates which contain breathing tubes and which act as gills.

The larva of Anopheles has no siphon and, when breathing, lies with its body parallel to the surface so that the spiracles on the 8th segment of the body communicate with the air.

The Pupa

The larva casts its skin four times and then becomes the pupa, which has a large rounded anterior part representing the head and thorax, and a narrow curved abdomen. The abdomen ends in a pair of flat plates which form a tail fin. Two small tubes or respiratory trumpets project from the upper side of the thorax. These breathing tubes pierce the surface film, from which the pupa hangs until it is disturbed, when it jerks itself downwards by curving movements of its abdomen. It recovers its position by floating passively to the surface.

The Imago

After a few days the skin of the pupa splits along the back and the mosquito climbs out. It rests on the pupa case for a short time to enable its wings to expand and harden, and then flies away. The male Culex, Anopheles and Theobaldia, die off in the autumn, but the females hibernate ; those of Anopheles shelter in farm buildings and those of Culex and Theobaldia in the cellars and attics of houses. In the case of other kinds of mosquitoes, the adults of both sexes die and the winter is passed in the larval condition.

Mosquito Control

Malaria is caused by the presence of a minute Protozoon in the blood of the infected person, and it is transmitted to others by the mosquito Anopheles maculipennis. When an infected person is bitten the malaria parasites pass with the blood into the stomach of the mosquito and undergo complicated changes in the body of the mosquito, which result in the production of young parasites in the salivary glands. When another victim is bitten the malaria organisms pass down the salivary duct of the mosquito and so into the blood of the victim.

In malaria-infested regions every effort is made to prevent the spread of the disease by controlling the mosquitoes. Suitable clothing and mosquito netting may prevent the insect from biting, but the best method of control is by draining the breeding grounds, or by destroying the larva?. Swamps, ponds and ditches are drained, and tanks and water butts are covered to prevent the entrance of egg-laying mosquitoes. Water which cannot be drained is sprayed with paraffin oil, which forms a thin film on the surface. The oil lowers the surface tension of the water so that when the larva rises to the surface to breathe it cannot hang from the surface film but sinks immediately. It dies through lack of air, for not only is it unable to remain at the surface to breathe but its siphon becomes blocked with oil.

The toll of human life and the reduction in efficiency from malaria may be extremely heavy. Much of the proverbial laziness of the African negroes may be ascribed to the heavy and repeated infection to which they are exposed. For centuries the inhabitants of Rome suffered yearly from epidemics of malaria owing to the invasion of mosquitoes from the Pontine Marshes, which in recent times have been scientifically drained and are beginning to yield useful crops. Mosquitoes also transmit yellow fever and the construction of the Panama Canal was for a time at a standstill through loss of labour, until the breeding grounds had been treated and the mosquitoes largely eliminated. New insecticides called D.D.T. and Gammexane are being used to stamp out mosquitoes while drugs such as quinine and paludrin are used to treat patients who have contracted malaria.

Practical Work on the Mosquito

Examine the eggs, larva, pupa and imago of Culex and Anopheles. Also examine prepared slides of the same.

Examine the habits of larva; and pupa; kept in an aquarium.

Examine prepared slides showing the head and mouth-parts of male and female mosquitoes.

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