A two-winged fly belonging to the family Culicidae in the insect order Diptera. Mosquitoes are slender, fragile insects (Fig. 1) and are recognized by their long legs, a long and slim abdomen, and narrow wings with characteristic scaled-wing venation. Most notably, adult mosquitoes possess a long proboscis, which is used for piercing and sucking. This anatomical structure is the key feature enabling the mosquito to be a vector (transmitting carrier) of organisms that cause diseases in humans and other animals (for example, dogs and horses). Mosquitoes also are holometabolous insects—that is, they undergo complete metamorphosis with a four-stage life cycle [egg (embryo), larva, pupa, and imago (adult)]—and all larval stages are aquatic. Mosquitoes inhabit all temperate and tropical regions of the world. They are even found in Arctic regions (although absent from Antarctica). See also: Diptera; Economic entomology; Insecta; Metamorphosis; Zoonoses
The mosquito family Culicidae is a relatively large group and includes more than 3500 species, which are classified into two subfamilies: Anophelinae (3 genera) and Culicinae (depending on the classification scheme, the number of genera ranges from approximately 40 to 110). The most important members of the mosquito family belong to three genera: Aedes, Anopheles, and Culex (Fig. 2).
Adult female mosquitoes lay their eggs on or near water. For example, members of the genera Culex and Culiseta deposit egg rafts on water surfaces; members of the genus Anopheles lay their eggs singly on the surface of water; and members of the genera Aedes and Psorophora lay batches of eggs on substrates near water. In fact, the latter two genera are called floodwater mosquitoes because their eggs require a period of drying for early embryonation to occur and hatch when rains and flooding cover them with water. Although mosquitoes may be found in nearly all aquatic habitats (including ponds, streams, salt lakes, tree holes, and artificial containers), the majority use still-water habitats.
Most larvae, or wrigglers, feed on algae and organic debris that they filter from the water with their oral brushes. However, certain genera (for example, Toxorhynchites and Psorophora) may be predaceous and feed on other mosquito larvae. Larvae go through three molts and four instars before pupation (note that the process of molting consists of the shedding or ecdysis of the outer cuticular layer of the body, whereas an instar is a stage between molts). Pupae, or tumblers, are active (but nonfeeding) stages in which metamorphosis to the adult stage occurs. Both larvae and pupae usually breathe through air tubes at the surface of the water. See also: Ecdysone; Insect physiology; Molting in arthropods
Adult male mosquitoes are relatively short-lived. They do not suck blood, but feed primarily on nectar and other plant juices. Females also feed on nectar as their primary energy source, but they require a blood meal for egg production in most species (Fig. 3). Some mosquito species are very host-specific, blood-feeding only on humans, birds, mammals, or even reptiles and amphibians, although many species will feed on any available host.
Mosquitoes are of major importance in both human and veterinary medicine. They can cause severe annoyance and blood loss when they occur in dense populations, and they are responsible for the transmission of a number of very serious pathogens. In particular, mosquitoes act as vectors of three significant groups of disease-causing organisms: Plasmodium, which is a protozoan parasite (transmitted exclusively by female Anopheles mosquitoes) that produces malaria; filarial worms, which are parasitic nematodes causing elephantiasis in humans and heartworm disease in canines; and arthropod-borne viruses (arboviruses), which are the causative agents of Zika virus disease, West Nile virus, chikungunya virus disease, dengue fever, yellow fever, Rift Valley fever, and a variety of encephalitis viral diseases. Collectively, the effects of these infectious diseases are immense, with millions of people and animals being infected and killed every year, particularly in countries with limited medical resources. See also: Arboviral encephalitides; Chikungunya virus disease; Dengue fever; Encephalitis (arboviral); Heartworm; Malaria; Medical parasitology; Pathogen; Rift Valley fever; West Nile virus; Yellow fever; Zika virus disease
As previously mentioned, the adult female mosquito requires a blood meal for egg production. Hence, it is the adult female that transmits the various disease-causing pathogens to susceptible hosts. The mosquito first acquires a pathogen when it feeds on an infected host. Within the vector mosquito, the agent undergoes development or multiplication. Typically, the pathogenic agent does not adversely affect the mosquito that it infects, which leaves the mosquito able to transmit the pathogenic agent to susceptible hosts during subsequent blood-feedings. The pathogen can enter these new hosts by being injected along with the saliva of the mosquito (as is the case with malaria and arboviral diseases), or it can break out of the insect's proboscis and crawl into the wound created during the blood-feeding (as is the case with heartworm infection in dogs). See also: Disease ecology
To prevent the occurrence of mosquito-borne diseases, it is imperative to reduce human and animal contact with mosquitoes (for example, through the use of mosquito netting, window screens, and repellents) and to remove or eliminate actual or potential mosquito breeding grounds and larval habitats. Vaccines for the prevention of Japanese encephalitis and yellow fever are available, and other prophylactic drugs have been developed. Most notably, vaccination pilot programs against the form of malaria caused by Plasmodium falciparum (the species responsible for the highest mortality and morbidity among victims of malaria) have been implemented in a number of African countries; however, this particular malaria vaccine does not offer any protection against the other forms of malaria caused by different species of Plasmodium. Overall, though, most mosquito-borne diseases can be prevented only by personal insect protection measures and the eradication or biocontrol of infected mosquitoes. See also: Public health; Vaccination