Mosquitoes

views updated

Mosquitoes

Mosquitoes belong to an order of insects called Diptera, which includes the common housefly. All together, the Diptera order, the flies, are responsible for carrying diseases to more than 50% of the world’s population. Some 120,000 species of Dipteran flies have been catalogued, which includes more than 2,500 species of mosquitoes. More than 15,000 species of flies and 150 species of mosquitoes are found in North America.

Some species of mosquitos in the genera Anopheles, Aedes, and Culex are responsible for infecting human beings with diseases such as malaria, filariasis, and yellow fever. While pest flies carry diseases to humans and animals and cause a considerable loss to agricultural

crops, their larvae also serve an important function in the process of decomposing dead plant and animal material, a role they share with many bacteria and fungi. Mosquito populations are a part of this process.

Characteristics

Mosquitoes have two pairs of wings, but their second pair of wings are reduced to short, peglike structures called halteres. Mosquitoes have thin, long bodies and three pairs of extremely long legs. They have scales along the veins of their wings and long beak like, sharp sucking mouth parts called a proboscis. These two features distinguish mosquitoes from other flies. Mosquitoes also have feathery or hairy antennae.

Female mosquitos are ready to mate within a few hours after reaching their adult stage, and males are usually ready within 24 hours. Mating typically occurs while the mosquitoes are in flight, but sometimes it occurs on the ground. The tone of the female wing beat attracts males, and they grab the females with their hind legs. Mating among mosquitoes is related to their swarming habits, which in some species, but not all, is the preliminary behavior to mating. Swarming usually occurs around sunset and near fences or other objects and can last from 10-30 minutes.

Mosquitoes feed on sweet nectar, fruit, and other sugary substances. The females of some mosquito species also feed on blood, which they need in order for their ovaries to mature and for their eggs to develop. The female blood meal can take place before or after she has mated. Female mosquitoes detect their blood hosts partly through the sense of smell and partly by sight. The distance over which a mosquito can detect a blood host can range from 20-90 feet (6-27 m). Research indicates that mosquitoes are attracted to hosts already under attack by other mosquitoes. Some adult male humans are more desirable to them than women or children. Human beings are not the only blood hosts that mosquitoes attack. This “vampire” of the insect world is known to feed on mammals, birds, lizards, fish, bats, and even caterpillars for its blood meal.

Life cycle

Mosquitoes have four stages in their life cycle beginning with the egg, then proceeding into a larva stage, followed by a pupa stage, and finally adulthood. Female mosquitoes deposit their eggs in a number of different environments depending on the particular species. While the larvae can only live in water, eggs are not always laid in water. Some species deposit their eggs in areas that may not be flooded for a number of years, but the eggs can survive for several years until the next flood. Other species deposit them separately on top of the water and others deposit them in groups on the water’s surface. These are called “rafts.” When the eggs are first laid they are white in color, but they change to black or brown in a short time. Some mosquito eggs, such as the Anopheles, have a hull-like shape with extensions on either side, giving them the appearance of tiny rafts. Some mosquito eggs are able to trap air bubbles. From 30-500 eggs are laid at one time by females, depending on the species. Most hatch in two or three days into aquatic larva.

During the larval stage, they feed on plankton and move by wiggling, hence the name “wiggler,” which is applied to mosquito larvae. The larva life span varies, again depending on the species. Some develop rapidly within days, while others may take months to develop. They also exhibit a variety of feeding habits, from scavenging dead food, bottom or surface feeding of plankton, to eating other living organisms. The next stage, the pupa, which occurs just before maturity, takes place on the surface of the water, where the pupa breaks out of the larva shell. The transformation from larva to pupa takes only several minutes and occurs at the surface of the water. The pupa stage lasts from two to three days in the tropics to several weeks in cold climates.

As adult mosquitoes leave the pupal stage they swallow air which helps them expand their abdomen and wings. Among the various species there are different ratios of male and female births. The life span of mature mosquitoes ranges from a few days to over a month depending on the species and the climate. Those living in hotter climates tend to have a shorter life span.

Malaria and yellow fever

Through her blood sucking, the female mosquito carries diseases like malaria and yellow fever to human beings. While it has been wiped out in some parts of the world, malaria is a disease that is still contracted in many places of the world. The illness is characterized by periodic fever and chills in the victim. Malaria is caused by a protozoa that was first identified by English physician Ronald Ross (1857-1932) in 1898. Not all mosquitoes carry the protozoa that causes malaria, but those that do deposit a parasite in the bloodstream that is capable of infecting the person. There are effective drug treatments and preventions for malaria today, but those who have had the disease are susceptible to relapses. The main vector of the malarial parasite are species of Anopheles mosquito. Open control measures for the disease include spraying against adult flies and treating water sources where the larvae develop.

Yellow fever is another mosquito-borne disease, which causes jaundice in the victim. The mosquito that carries the yellow fever virus to humans during its blood meal is Aedes aegypti. Jaundice causes a person to appear yellow in color, hence the name. While there is no exact treatment for yellow fever, most people recover and have an immunity to the disease for the rest of their lives. Immunization against the disease is available through vaccination, but the most effective method used to prevent yellow fever is through measures to control mosquito populations that transmit the disease to humans.

Much time and effort have been dedicated to mosquito research. Because they are such a nuisance to people, and because they transmit so much disease globally, billions of dollars are spent in finding ways to control their numbers. One area of research has been in finding the genetic basis of insecticide resistance. In the past, pesticides like DDT were very effective against mosquitoes and helped reduce the incidence of disease and death in malaria-stricken areas of the world. Mosquito populations, however, developed resistance to insecticides relatively rapidly so that now, many insecticides are ineffective and the toxic effects to the environment and to humans directly outweigh their use in killing mosquitoes. Research is underway at many institutions to identify the genes within mosquitoes that allow them to develop resistance to chemical insecticides. Once identified,

KEY TERMS

Halteres —Shortened appendages in flies and mosquitoes at the rear of the thorax, where other insects have wings. Halteres help flies balance themselves.

Larva —The initial stage of a mosquito after it hatches from its egg.

Pupa —An insect in the nonfeeding stage during which the larva develops into the adult.

scientists hope to create transgenic, or genetically engineered, mosquitoes that will no longer carry the resistance genes. The new susceptible mosquitoes would then be bred in mass numbers and released into the wild to breed with existing mosquitoes, breeding-out the ability to adapt to pesticides in the process.

Similarly, geneticists hope to use mosquitoes as vectors to breed out the ability to transmit malaria. By creating transgenic mosquitoes that lack the ability to act as a host to the parasite causing malaria in humans, scientists hope to breed-out, or replace, natural populations with new mosquitoes that do not transmit malaria. Genes conferring resistance to malarial infection have already been identified in malaria transmitting species of mosquitoes. Research is now underway to splice the genes into developing mosquito eggs in order to create the new malaraia-resistant transgenic mosquitoes to be released into the wild.