MOSQUITOES

DEFINITION: any of a large family (Culicidae) of two-winged dipteran insects, the females of which have skin-piercing mouth parts used to extract blood from animals, including humans: some varieties are carriers of certain diseases, as malaria and yellow fever.

More than just annoying insects, some mosquitoes are responsible for transmitting diseases that can result in serious illness and even death. Mosquitoes were once viewed merely as a nuisance because of the itching and irritation that resulted from their bites. In the early 1900s, however, they were recognized as carriers of yellow fever, malaria, and other diseases.

The mosquito is in the family Culicidae and belongs to the same order of insects as flies and gnats the order Diptera and has the same anatomical structure. Its soft body is covered by an exoskeleton (an external supportive covering) and divided into three parts: the head, thorax, and abdomen. It has two narrow wings and a pair of knob-like structures, known as halters, that are present in place of a second pair of wings. Unlike other Diptera, the wings of the mosquito have tiny scales on the veins.

The mosquito's head is rounded and supported by a slender neck. It has large compound eyes, complex mouth parts, and two antennae, usually divided into 15 segments. The antennae of the male are more feathery in appearance than those of the female. The major body segment behind the head is called the thorax, to which the wings and six legs are attached. The legs are long, slender, and segmented. The final segment of the mosquito's body is the soft, cylindrical abdomen. It has ten segments, the last ones bearing the openings for the anus and reproductive organs.

Proboscis, snout, trunk, or other tubular organ projecting from the head of an animal.

The most dangerous parts of a mosquito's anatomy are the female's mouth parts These are modified into a proboscis for piercing and sucking. The proboscis looks like a single thin tube and is straight in most species. It actually consists of a sheath (the labium) that encloses saw-tipped daggers (the mandibles and maxillae), an injection tube (the hypopharynx), and a sucking tube (formed by closing the labium against the hypopharynx). The construction of the proboscis is ideal for removing blood from beneath the skin of animals. The mouth parts of the male mosquito are modified for feeding on plant juices; male mosquitoes do not bite.

Mosquito Bites

Not all species of mosquitoes suck blood. However in some species a blood meal by the female is essential to the reproductive cycle. In most species the females, like the males, suck nectar and other juices from plants for nourishment. The bloodsucking species feed primarily on mammals or birds, though some mosquitoes will feed on reptiles and amphibians. Some species are particular in their choice of host species, whereas others appear to be less selective. The feeding periods of many types of mosquitoes are restricted to particular times of the day or night.

To obtain a blood meal, a female mosquito selects a likely spot on her victim, brings her labium against it, and begins sawing through the skin with her mandibles and maxillae.

Through her hypopharynx she injects saliva into the wound to prevent the blood from clotting so that it flows freely into her labro-hypopharyngeal tube. She then sucks up a supply of blood, stores it in her abdomen, and flies away.

The itching of a mosquito bite is caused primarily by the saliva that has been injected. If the mosquito completes her withdrawal of blood before being driven away, much of the saliva will be removed and the itching may be less severe.

Life Cycle and Habitats

A mosquito's life cycle is one of complete metamorphosis it consists of four distinct stages: egg, larva, pupa, and adult though the pattern of development may vary between species. The female mosquito typically lays her eggs in standing water, where they float on the surface in a tiny cluster. The eggs may also be deposited singly or attached to vegetation, depending upon the species of mosquito. Some mosquitoes lay their eggs in the vicinity of water rather than directly in water, and the eggs develop when the area becomes flooded.

During warm weather the eggs develop into larvae within two or three days. Mosquito larvae are long, transparent, and constantly wriggling as they move up and down in a water column. They feed on organic matter, including small animals, bacteria, dead plant material, and algae. Some species feed on other mosquito larvae.

Pupa, quiescent stage between larva and adult in insect metamorphosis.

As the larvae grow, they periodically shed their skins (called moulting) in order to accommodate their larger bodies. Mosquito larvae normally moult four times. After the final moult the animal emerges as a pupa. The pupa has an enlarged anterior portion, composed of a head and thorax, and a curved, elongate abdomen. The pupa is aquatic but does not feed. Both the larvae and pupae of most species must come to the water's surface to breathe. After two or three days the pupa develops into an adult, emerges from its pupal case, and flies away.

Mosquitoes vary in their courtship and mating habits. Many species mate while in flight. The males of some congregate in huge swarms, to which the females are then attracted.

The humming sound made by mosquitoes is often a signal to attract mates.

In cooler temperate regions, adult mosquitoes hibernate, emerging in the spring to lay eggs. In some species mating occurs before the approach of winter and the males die, leaving only fertilized females. In others, eggs are laid in the fall and survive the winter without harm to hatch in the spring.

Mosquitoes are found almost everywhere in the world except open ocean areas, the most arid deserts, and the polar regions. Because of their dependence on water for development during their first stages of life, mosquitoes are most abundant in wet regions of the world.

Nevertheless, of the more than 150 species of mosquitoes that inhabit the United States, many persist in arid regions of the South west Some species thrive in the extremely cold climates of Canada and Alaska, where vast swarms can sometimes be seen around some of the larger lakes and marshes.

Mosquitoes live in a wide variety of aquatic habitats. Besides lakes, ponds, and marshes, some mosquitoes lay their eggs in small depressions where water has collected temporarily. For example, many species use tree holes or fallen leaves, where water has accumulated after rains. In urban areas, common egg-laying sites for mosquitoes are empty containers that have collected water. Furthermore, mosquitoes are not restricted to fresh water for egg laying salt marshes are also a common habitat of many species.

1902: Cure for yellow fever. Walter Reed was a physician and bacteriologist in the service of the United States Army when he proved that yellow fever is transmitted by mosquito bites. Throughout the 19th century the general assumption was that yellow fever was transmitted by contact with such articles as clothing or bedding touched by someone who had the disease.

A Cuban doctor, Carlos Juan Finlay, theorized that the disease was carried by insects, but he had not been able to prove it. In 1896 an Italian scientist, Giuseppe Sanarelli, isolated the organism Bacillus icteroides from yellow fever patients. Reed, along with physicians James Carroll and Aristides Agramonte, was assigned the task of investigating the bacillus. At the same time, a yellow fever outbreak started in the American military garrison in Havana, Cuba. The three travelled there in the summer of 1900 and, by 1902, proved that mosquitoes were the carriers of the disease.

Shortly afterwards an insect extermination program was undertaken, and Havana was freed of yellow fever within 90 days. Colonel William Crawford Gorgas of the U.S. Army Medical Corps later used Reed's techniques to rid Panama of yellow fever, making way for the construction of the Panama Canal.

Mosquitoes and Disease

Mosquito-transmitted diseases differ in their geographic distribution, specific causes and effects, and in the types of mosquitoes that transmit them. Yellow fever is caused by a virus that is transmitted primarily by the mosquito species Aedes aegypti, found in tropical and warm temperate regions of Africa and the Americas.

The primary mechanism of transfer of the yellow-fever virus (as well as other disease-causing organisms) is the mosquito bite specifically, when a mosquito bites an infected person and then bites a healthy one. The virus is thus passed from one person to another through the fluids from the mosquito's mouth. The yellow-fever virus can also be present in other mammals, including monkeys, armadillos, and rodents, and a mosquito can transmit the disease to humans after biting an infected animal. Yellow fever attacks the liver, kidneys, and digestive tract, producing high fever and jaundice, a yellow skin colour from which the disease gets its name. More than half of the victims of yellow fever die within a few days. Those who recover are immune thereafter.

Malaria, disease consisting usually of successive chill, fever, and "intermission" or period of normality.

Malaria is another disease transmitted by mosquitoes. It is caused by microscopic protozoan parasites of the genus Plasmodium. The transmission of malaria is more complicated than that of yellow fever because the parasite must spend a portion of its life cycle inside a mosquito and the other part inside a human. (Yellow fever is dependent on the mosquito only as a transmitting agent.) Malaria is transmitted by mosquitoes in the genus Anopheles.

When an Anopheles mosquito bites a person infected with malaria, it may ingest blood that contains parasites in the sexually reproductive stage, called gametocytes. These gametocytes unite in the mosquito's digestive tract and produce egg-like cells that burrow into the intestinal wall. They then hatch into free-swimming forms that travel to the mosquito's salivary glands.

When the mosquito bites an uninfected human, the free-swimming parasites are transmitted to the victim through the mosquito's saliva. These tiny parasites then enter the victim's red blood cells and begin to divide to form new parasites. Eventually, the affected blood cells burst, and the parasites are released to enter new blood cells within the host and repeat the process of growth and division.

Within one to two weeks millions of these parasites are being released from burst blood cells, resulting in the characteristic symptoms of malaria: periodic chills and fever. Within ten days to two weeks after the initial infection, a new generation of sexually reproductive parasites develops in the blood of the victim. These parasites produce gametocytes, and victims can then infect any Anopheles mosquito that bites them. In this way the cycle of the disease is perpetuated.

In many areas of the world, including North America, mosquitoes of the genus Culex are transmitters of viral encephalitis (sleeping sickness) and other diseases. Dengue, or "break bone fever," is a common tropical disease that results in muscular pains and eruptions of the skin. It is transmitted by Aedes and Anopheles mosquitoes.

Filariasis, disease caused by roundworms and transmitted by mosquitoes.

Roundworm, worm of the phylum Aschelminthes and the class Nematoda.

Filariasis, a disease that affects the lymph glands, is caused by parasitic roundworms and is transmitted by several different mosquito species in tropical regions.

Mosquito-transmitted diseases can be controlled through the elimination of mosquitoes or their egg-laying sites, medical treatment of victims, and prevention of mosquito bites through the use of insect repellent or protective clothing. As early as the 1700s South Americans recognized that quinine, an alkaloid obtained from the bark of the cinchona tree, alleviated the symptoms of malaria, though they did not know how the disease was transmitted.

In the late 1800s mosquitoes were implicated in the transmission of yellow fever in Cuba, and in the transmission of malaria in India. The United States Army initiated the first major effort to eradicate a mosquito-transmitted disease when it launched its campaign to quell the Cuban yellow-fever epidemic. Once the relationship between mosquitoes and yellow fever was understood, major projects were undertaken to eliminate the egg-laying sites of the Aedes mosquito. Similar measures were taken in malaria-infested areas of the world.

In addition to eliminating mosquito habitats, large-scale production began of chemical products that would kill mosquitoes or their eggs. Aerial sprays have been developed to kill adult mosquitoes. Toxic chemicals and oil products have been used in aquatic habitats to kill mosquito eggs, larvae, and pupae. Such chemicals must be used with caution because of their potentially damaging environmental effects. Mosquito bites can be prevented effectively with the use of a wide variety of insect repellents.

Assisted by J. Whitfield Gibbons, Senior Research Ecologist and Professor of Zoology, Savannah River Ecology Laboratory, University of Georgia.