Suborder Nematocera (Flies - Order: Diptera)

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A photograph of St Mark's Fly (_Bibio marci_).

St Mark's Fly (Bibio marci) - named because it is said to appear each year around St Mark's Day in the Christian calendar (March 21st)

These are the more 'primitive' crane flies and gnats. They have long segmented antennae, and their larvae have a well developed head and mouthparts.

Members of the Tipulidae - crane flies, or daddy long legs - number around 300 British species whose wingspans range from large (the wings of the crane fly Tipula maxima are 65mm across, larger than any other British fly) to very small (gnat-like flies have tiny wingspans of 1.5mm).

The crane flies have long, stilt-like legs. Their larvae live in soil and are called leather jackets - e.g. Tipula paludosa and T. oleracea. Some feed on decaying wood or leaf litter, or even at the bottom of ponds and streams. The larger ones are herbivorous, but some are carnivorous. The adults are harmless.

The Psychodidae are known as owl midges or moth flies, and there are about 80 British species of them - all are tiny, with hairy wings. This family includes the sand flies, which feed on the blood of vertebrates and in warmer climes are responsible for spreading the disease leishmaniasis in man.

The mosquitoes and gnats belong to the family Culicidae. They are almost all blood suckers. In temperate regions (such as the British isles, where there are 33 species) they are mostly just an annoyance - their hum can sometimes be heard in a dark room as people are about to go to sleep! In warmer climes, however, they can be responsible for the spread of diseases such as malaria, yellow fever and elephantiasis.

Only the females suck blood, and their mouthparts are needle like and suited to the purpose. The males feed on nectar. There are two main sub-families of mosquitoes - the anophelines, which rest with their bodies at an inclined angle, and the cilicines, to which most of the British species belong, which rest with their abdomens parallel to the surface they are resting on.

Mosquitoes lay their eggs on the surface of water, sometimes glued together to form tiny rafts. Leaving containers of water out in summer often attracts them to lay eggs, and their larvae can be seen wriggling through the water, hanging at an angle (Culicinae) or parallel (anophelines) to the surface of the water as they take in air through their spiracles. They feed on protozoans and other tiny creatures in the water, which they waft into their mouths with tiny hairs.

There are 400 British species of Chironomidae, or non-biting midges, and these look similar to the biting midges, Ceratopogonidae, which are found close to water.

The blackflies (Simulidae) live near running water where their larvae live, attaching themselves to stones by means of hooks or silken webs. These can be a problem in some parts of the world as the females are blood suckers and can transmit serious diseases. Immense populations build up in the Arctic in the summer.

A related family, the Bibionidae, includes the March Fly, or St Mark's Fly (Bibio marci) - named because it is said to appear around St Mark's Day in the Christian calendar (March 21st). This is a common and harmless species often found is numbers in the UK in the spring. Bibionidae larvae feed on decaying matter on land and the adults are probably important plant pollinators.

Another family worth mentioning from among the Nematocera is the Mycetophilidae, or fungus gnats. These small, delicate flies have long slender antennae and long legs and resemble mosquitoes. Their larvae mostly feed on fungi, though some eat tiny insects and worms.

Other suborders of flies:

A to Z of insects

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