Advanced biology GCSE Biology 3rd edition
IGCSE Biology 2nd edition Tropical biology

The Mosquito, an Introduction

Mosquito Adult
For illustrations to accompany this article see Insect Life-Cycles
See also: Videos of Mosquitos
Mosquitoes, midges, tsetse-flies and house-flies are familiar insects included in a group called Diptera. They have only one pair of wings, the second pair being reduced to tiny stumps called halteres.

Culex.

One of the commonest kinds of mosquito is the Culex mosquito. Other genera of mosquitoes are Anopheles and Aedes. The females of many species of these mosquitoes pierce the skin of humans and suck blood from the capillaries, often leaving a painful swelling in the region of the puncture. In tropical countries many species of mosquito carry the organisms which cause malaria, yellow fever and elephantiasis (filariasis). A study of the life cycles of the mosquitoes is thus very important.

Eggs.

The female Culex lays her eggs in stagnant water such as in drains, marshy swamps, ditches and shallow ponds. About 200 eggs are laid at a time. As they are laid they are pushed into position by the hind legs and stick together to make a floating raft. Bubbles of air trapped between the eggs keep the raft buoyant even if the water is disturbed. After one or two days the eggs hatch, the larvae push the bottoms out of the eggs and enter the water.

Larvae.

The larvae which hatch out are without legs but can propel themselves through the water by a vigorous flicking-wriggling movement. They have a distinct head, thorax and abdomen, and from the eighth segment of the abdomen a breathing tube projects. When undisturbed, the larva swims to the surface and remains motionless. The breathing tube breaks through the surface film and opens to the air so that the larva is suspended head downwards from the water surface. The breathing tube leads to the two main tracheae of the body and these carry air from the atmosphere to all parts of the insect. There are five little flaps at the entrance to the breathing tube, and these close and prevent the entry of water when the larva leaves the surface. The cuticle in this region is particularly water-repellent so that when the larva is breathing at the surface, water cannot enter and block the tracheae.

While it hangs at the surface, the mosquito larva feeds by continuous whisking movements of its mouthparts. These bear a dense fringe of bristles and set up a current of water which brings microscopic plants to the mouth of the larva. These micro-organisms are filtered out and eaten. If disturbed, the larva flicks its abdomen from side to side and swims to the bottom of the pond. Similar wriggling, swimming movements serve to bring it to the surface again.

The many long bristles on its body are thought to give some protection against being eaten by fish that thus find them distasteful. Nevertheless, some fish such as guppies eat mosquito larvae in great numbers. The tufts of bristles on the last abdominal segment may help the larva to steer when swimming.

Pupation.

After three or four ecdyses, the larva changes to a pupa. The last larval moult reveals a grotesque, comma-shaped pupa which, unlike the butterfly chrysalis, does not remain motionless but is able to swim vigorously by flicking its abdomen. When at rest it is buoyant and floats passively to the water surface where it hangs, head uppermost, from the surface film. The pupa breathes air through a pair of breathing tubes which pass through the pupal case above the thorax. During the pupal stage, the insect does not feed.

Just before the last larval moult, and during pupation, larval organs are digested away and special cells that have so far remained dormant multiply and form the organs of the adult insect. In the pupa, some of the structures of head and thorax can be seen through the pupal skin; the compound eyes, the antennae, legs and wings for example. The abdomen shows little change except for the appearance of a pair of plate-like flaps on the last segment, which probably assist the swimming movements.

After a few days the pupal case splits above the head and thorax. The margins of the broken cuticle spread out into a raft-like float on the surface. The mosquito pushes its head and thorax out of the pupal case and then withdraws the legs and mouth parts. Balanced on the floating pupal case or surface film the imago rests while the wings enlarge and harden, after which it flies away.

Adult.

The adult mosquito has one pair of wings. The second pair is reduced to tiny protrusions
called halteres. These vibrate rapidly while the insect is in flight and constantly stimulate the wing muscles, so maintaining their muscular tone.

The antennae in the female are thread-like. Those of the male have many filaments branching from them and look bushy. Both sexes have elongate mouth parts through which only fluids can be sucked. Whereas those of the females are able to pierce the skin, those of the males cannot do so and are used only to suck up plant juices such as nectar.

The fine, sharp mouth parts of the female are enclosed in a grooved labium. When she takes a blood meal the two lobes at the end of the labium are placed on the skin of the victim and the tubes and stylets are pushed through the skin until they reach a blood vessel. Exactly how such fine structures can penetrate the tissues without buckling is not yet fully understood. Unless a skin sensory cell is touched the victim is usually quite unaware of the penetration.

Saliva is passed down the salivary duct in the hypopharynx; it contains a chemical that prevents the blood from clotting. Some biologists think that the saliva also produces a response in the punctured tissues, resulting in swelling and local irritation. Blood is pumped up through the food channel in the labrum by the action of muscles in the gullet. The labium does not penetrate the skin but acts as a kind of guide and support for the sharper mouth parts. It becomes doubled back as the mouth parts penetrate deeper into the tissues.

It seems that a blood meal is taken by a female before fertile eggs are laid. This, however, is not always necessary, for the first batch of eggs at least, and the females, like the males, may also feed on nectar and fruit juices.

Malaria.

In tropical countries, the Middle East and Southern Asia, certain species of the mosquito transmit the disease malaria. Malaria is a disease of the red blood cells. A microscopic, single-celled animal invades the cells and in feeding on their cytoplasm destroys them, producing regular fevers and other symptoms.

If a mosquito sucks up blood from a person suffering from malaria, red cells containing the malarial parasite are taken in. These parasites are released from the blood cell by the insect's digestive juices and burrow into the insect's stomach wall, where they grow and multiply. The offspring eventually move to the salivary glands. Later, when this mosquito feeds on another person, it injects saliva containing the parasite into his or her blood stream and the person so bitten develops malaria.

The disease is responsible for many deaths and prolonged suffering. In Africa alone, up to three million people die every year from malaria, and it is estimated that a quarter of the population is seriously ill with it for two months in each year.

Drugs such as chloroquine are used to kill the parasites in the blood stream but the parasite quickly develops resistance to it. Much money, time and research have also been applied to exterminating the mosquito which spreads the disease. Methods of control include the draining of swamps and marshes where the larvae live; turning sluggish waterways into swiftly flowing streams; spraying lakes and ponds with oil films and insecticides which poison the larvae; spraying dwelling-places with insecticides to kill the adult mosquitoes.

The World Health Organisation has organized a world-wide campaign against malaria, based on (a) killing the mosquito; (b) destroying the parasite with drugs.

For illustrations to accompany this article see Insect Life-Cycles
See also: Videos of Mosquitos

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