Housefly

The housefly (Musca domestica) is a fly of the suborder Cyclorrhapha. It possibly originated in the Middle East, and spread around the world as a commensal of humans. Adults are gray to black, with four dark, longitudinal lines on the thorax, slightly hairy bodies, and a single pair of membranous wings. They have red compound eyes, set farther apart in the slightly larger female.

The female housefly usually mates only once and stores the sperm for later use. It lays batches of about 100 eggs on decaying organic matter such as food waste, carrion, or feces. These soon hatch into legless white larvae, known as maggots. After two to five days of development, these metamorphose into reddish-brown pupae, about 8 millimetres (3⁄8 inch) long. Adult flies normally live for two to four 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 can carry pathogens on their bodies and in their feces, contaminate food, and contribute to the transfer of food-borne illnesses, while, in numbers, they can be physically annoying. For these reasons, they are considered pests.

Houseflies, with short life cycles and ease with which they can be maintained, have been found useful for laboratory research into aging and sex determination. Houseflies appear in literature from Ancient Greek myth and Aesop's "The Impertinent Insect" onwards. Authors sometimes choose the housefly to speak of the brevity of life, as in William Blake's 1794 poem "The Fly", which deals with mortality subject to uncontrollable circumstances.

Description

Adult houseflies are usually 6 to 7 mm (1⁄4 to 9⁄32 in) long with a wingspan of 13 to 15 mm (1⁄2 to 19⁄32 in). The females tend to be larger winged than males, while males have relatively longer legs. Females tend to vary more in size and there is geographic variation with larger individuals in higher latitudes. The head is strongly convex in front and flat and slightly conical behind. The pair of large compound eyes almost touch in the male, but are more widely separated in the female. They have three simple eyes (ocelli) and a pair of short antennae. Houseflies process visual information around seven times more quickly than humans, enabling them to identify and avoid attempts to catch or swat them, since they effectively see the human's movements in slow motion with their higher flicker fusion rate.

The mouthparts are specially adapted for a liquid diet; the mandibles and maxillae are reduced and not functional, and the other mouthparts form a retractable, flexible proboscis with an enlarged, fleshy tip, the labellum. This is a sponge-like structure that is characterized by many grooves, called pseudotracheae, which suck up fluids by capillary action. It is also used to distribute saliva to soften solid foods or collect loose particles. Houseflies have chemoreceptors, organs of taste, on the tarsi of their legs, so they can identify foods such as sugars by walking over them. Houseflies are often seen cleaning their legs by rubbing them together, enabling the chemoreceptors to taste afresh whatever they walk on next. At the end of each leg is a pair of claws, and below them are two adhesive pads, pulvilli, enabling the housefly to walk up smooth walls and ceilings using Van der Waals forces. The claws help the housefly to unstick the foot for the next step. Houseflies walk with a common gait on horizontal and vertical surfaces with three legs in contact with the surface and three in movement. On inverted surfaces, they alter the gait to keep four feet stuck to the surface. Houseflies land on a ceiling by flying straight towards it; just before landing, they make a half roll and point all six legs at the surface, absorbing the shock with the front legs and sticking a moment later with the other four.

The thorax is a shade of gray, sometimes even black, with four dark, longitudinal bands of even width on the dorsal surface. The whole body is covered with short hairs. Like other Diptera, houseflies have only one pair of wings; what would be the hind pair is reduced to small halteres that aid in flight stability. The wings are translucent with a yellowish tinge at their base. Characteristically, the medial vein (M1+2 or fourth long vein) shows a sharp upward bend. Each wing has a lobe at the back, the calypter, covering the haltere. The abdomen is gray or yellowish with a dark stripe and irregular dark markings at the side. It has 10 segments which bear spiracles for respiration. In males, the ninth segment bears a pair of claspers for copulation, and the 10th bears anal cerci in both sexes.

A variety of species around the world appear similar to the housefly, such as the lesser house fly, Fannia canicularis; the stable fly, Stomoxys calcitrans; and other members of the genus Musca such as M. vetustissima, the Australian bush fly and several closely related taxa that include M. primitiva, M. shanghaiensis, M. violacea, and M. varensis. The systematic identification of species may require the use of region-specific taxonomic keys and can require dissections of the male reproductive parts for confirmation.

Distribution

The housefly is probably the insect with the widest distribution in the world; it is largely associated with humans and has accompanied them around the globe. It is present in the Arctic, as well as in the tropics, where it is abundant. It is present in all populated parts of Europe, Asia, Africa, Australasia, and the Americas.

Evolution and taxonomy

Though the order of flies (Diptera) is much older, true houseflies are believed to have evolved in the beginning of the Cenozoic Era. The housefly's superfamily, Muscoidea, is most closely related to the Oestroidea (blow flies, flesh flies and allies), and more distantly to the Hippoboscoidea (louse flies, bat flies and allies). They are thought to have originated in the southern Palearctic region, particularly the Middle East. Because of their close, commensal relationship with humans, they probably owe their worldwide dispersal to co-migration with humans.

The housefly was first described as Musca domestica in 1758 based on the common European specimens by the Swedish botanist and zoologist Carl Linnaeus in his Systema naturae and continues to be classified under that name. A more detailed description was given in 1776 by the Danish entomologist Johan Christian Fabricius in his Genera Insectorum.

Life cycle

Each female housefly can lay up to 500 eggs in her lifetime, in several batches of about 75 to 150. The eggs are white and are about 1.2 mm (1⁄16 in) in length, and they are deposited by the fly in a suitable place, usually dead and decaying organic matter, such as food waste, carrion, or feces. Within a day, larvae (maggots) hatch from the eggs; they live and feed where they were laid. They are pale-whitish, 3 to 9 mm (1⁄8 to 11⁄32 in) long, thinner at the mouth end, and legless. Larval development takes from two weeks, under optimal conditions, to 30 days or more in cooler conditions. The larvae avoid light; the interiors of heaps of animal manure provide nutrient-rich sites and ideal growing conditions, warm, moist, and dark.

At the end of their third instar, the larvae crawl to a dry, cool place and transform into pupae. The pupal case is cylindrical with rounded ends, about 8 mm (5⁄16 in) long, and formed from the last shed larval skin. It is yellowish at first, darkening through red and brown to nearly black as it ages. Pupae complete their development in two to six days at 35 °C (95 °F), but may take 20 days or more at 14 °C (57 °F).

When metamorphosis is complete, the adult housefly emerges from the pupa. To do this, it uses the ptilinum, an eversible pouch on its head, to tear open the end of the pupal case. Having emerged from the pupa, it ceases to grow; a small fly is not necessarily a young fly, but is instead the result of getting insufficient food during the larval stage.

Male houseflies are sexually mature after 16 hours and females after 24. Females produce a pheromone, (Z)-9-tricosene (muscalure). This cuticular hydrocarbon is not released into the air and males sense it only on contact with females; it has found use in pest control, for 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 semiochemical that is deposited by females on their eggs attracts other gravid females and leads to clustered egg deposition.