These are the gnats and mosquitoes there are about 3000 species world wide, and 34 British species, many of medical and veterinary importance, e.g. some species transmit the pathogens which cause malaria (see Anopheles maculipennis below), yellow fever (see Aedes aegypti below right), filariasis and dengue. It is the females who suck blood to get the protein needed to produce eggs. Dengue fever was eradicated from the USA in the 1950s by DDT spraying; it has now made a comeback, and is being transmitted today in Houston, and perhaps other places. Dengue in its acute form is thought to affect 110,000 - 200,000 people annually in the USA.
Before modern medicine, mosquitoes were used in the treatment of syphilis. The malarial fever passed on by the mosquito to the human could raise the human body temperature above 40oC, and this halted the progress of syphilis. Having malaria was very much the lesser of two evils.
Most of the British adults are less than 6 mm long, have long legs and clear wings.
The adults have mouthparts adapted to both pierce and suck. The antennae have 13 segments and are plumed in the male. The way to tell a male (non-biting) from a female (biting) is to look at the antennae. The male has very bushy, plumose antennae, whilst the female has simple antennae.
The antennae of the male is precisely tuned to pick up the exact frequency of the female's whining flight. This enables him to locate a suitable mate.The larvae and pupae are active swimmers. Above right is the larva of the common gnat (Culex pipiens). The eggs are laid on stagnant water surface (in the U. K. this is usually from April onwards) and the larva are aquatic. The larva feed on protozoans and other small organisms. They breathe through a tiny snorkel-like tube, so an oil film spread on water will cause them to die as the tube cannot break through the film.
After pupating the males feed on nectar and plant juices. Generally the males hatch a few minutes before the females, and dance in the air waiting for females to emerge. In most species the females need a blood meal in order to lay eggs.
Individuals rarely fly more than a few hundred metres from their hatching site, however mass invasions of towns can occur from as far as 100 kilometres.
The common gnat usually gets her blood meal from birds, not man. Females of the common gnat hibernate as adults though the winter. They will have already mated and the males will all die once the weather gets cold.
When the female punctures skin she injects an anaesthetic which deadens the pain, and an anticoagulant to stop the blood clotting and to thin it making flow up her tubular tongue. She can drink up to three or four times her body weight in blood. In the U. K. the Culex pipiens does not transmit disease, but in some countries it transmits West Nile Virus.
Adult males live for just over a week, but females can live as long as a month, and if it gets cold before she has laid her eggs she can hibernate. In temperate climates there can be 15 generations in the spring, summer, autumn period.
Above left is the pupa of a Culex sp. mosquito. It has breathing tubes, and at rest it lies suspended in the water with its breathing tubes just pushed slightly above the surface.
On the right is an adult Anopheles maculipennis, a malarial mosquito. The abdomen has 8 segments.
The female lays her eggs on, or very near water in batches of 40 - 100. The larvae hatch in 2 or 3 days and feed on particles (mainly algae) in the water. When conditions are favourable the transition from larva to pupa can take as little as a week, during which the larva will moult 4 times. The pupal stage is more active and lasts for 2 or 3 days. When the adult is about to emerge the pupa straightens, sucks in air and floats to the surface. The skin splits longitudinally along the thorax, and the adult emerges.