Pest Library: Flies

The housefly, Musca domestica, is a fly of the suborder Cyclorrhapha. It is believed to have evolved in the Cenozoic era, possibly in the Middle East, and has spread all over the world as a commensal of humans. It is the most common fly species found in houses. Adults are grey to black with four dark longitudinal lines on the thorax, slightly hairy bodies and a single pair of membranous wings. They have red eyes, set further apart in the slightly larger female.

The female housefly usually mates only once and stores the sperm for later use. She lays batches of about 100 eggs on decaying organic matter such as food waste, carrion or faeces. These soon hatch into legless white larvae, maggots. After 2 to 5 days of development, these metamorphose into reddish-brown pupae, about 8 mm (0.3 in) long. Adult flies normally live for 2 to 4 weeks, but can hibernate during the winter. The adults feed on a variety of liquid or semi-liquid substances, as well as solid materials which have been softened by their saliva. They carry pathogens on their bodies and in their faeces, and can contaminate food and contribute to the transfer of food-borne illnesses. For these reasons they are considered pests.

Male houseflies are sexually mature after 16 hours and females after 24. Females produce a pheromone, (Z)-9-Tricosene (muscalure), which attracts males; it has found use as a pesticide, luring males to fly traps. The male initiates the mating by bumping into the female, in the air or on the ground, known as a “strike”. He climbs on to her thorax, and if she is receptive a courtship period follows, in which the female vibrates her wings and the male strokes her head. The male then reverses onto her abdomen and the female pushes her ovipositor into his genital opening; copulation, with sperm transfer, lasts for several minutes. Females normally mate only once and then reject further advances from males, while males mate multiple times. A volatile oviposition pheromone to assist in egg clustering is deposited along with the eggs.

The larvae depend on warmth and sufficient moisture to develop; generally, the warmer the temperature, the faster they will grow. In general, fresh swine and chicken manure present the best conditions for the developing larvae, reducing the larval period and increasing the size of the pupae. Cow, goat and horse manure produce fewer, smaller pupae, while fully composted swine manure, with a water content of under 40%, produces none at all. Pupae can range from about 8 to 20 milligrams (0.0003 to 0.0007 ounces) under different conditions.

The life cycle can be completed in seven to ten days under optimal conditions but may take up to two months in adverse circumstances. In temperate regions, there may be twelve generations per year, and in the tropics and subtropics, more than twenty. Houseflies feed on liquid or semiliquid substances beside solid material which has been softened by saliva. They deposit faeces frequently and regurgitate semi-digested food. Although they are domestic flies, mainly confined to human habitations and farm buildings, they can fly for several miles from the breeding place.

Houseflies carry a wide variety of organisms on their hairs, mouthparts, vomitus and faeces. Parasites carried include cysts of protozoa e.g. Entamoeba histolytica, Giardia lamblia and eggs of helminths, e.g., Ascaris lumbricoides, Trichuris trichiura, Hymenolepis nana, Enterobius vermicularis. Houseflies are capable of carrying over 100 pathogens, such as those causing typhoid, cholera, salmonellosis, bacillary dysentery, tuberculosis, anthrax, ophthalmia and pyogenic cocci. Disease-carrying organisms on the outside surface of the fly may survive for a few hours, but those in the crop or gut may still be viable several days later. There are usually too few bacteria on the external surface of the flies (except perhaps for Shigella) to cause infection and the main routes to human infection are through the fly’s vomit and particularly its faeces.