The origin of the housefly is unknown, but its biology and worldwide distribution suggest it began in a subtropical or tropical part of the world such as East Africa. M. domestica, troublesome because it is dependent on human settlements and domestic animals, frequently enters houses.

M. domestica, as an adult, is 6-8 mm long with a 13-15 mm wingspan. It has a gray thorax with four longitudinal dark stripes. The front half of the abdomen is buff-colored and occasionally transparent at the sides with a central dark band broadening to cover the last abdominal segments.

The housefly passes through complete metamorphosis as egg, larva, pupa and adult. Development is temperature dependent, but under most conditions the life cycle is complete in 10-20 days. In the third of three larval stages, the mature larvae, or maggots, crawl away from their breeding place and burrow into loose ground nearby where they transform to the next stage, the pupa. When pupal development is complete and the adult fly is ready to emerge, it pushes the end of the pupa open with an extrudable saclike structure on the head. The adult continues to alternate expansion and contraction of this structure to clear a path through the soil to the surface.

M. domestica is extremely prolific and has tremendous potential for population increases. For example, in 1911 C.F. Hodge made the following estimation. Assume that one adult female deposits 120 to 150 eggs per "lot," with at least six lots at intervals of three or four days. A pair of flies beginning "operations" in April may be progenitors, if all were to live, of 1.91 x 10 flies in August.

That's almost 200 million trillion flies, or 191,000,000,000,000,000,000 flies.

That many flies would cover the earth with a layer at least 2.5 feet deep. While Hodge's estimation is unlikely to happen, it demonstrates how rapidly a small population of houseflies can increase in size. It also illustrates the importance of early season fly control measures.

The housefly is well adapted by structure and behavior to transmit disease organisms. Its body is covered with fine hairs and bristles that readily pick up filth particles. At the base of each leg is a cushion-like structure covered with glandular hairs. The sticky secretions from the glandular hairs gather bacteria and other organisms.

The housefly excretes and regurgitates whenever it comes to rest. Regurgitation is the process of digestion during which food is brought up bit by bit from the fly's crop and mixed with saliva before being passed on to the digestive tract. Because the maximum flight range of houseflies exceeds five miles, they have ample geographic opportunity to spread disease pathogens prior to landing on, and contaminating human food.