Appearance: The adult flies are 7-8mm in length and greyish in colour, with four narrow black stripes on the thorax. Males have a pair of yellowish patches at the base of the abdomen.

Life-cycle: The eggs are laid in almost any vegetable or animal matter provided that it is not too dry and can be readily swallowed and digested by the larvae. Stacks of fermenting horse-dung are frequently used for breeding but breeding will take place freely in human faeces, pig dung and household refuse in unemptied dustbins. When egg-laying takes place in dung the latter must not be more than 72 hours old. Adult flies live from 4 to 12 weeks. In the early summer there is a rapid re-colonization of all available breeding sites.

Egg: The eggs are about 1 mm long, glistening white, and are laid in batches of 120 to 150 by the female fly, which may eventually produce a total of 600 to 900 eggs over a period of from 4 to 12 days. The time taken for the eggs to hatch varies according to temperature from about 8 hours to 3 days.

Larva: The larva is white, legless and conical in shape, tapering at the head end. In warm weather it may be fully developed in 3 days, reaching 10 to 12 mm in length, but under less satisfactory conditions it may take 8 weeks to complete its development. When fully fed, the maggot, as the larva is known, appears waxy and ivory –yellow in colour, and it abandons its larval environment in search of a cooler and drier place in which to pupate. The larva may travel considerable distances before it finds a suitable spot and will readily crawl up a smooth vertical surface if it is moist.

Pupa: Pupation usually takes place in the soil where the larva may bury itself to a depth of 7-60cm, depending on the nature of the soil. The pupa is formed within the last larval skin and is barrel-shaped with rounded ends. It is at first pale yellow in colour but then darkens to reddish-brown and finally to dark brown or black. The pupal period varies from 3 to 28 days according to temperature. The total time taken to complete the life-cycle varies considerably with the temperature, humidity and the nature and abundance of the food supply.

Economic importance: In the past, it would have been difficult to exaggerate the importance of the Common House Fly as a pest in the home. This is on account of its being a carrier of disease, brought about by its habit of flying between human faeces and human food. The disease organisms of typhoid, dysentery, summer diarrhoea, probably infantile paralysis and other diseases are transferred from faecal matter to food by vomit drops, in fly excrement or by organisms adhering to the fly’s feet. The eggs of parasitic worms are also transferred in this latter way. In tropical and sub-tropical areas, in addition to these diseases, the House Fly is responsible for the spread of cholera, yaws and opthalmia. The adult fly feeds on both solid and liquid matter which can be lapped up by the sponge-like proboscis; the familiar ‘fly-spots’ are drops of liquid regurgitated and deposited on the surface by the fly.