House fly adult is about 1/8-1/4 in (4-7.5 mm) long, female larger than male. Mature larva is about 1/4-3/8 in (7-10mm).
Adult face has 2 velvety strips, silver above and gold below; thorax has 4 narrow stripes; no pale spot behind head or rear tip of thorax; sides of abdomen usually pale; sponging mouth parts. Larva is eyeless, legless, tapered from rear to head, which is a pair of dark hooks.
Adult is dull gray. Larva is cream colored and greasy looking.
Comparison with other species
Face fly males have eyes that almost touch. Flesh flies have only 3 dark stripes on thorax, and tip of abdomen usually red/pink, sides of abdomen not pale. Stable flies have piercing mouthparts and thorax with pale spot behind head. False stable flies have pale rear tip of thorax. Cluster fly has golden hairs on thorax, which lacks 4 dark stripes.
Females lay eggs on almost any warm, moist material with adequate food supply for egg-laying and larval development. Most house flies stay within 1-2 miles of their larval habitat if food is available, though some have migrated up to 20 miles. During the day, they rest less than 5 ft. from the ground; at night, mainly above 5 ft., but near their food sources.
House flies are general feeders on liquids; attracted to many substances, from excrement to human foods. House flies can liquefy solid foods by regurgitation.
Adult female lays oval, white eggs singly in clusters of 20-50, laying 350-900 eggs in her lifetime. Eggs hatch in 8-20 hours. Larvae go through 3 instars in 3-7 days at 70-90 degrees F. Full grown larvae go to a cool, dry place to pupate, traveling up to 150 feet in 3-4 days. Pupa goes from yellowish to black during a period of a few days to a month, depending on temperature and humidity. After emerging, the body hardens and the wings dry within about an hour. Development from egg to adult can be as few as 6 days, with 10-12 generations per summer. Adults live 15-25 days.
House flies excrete and regurgitate whenever stopping to rest. They have many hairs and bristles, as well as sticky pads at the base of the leg claws. Therefore, they tend to transmit disease organisms, especially those associated with filth.