Houseflies are insects included in the order Diptera (Gk. di – two, pteron – wing). Most other orders have two pairs of wing (i.e. four in all) but houseflies, bluebottles, hover flies, mosquitoes, tetse flies, crane flies (‘daddy long legs’) etc. have only the single pair. Houseflies are of interest principally because they are associated with the transmission of certain diseases.
Apart from the single pair of wings, houseflies share the typical insect features; their bodies are divided into segments which are grouped into a head, thorax and abdomen. The thorax carries three pairs of jointed legs as well as the wings. There are no appendages on the abdomen.
Houseflies lay their eggs in rotting organic matter, with a preference for stable manure. The eggs hatch into white, legless larvae, (maggots), which feed on fluids and small particles in the organic waste. As they grow, they shed their skins several times but retain the last one as a hard pupal case in which they undergo the drastic changes (metamorphosis) from a maggot to a fully formed housefly. The immature fly pushes its way out of the pupal case and allows time for its wings to expand and harden before it flies off.
The housefly poses a health hazard because of its indiscriminate choice of food ranging from human and animal faeces to food awaiting human consumption. Houseflies cannot take in solid food. They have a proboscis which can be extended on to the potential food. At the end of the proboscis are two flat lobes with a series of parallel channels. The fly places the lobes on the food and secretes saliva through the channels. The saliva contains enzymes which digest the food to a liquid which is then pumped back through the proboscis to the gut.
Any bacteria in the food accumulate in the proboscis channels and are flushed out with the saliva onto the next substance on which the fly alights. If the first food sample happens to be human faeces and the next sample a slice of bread, any bacteria from the faeces will be transferred to the bread and eaten. If the bacteria are those which cause intestinal disease, there is the risk that they will cause that disease in the person who eats the bread. Cholera, typhoid and salmonella food poisoning are diseases that can be spread by houseflies. In addition to the feeding mechanism, the bacteria can also be transferred on the housefly’s legs.
There are several ways of reducing the spread of disease by houseflies. The most obvious is to store all food for human consumption in situations which houseflies cannot reach. It is also vital to dispose of human waste in such a way that houseflies cannot pick up bacteria from it. This can be achieved by modern methods of sanitation and sewage treatment.
Flies have a range of about one mile so it is best if their potential breeding sites, e.g. manure heaps, are not allowed to accumulate near to urban populations.