Culex

Culex, (genus Culex), genus of about 770 species of mosquitoes, some of which are serious biting nuisances and vectors of disease that can transmit potentially deadly pathogens (disease-causing organisms) to humans and other animals. Culex is the most widespread genus of mosquitoes in the world, and species are found on every continent except Antarctica. They are native to Africa, Asia, and Europe and have been introduced to the Americas and Australia. They commonly bite at dawn, dusk, and overnight. Certain species have been known to carry diseases, including West Nile virus, St. Louis encephalitis, and avian malaria.

Physical description

Culex mosquitoes tend to be brown and medium-sized (4–10 mm [0.16–0.4 inch] long), with slender bodies. Like other mosquitoes, they are characterized by long fragile-looking legs and elongated piercing mouthparts. The wings, with scales on the veins and the margin, are uniform in color. A bloodsucking female typically holds its body parallel to the resting surface, and its proboscis is bent downward relative to the surface. The tip of the female’s abdomen is blunt and has retracted cerci (sensory appendages).

A Culex mosquito is distinguished from other kinds by the presence of two conspicuously large hooks at the tip of each leg and pulvilli (soft adhesive pads between the hooks). In addition, it lacks bristles near the spiracles (breathing orifices) on the sides of the thorax.

Natural history

Like other mosquitoes, Culex species go through four stages in their life cycle: egg, larva, pupa, and adult. Female Culex mosquitoes lay their buoyant eggs on a freshwater surface, often about 200 eggs at a time. The eggs, grouped in masses, usually take 1–10 days to hatch, depending on the ambient temperature. The eggs and larvae may be found on almost any body of fresh water, including ponds, rivers, and puddles, as well as in artificial bodies of water, such as at construction sites and in ditches and water barrels. They tolerate even polluted water. Adults rarely move more than 500 meters (0.3 mile) from their breeding sites.

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mosquito: Culex mosquitoes

The long and slender Culex larva has a breathing tube near the tip of the abdomen that bears distinctive sensory hairs or hair tufts, which are useful for species identification. The breathing tube rests at the surface of the water, and the larva hangs head downward at an angle of 45° from the water surface and feeds on microorganisms such as bacteria and algae. Each growing larva molts four times before emerging as a pupa. The pupal stage lasts up to 48 hours, during which the insect does not feed and floats near the surface of the water. At the end of this stage, the pupa molts again and emerges as an adult.

(Read Britannica’s essay “What Purpose Do Mosquitoes Serve in Ecosystems?”)

As adults, the males, and sometimes the females, feed on nectar and other plant juices and are known pollinators. In most species, however, the females require the proteins obtained from a blood meal in order to mature their eggs, and hence they use their sucking proboscis to pierce the skin of their host and withdraw blood. Different species show different host preferences and, in many cases, have narrow restrictions as to host animals, though some are generalists, able to use the blood from a number of animals. C. erraticus, for example, feeds on humans, large mammals, birds, reptiles, and amphibians. The majority of Culex mosquitoes rarely bite humans.

Two species of major importance to humans are the southern house mosquito (Culex quinquefasciatus), which is found mostly at low to moderate elevations in tropical and subtropical regions, and the abundant northern, or common, house mosquito (C. pipiens), which tends to live in more-temperate areas in the Northern Hemisphere.

Diseases and outbreaks

Culex mosquitoes are the primary vectors of several varieties of encephalitis, especially Japanese encephalitis, both Eastern and Western equine encephalitis, and St. Louis encephalitis. The insects contract the causative viruses by feeding on infected birds (especially sparrows, pigeons, blue jays, and robins). Then they pass the viruses along to humans. Japanese encephalitis was first documented in Japan, in 1871, and St. Louis encephalitis first broke out in St. Louis, Missouri, in 1933, spreading to more than 1,000 people.

Culex mosquitoes are also the dominant vectors of West Nile virus, a serious viral disease of birds that can be transmitted to humans. The first outbreak of the virus occurred in Uganda in 1937. Historically, West Nile virus was confined largely to Africa, the Middle East, and parts of Russia, India, and Indonesia, where it caused occasional, usually minor, epidemics of denguelike illness or sporadic encephalitis. However, the virus eventually was imported more broadly into Europe by migratory birds, and in 1999 it reached the United States and emerged in New York City, where it was detected in both birds and people. In the United States, major Culex vectors of West Nile virus are C. pipiens, C. tarsalis, C. salinarius, and C. restuans (which feeds primarily on birds).

The insects do not transmit human malaria, but they do spread avian malaria in birds. In the 1820s Culex mosquitoes were responsible for an avian malaria outbreak in Hawaii that caused a massive decline in native bird populations.