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The Mosquito

Mosquito is an insect that spreads some of the worst diseases of people and animals. Certain kinds of mosquitoes carry the germs that cause such serious diseases as encephalitis, malaria, filariasis, and yellow fever. When a mosquito "bites," it may leave germs behind. Many kinds of mosquitoes do not spread diseases, but they have painful "bites." Many of the mosquitoes that are associated with disease live in the hot, moist lands near the equator. But mosquitoes are found in all parts of the world, even in the Arctic.

There are more than 3,000 species of mosquitoes. Biologists classify species of mosquitoes into about 35 groups, each called a genus. For example, a common species of mosquito, which may transmit certain kinds of parasitic worms, belongs to the genus Culex. This species and other members of its genus may also carry encephalitis viruses. Some mosquitoes in the genus Anopheles carry malaria, and some in the genus Aedes transmit yellow fever, Zika Virus, Dengue, and Chikungunya virus.

Chemical insecticides kill mosquitoes when sprayed in homes, garages, and other buildings. Thick mists of insecticides may be sprayed into fields, forests, and gardens. People also control mosquitoes by destroying the places where they breed. Mosquitoes lay their eggs in marshes, swamps, and pools of still water, and tree holes, tyres, and tin cans that contain standing water. Such places may be drained, or the surface of the water may be covered with thin layers of oil or insecticides.

Since the 1960's, scientists have turned increasing attention to the biological control of insects, including mosquitoes. Programmes are designed to control certain insects without damaging other elements of the environment. One such programme uses fish that eat mosquito larvae (young). Another uses the spores of a bacterium, Bacillus thuringiensis, to kill the larvae.

Most kinds of mosquitoes are from 3 to 6 millimetres long. One of the largest is the American gallinipper. It grows to about 16 millimetres long.

The hum of a mosquito is the sound of its wings beating. A mosquito's wings move about 1,000 times a second. A female's wings make a higher tone than a male's wings, and the sound helps males find mates.

Mosquitoes are flies (insects with two wings). The word mosquito is Spanish and means little fly.

The mosquito's slender body has three parts: (1) the head, (2) the thorax, and (3) the abdomen. The insect's body wall is thin and elastic. Fine hair and thin scales grow on the body and on the wings. Most kinds of mosquitoes are black, brown, grey, or tan. Many species have white or light-coloured markings on their backs, legs, or wings. A few kinds are bright blue or green, and seem to shine with coppery or golden lights.

Head. The mosquito has a large, round head that is joined to the thorax by a short, thin neck. Two huge compound eyes cover most of the head. These eyes, like those of most other kinds of insects, are made up of thousands of six-sided lenses. Each lens points in a slightly different direction and works independently. A mosquito cannot focus its eyes for sharp vision, but it quickly sees any movement. The eyes are always open, even when the insect rests.

A mosquito hears and smells with its two antennae, which grow near the centre of its head between the eyes. The antennae of a female mosquito are long and covered with soft hair. The male's antennae are also long, and have bushy hairs that give a feathery look.

The mouth of a mosquito looks somewhat like a funnel. The broadest part is nearest the head, and a tubelike part called the proboscis extends downward. A mosquito uses its proboscis to "bite," and as a straw to sip liquids, its only food. The males and females of many species sip plant juices.

How a mosquito "bites." Only female mosquitoes "bite," and only the females of a few species attack human beings and animals. They sip the victim's blood, which they need for the development of the eggs inside their bodies.

Mosquitoes do not really bite because they cannot open their jaws. When a mosquito "bites," it stabs through the victim's skin with six needlelike parts called stylets, which form the centre of the proboscis. The stylets are covered and protected by the insect's lower lip, called the labium. As the stylets enter the skin, the labium bends and slides upward out of the way. Then saliva flows into the wound through channels formed by the stylets. The mosquito can easily sip the blood because the saliva keeps it from clotting. Most people are allergic to the saliva, and an itchy welt called a "mosquito bite" forms on the skin. After the mosquito has sipped enough blood, it slowly pulls the stylets out of the wound, and the labium slips into place over them. Then the insect flies away.

The amount of blood taken varies greatly among individual mosquitoes. Some may sip as much as 11/2 times their own weight at a time.

Thorax. The mosquito's thorax is shaped somewhat like a triangle, with the broadest part above and the narrowest part underneath. Thin, flat scales of various colours form patterns on the upper part of the thorax of certain kinds of mosquitoes. These patterns help identify different species. One kind of mosquito that spreads yellow fever has a U-shaped pattern formed by white scales on a background of dark scales.

Strong muscles are attached to the inside wall of the thorax. These muscles move the mosquito's legs and wings. A mosquito has six long, slender legs, and each leg has five major joints. A pair of claws on each leg helps the insect cling to such flat surfaces as walls and ceilings. The mosquito uses all its legs when it walks, but usually stands on only four of them. Many kinds of mosquitoes rest on their four front legs. Some kinds hold their two hind legs almost straight out behind them, but others curve their legs over their backs. White scales form bands on the legs of some species.

Mosquitoes have two wings, unlike most other kinds of insects, which have four wings. The wings are so thin that the veins show through. The veins not only carry blood to the wings, but also help stiffen and support them. Thin scales cover the veins and the edges of the wings. The scales rub off like dust when anything touches them. Some species of mosquitoes may have scales of beautiful colours.

Instead of hind wings, which most other insects have, a mosquito has two thick, rodlike parts with knobs at the tips. These parts, called halteres, give the mosquito its sense of balance. The halteres vibrate at the same rate as the wings when the insect flies.

A mosquito lifts itself into the air as soon as it beats its wings. It does not have to run or jump to take off. In the air, the mosquito can dart quickly and easily in any direction. The halteres keep the insect in balance. A mosquito must beat its wings constantly while it is in the air. It does not glide during flight or when coming in for a landing as do butterflies, moths, and most other flying insects. A mosquito beats its wings until its feet touch a landing place.

Abdomen of a mosquito is long and slender, and looks somewhat like a tube. Some kinds of mosquitoes have an abdomen with a pointed end. Other kinds have an abdomen with a rounded end. The shape of the abdomen helps scientists identify the species.

A mosquito breathes through air holes called spiracles along the sides of its body. The abdomen has eight pairs of spiracles, and the thorax has two pairs. Air flows into the holes, and tubes carry the air from the spiracles to all parts of the mosquito's body.

The life of a mosquito

A mosquito's life is divided into four stages: (1) egg, (2) larva, (3) pupa, and (4) adult. At each stage the mosquito's appearance changes completely, and the insect lives a different kind of life. In warm climates, some species develop from newly hatched eggs into adults in only a week. In the cold climate of the far north, mosquito eggs may remain dormant from autumn until late spring. They hatch in May or June, and take a month or more to grow into adults.

Egg. A female mosquito lays from 100 to 300 eggs at a time, depending on the species. One female may lay as many as 3,000 eggs during her lifetime. The eggs are laid through an opening at the tip of the abdomen.

The females of most species of mosquitoes lay their eggs in water or near it, but each species has a favourite spot. Some like quiet swamps, and others prefer salt marshes. Still others lay their eggs in hidden pools that form in tin cans, rain barrels, gutters, fallen logs, or hollow tree stumps.

Among some species, the females drop their eggs one at a time. Frilly, transparent parts on the shell keep each egg afloat until it hatches. The females of other species arrange their eggs in groups that look somewhat like rafts. The female rests on the surface of the water while she lays her eggs, which are narrow at the top. With her hind legs, she carefully pushes the eggs, wide ends downward, into raftlike groups. The eggs of most kinds of mosquitoes hatch in two or three days in warm weather.

All mosquito eggs must have moisture to hatch, but not all species lay their eggs in water. Certain mosquitoes, called floodwater mosquitoes, drop their eggs in moist soil on flood plains and on irrigation sites. The eggs hatch after a flood takes place perhaps a year later. Other species, sometimes called pond mosquitoes, lay their eggs in hollow places left by ponds that have dried up. The eggs hatch after rains fill the ponds with water. Not all of the eggs of these mosquitoes hatch after the first rain. The eggs must be soaked by a second or even a third rain before they hatch into larvae.

Larva of a mosquito is often called a wriggler because it is so active. The wrigglers of most species move about by jerking their bodies through the water.

A wriggler looks somewhat like a worm or a caterpillar. A thin, skinlike shell covers its body. The wriggler has a broad head, with two short, bushy antennae on each side. It has two eyes behind the antennae, near the back of the head. Its mouth is on the underside of the head, near the front. Long hairs called mouth brushes grow around the jaws and sweep food into the wriggler's mouth. Unlike an adult mosquito, a wriggler can open its jaws and chew its food. It eats small plants and small animals that live in the water, including other wrigglers and one-celled animals called protozoans.

A wriggler breathes through a tubelike siphon (air tube) at the rear of its body. To get air, it pushes its siphon above the surface of the water.

The larvae of certain swamp mosquitoes do not have to come to the surface for air. They get air from the leaves, stems, and roots of various underwater plants. The larva of one kind of swamp mosquito has a breathing tube with two sharp tips. It uses one tip to hold itself to the plant, and moves the other tip back and forth in the plant tissue to get the oxygen stored there.

The larvae of many species of mosquitoes grow quickly. They moult (shed their skins and grow new ones) four times in 4 to 10 days. After the last moult, the larvae change into pupae. The larvae of some species spend the winter in hibernation. They change into pupae early in spring.

Pupa. A mosquito pupa is shaped somewhat like a comma. The head and thorax are rolled into a ball, and the abdomen hangs down like a curved tail. A thin "skin," like that of the larva, covers the pupa's body. The pupa breathes through trumpet-shaped tubes attached to the top of its thorax. The pupa sticks these tubes out of the water to get air. The pupa of certain swamp mosquitoes, whose larva gets air from underwater plants, pushes its tubes into the plant. After this pupa has changed into an adult, it pulls out the tubes or breaks them off and leaves them in the plant. The pupa then swims to the surface.

The pupae of most species of insects do not move, but almost all kinds of mosquito pupae can swim. These pupae are sometimes called tumblers because they roll and tumble in the water.

A mosquito pupa does not eat. It changes into an adult in two to four days. The pupal "skin" splits down the back, and the adult mosquito pushes its head and front legs out. The insect then pulls out the rest of its body.

Adult. After the adult mosquito leaves the pupal "skin," its wings dry quickly and it flies a short distance away. Most species of mosquitoes spend their whole lives within about 1.5 kilometres of the place where they hatched. A few kinds may travel as far as 30 kilometres away to find food or mates.

A female mosquito attracts a mate by the high-pitched sound made by her wings. The males are deaf for the first 24 to 48 hours of their lives, until the hairs on their antennae are dry.

The females of some species must sip blood before they can lay eggs that will hatch. Each species of female prefers the blood of certain kinds of animals. Some feed only on frogs, snakes, or other cold-blooded animals. Others prefer birds. Still others suck the blood of cows, horses, and people.

Male mosquitoes may live only about 7 to 10 days, but females may live up to 30 days or more. The females of some species live through the winter in barns, garages, houses, caves, or in the bark of logs. Some species spend the winter as eggs or as larvae. They develop into adults in spring.

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