The housefly (Musca domestica) is the most common fly occurring in homes and indeed one of the most widely distributed animals and the most familiar of all flies; it is a pest that can facilitate serious diseases.
The adults are 5-8 mm long. Their thorax is grayish, with four dark longitudinal lines on the back. The underside of the abdomen is yellowish. The whole body is covered with hair. They have reddish compound eyes. The females are slightly larger than the males and have a much larger space between the eyes.
Like most Diptera, houseflies have only one pair of wings; the hind pair is reduced to small halters that aid in flight stability. Characteristically, the fourth long vein of the wing shows a sharp upward bend.
Species that appear similar to the housefly include:
- the lesser house fly (Fannia canicularis), somewhat smaller and more slender than M. domestica, fourth long vein of the wing is straight.
- the stable fly (Stomoxys calcitrans) looks similar to M. domestica but has a longer piercing mouthpart, used to penetrate the skin of humans and animals in order to suck blood.
Each female fly can lay up to 500 eggs (in five batches of 100 eggs each). Within a day, the larvae (maggots) hatch from the eggs; they live and feed in (usually dead and decaying) organic material, such as garbage or feces. They are pale whitish and have no legs. After several molts, the maggots crawl to a dry cool place and transform into pupae, colored reddish or brown. The adult flies then emerge from the pupae. (This whole cycle is known as complete metamorphosis.) The adults live from half a month to a month. After having emerged from the pupae, the flies cease to grow. Small flies are not young flies but the result of little food during the maggot stage.
Some 36 hours after having emerged from the pupa, the female is receptive for mating. The male mounts her from the back to inject sperm. Normally the female mates only once, storing the sperm to use it repeatedly for several sets of eggs. Males are territorial: they defend a certain territory against other males and try to mount any female that enters.
The flies depend on warm temperatures; generally, the warmer the temperature the faster the flies will develop. In the winter, most of them survive in the larval or pupa stage in some protected warm location.
Some species of wasps can parasitize and kill the pupae.
Houseflies can only take in liquid foods. They spit out saliva on solid foods to pre-digest it, and then suck it back in. They also throw up partially digested matter and eat it again.
The flies can walk on vertical planes, and can even hang upside down from ceilings. This is accomplished with the surface tension of liquids secreted by glands near their feet.
Lacking eyelids, the flies continually clean their eyes with their forelegs. Most of their taste and smell sensor cells are on hairs on their legs, and that is why they also keep rubbing their legs together.
Flies have a very highly-evolved evasion reaction which helps to ensure their survival. It is possible to confuse a fly’s evasion system by swatting it with two objects simultaneously from different directions.
Flies and humans
Houseflies originated in Africa. In colder climates, houseflies only occur together with humans. They have a tendency to aggregate and are difficult to dispel. They are capable of carrying over 100 pathogens, such as typhoid, cholera, Salmonella, bacillary dysentery, tuberculosis, anthrax ophthalmia, and parasitic worms. Some strands have become immune to common insecticides.
The housefly is an object of biological research, mainly because of one remarkable quality: the sex determination mechanism. Although a wide variety of sex determination mechanisms exists in nature (e.g. male and female heterogamy, haplodiploidy, environmental factors) the way sex is determined is usually fixed within one species. However, the housefly exhibits many different mechanisms for sex determination, such as male heterogamy (like most insects and mammals), female heterogamy (like birds) and maternal control over offspring sex. This makes the housefly one of the most suitable species to study the evolution of sex determination.