True flies have only one pair of membranous wings (a few parasitic species are wingless)
The hind wings have been reduced to form club-shaped halteres or gyroscopic balancers. These are most easily seen in the crane flies or Tipulidae (see below). Without halteres flies just fall to the ground.
Flies are Holometabolous, i.e. have 4 distinct life stages, egg, larva, pupa, and adult.
Over 152,000 species have been described world wide, but many more are awaiting description.
Over 19,000 species are found in Europe.
over 7, 000 species are found in the UK.
They are of huge medical and veterinary importance both for good and bad. Millions of humans die each year from malaria, yellow fever, sleeping sickness and other diseases caused by parasites carried into the human by flies when they suck blood. And were it not for the scavenging activity of flies we would be knee deep in dung and other decaying matter.
All are liquid feeders, but there is great variation in their mouthparts (see below).
Their antennae vary greatly in shape and size.
Most lay small, cylindrical eggs, which hatch to produce legless larvae.
They have chemoreceptors for taste and smell on their legs as well as on their mouthparts.
On the left is a fly belonging to the Tipulidae family, these are commonly known as daddy-long-legs or crane flies. There are around 15 000 species world wide, and 87 species in the British Isles.
The adults are fragile and slender ranging from 0.6 - 6 cm in body length. All adults have the same thin body shape, with narrow wings and long legs. The legs are shed easily if they are caught, but this does not seem to hamper the fly much as they cannot walk or run. You can tell the sex of an adult by looking at the tip of the abdomen - females have a pointed tip, and males have a blunt tip. You will often see a female hopping around over lawns, wet ground and even water. She is laying eggs.
They have the most inelegant flight of all insects, with legs left dangling and flapping in every direction. Consequently the legs get trapped in spider's webs and vegetation. In order for the fly to escape it simply snaps off the trapped leg and carries on. The loss of 1, 2 or even 3 legs does not seem to inconvenience the daddy long legs who is quite content to manage as long as it has 3 legs.
Right is the ovipositor. The eggs are small, oval and usually black. Each female can lay several hundred in grassland, moist soil, bog and even over water. On land she pushes her ovipositor into the soil, but over water she places the eggs on, or just below the surface. This is often at the side of a pond among the aquatic plants.
The adults are short-lived and feed on nectar and other fluids. In some of the smaller species the adult males gather together in small swarms to dance in the late afternoon, these are sometimes called bobbing gnats.
Some adult females have no wings or only small vestigial wings. They can be found in the autumn waiting on walls for males to find them. Many species are nocturnal and are attracted to lights.
Tipulid larva (above) are commonly known as leatherjackets and live in the soil, rotting wood, bogs, other moist habitats, and some are aquatic. Fully grown they can reach up to 40 mm long.
They are a very important food source for some birds, and can occur in great numbers. They can cause damage to lawns, and can be a pest where potatoes and oats are grown. They are usually grey or greyish-brown in colour.
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.
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 anesthetic 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.
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.