Mosquitoes

Although mosquitoes are found all over the world, as far north as the Arctic Circle, the vast majority of them live in the tropics and subtropics. The adults have wings and are able to fly, but the immature stages, or larvae, are wormlike and are confined to small bodies of water. Adult male mosquitoes do not have mouthparts that are capable of penetrating skin, so they feed primarily on nectar from flowers. However, adult females have piercing mouthparts that easily pass through the skin of larger animals so they can feed on their blood. About 200 million years ago they probably evolved from other insects that used their mouthparts to feed on plants.

The Life Cycle of Mosquitoes

Female mosquitoes lay their eggs in water or in areas that will be flooded with water after rainfall accumulates or the snow melts. The eggs hatch after a few hours of being submerged, and this is the reason for the usually large numbers of mosquitoes in the wet spring months. The larvae that hatch from the egg must remain in the water to develop, but they are only able to breathe when they come to the water’s surface. At the end of the larval abdomen is a strawlike siphon tube that allows air to enter. By hanging with its head down and its siphon tube at the surface, the larva can continue to feed underwater and breathe at the same time. The larvae feed by filtering out small particles of food from the water and grow by periodically producing a new skin and molting the old skin. After three larval molts over the course of about one week, the larva changes into a comma-shaped pupa that does not feed but is transformed into the adult. The pupa comes to the water surface when its development is complete and the adult mosquito escapes from the pupal skin and flies from the water surface.

Adult mosquitoes often mate shortly after they emerge. The males of many mosquito species form swarms over trees and bushes into which the female enters. When she enters the swarm of males, her wingbeats produce a tone that attracts the male. Most females only mate once during their lives and store the sperm in a special sac that is used to fertilize the eggs just before they are laid. Males live only about two weeks and feed on sugars from plants. Female mosquitoes live for about a month. Unlike the males, they have needlelike mouthparts consisting of two elongated maxillae, two long mandibles, a labium, and a hypopharynx that has a hollow tube running its length. When the mosquito feeds, the sheathlike labium that encloses the other mouthparts folds away and the six needlelike mouthparts then penetrate the skin. The tube within the hypopharynx is connected to the salivary glands and allows saliva to be injected into the animal being fed upon. The saliva prevents the blood from clotting in the mouthparts and may also lessen the pain so the mosquito can feed without being detected. The saliva that is injected also produces the common symptoms of itching and redness that usually follow the mosquito bite. Feeding on blood is necessary if the female is to develop her eggs. By using the protein contained in the blood, she is able to mature about one hundred eggs within two days of feeding. She can take another blood meal and develop another egg batch every two days. Feeding behavior usually occurs at dusk or during the evening hours, when its hosts are asleep and less likely to defend themselves; however, some mosquito species are most active during the daytime. A few mosquito species do not feed on blood at all and acquire the protein they need to develop eggs from the larval stage.

Adult female mosquitoes have special sensory receptors on their antennae that can detect the odors of potential hosts. The carbon dioxide that animals give off as they breathe is one of the major signals the female uses to locate a host for a blood meal, but other chemicals produced by the skin are also important. The female mosquito may also use body heat and the visual image of the host to locate it. Darker colors are generally more attractive than are lighter colors. Common mosquito repellents are believed to act as chemical masks that disrupt the pattern of host stimuli so the female no longer recognizes the host.

Mosquitoes and Disease

Mosquitoes are not just nuisances when they feed on humans. Female mosquitoes are able to acquire parasites, such as viruses and protozoa, when they feed on an animal that already has an infection in its blood. Some of these parasites are able to infect the tissues of the mosquito and then invade its salivary glands . Once the salivary glands contain the parasites, parasitic diseases can be transmitted to an uninfected host the next time the mosquito injects its saliva and feeds. Many diseases of humans and other animals are transmitted by mosquitoes when they feed on blood. Malaria is the most important of these, causing nearly a half million human deaths each year. Other important mosquito-transmitted diseases include dengue, yellow fever, viral encephalitis,West Nile virus, chikungunya, and Zika virus disease, all caused by viruses. Heartworm is an important disease of dogs that is caused by a nematode worm transmitted by mosquitoes. Filariasis is a similar disease in humans that produces grossly enlarged limbs.

In 2018, the Centers for Disease Control and Prevention (CDC) announced that in the period from 2004 through 2016, cases of mosquito-, tick-, and flea-borne diseases in the United States had tripled. Tick-borne illnesses accounted for approximately 75 percent of cases, while mosquito-borne illnesses made up most of the remaining 25 percent. Unlike tick-borne illnesses, which increased steadily, mosquito-borne illnesses fluctuated in frequency as epidemics of diseases such as West Nile virus, dengue fever, chikungunya, and Zika broke out and subsided. Many of these outbreaks occurred in Puerto Rico.

The best way to control mosquitoes is to limit the places larvae can breed. This involves preventing water from collecting in small containers and channeling water runoff so it does not accumulate in puddles that might last long enough for the larvae to develop into adults. Other ways to reduce the risk of mosquito bites include placing a mosquito net over the bed while sleeping, placing screens on windows and doors, wearing loose and long-sleeved shirts and pants, and wearing insect repellent when outdoors.

Principal Terms

labium: the sheath that contains the slender stylet-like mouthparts of the mosquito, including the mandibles, maxillae, and hypopharynx

saliva: the liquid containing enzymes secreted by the salivary glands that is injected into the host when the adult mosquito feeds

siphon tube: tube that extends from the rear of the larval abdomen at the air-water interface and allows the larva to breathe air

Bibliography

"Avoid Mosquito Bites." Centers for Disease Control and Prevention. US Dept. of Health and Human Services, 24 Aug. 2016. Web. 3 Oct. 2016.

Becker, Norbert. Mosquitoes and Their Control. 2nd ed. Berlin: Springer, 2010. Print.

Busvine, James R. Disease Transmission by Insects. New York: Springer-Verlag, 1993. Print.

Eldridge, Bruce F., and John D. Edman, eds. Medical Entomology. Boston: Kluwer, 2000. Print.

Harrison, Gordon. Mosquitoes, Malaria and Man: A History of the Hostilities since 1880. New York: E. P. Dutton, 1978. Print.

Klowden, M. J. “Blood, Sex, and the Mosquito: The Mechanisms That Control Mosquito Blood-Feeding Behavior.” BioScience 45 (1995): 326-331. Print.

Rosenberg, Ronald, et al. "Vital Signs: Trends in Reported Vectorborne Disease Cases—United States and Territories, 2004–2016." Morbidity and Mortality Weekly Report, vol. 6, no. 17, 2018, pp. 496–501. Centers for Disease Control and Prevention, www.cdc.gov/mmwr/volumes/67/wr/mm6717e1.htm?s‗cid=mm6717e1. Accessed 11 June 2018.