The house fly, Musca domestica (Fig. 1) , is one of the best known and most widely distributed insects known to humans. It is a classic example of a synanthropic animal, one that lives in association with humans and their domesticated animals. House flies occur and thrive wherever humans are found but are very rare in natural or wild areas throughout the world. “Insects will survive long after humans disappear” is a common expression, but it is not true of the common house fly. House flies would likely not be able to survive in the absence of humans because their relationship is so closely linked.
FIGURE 1 M. domestica.
LIFE CYCLE AND BEHAVIOR
Mating Behavior
Courtship and copulatory behaviors are the most important and the most complex behaviors exhibited by the house fly. Visual, chemical, tactile, and auditory cues are all used, to various degrees, in courtship and copulation. The elimination of a male’s production of, or a female’s reception of, any one stimulus may not greatly affect mating success. However, if combinations of stimuli are simultaneously eliminated, mating can be significantly affected. In general, males mate as often as they can, whereas females mate just once.
The courtship behaviors of M. domestica, and many other calyp-trate Diptera, are initiated when the male first sights a prospective femalelike object. Males are not very discriminatory in their initial choice of partners and strike other males, other species of flies, and small inanimate objects moving through their visual field.
The discovery of cuticular hydrocarbons that serve as sex phe-romones in the house fly triggered a burst of investigations into the role of such pheromones in the mating behavior of the calyptrate Diptera. No evidence has been found that these pheromones are olfactory stimulants; rather, they appear exclusively to be contact excitants. In house flies, chemosensilla involved with contact chem-oreception (or “taste”) are located on both the mouthparts and the tarsi. Thus, house flies can “taste” with their feet. When a male touches a female with his tarsi, as he grasps her upon initial contact, he can use the female-produced sex pheromone to determine whether a potential mate is of the correct species, sex, and even mating status. This pheromone is a very important stimulus for the male, and a male repeatedly attempts to copulate with an object that is of appropriate size and ” tastes right. ”
Behaviors that involve the touching or bodily movement of males and females beginning after the initial contact may be elicited by tactile cues. Tactile cues may be given by either sex during courtship, but it appears that the male’s role is much more complex than that of the female. High-speed photography has shown that the house fly’s courtship is extremely brief and complex. The highly ritualized sequence of movements that the male performs immediately after contact with the female seems to be very important to the female in her choice of potential mating partners. Males often strike females in midflight and perform the courtship ritual during their plunge to the ground. If the courtship is performed to the female’s satisfaction, she allows the male to mate with her. If not, she can dislodge the male and stop mating from occurring by performing one of several different rejection maneuvers.
Mating pairs of M. domestica are normally quiescent during copulation. However, if the pair is disturbed, they move, and the female flies short distances carrying the male on her back.
Development
The number of eggs that mature in a fly’s ovaries at one time is about 120. The female requires both sugar and protein meals for egg production. After copulation, egg laying takes place in 4-8 days. The female requires nearly a day to deposit the eggs, which may be deposited in a single mass or distributed in a number of locations. Each female is capable of developing several batches of eggs during her lifetime. Animal manure is the preferred ovipositional substrate, although a variety of decaying organic material can be used if fecal material is not available. Egg-associated bacterial symbionts appear to serve as resource-partitioning cues in the ovipositional behavior of gravid females.
The developmental time is highly temperature dependent. Hatching usually takes place within 1 day after oviposition. Larval development occurs rapidly, with the larva (maggot) normally passing through all three instars in 5-9 days. After full larval development, the third instar turns into a dark, cylindrical puparium, which is composed of the scle-rotized skin of the last (third) larval stage. The process of pupation normally lasts about 5 days.
Most adults live for 2-3 weeks at normal summertime temperatures in the temperate regions. Adult flies in the laboratory show longest life spans when fed a diet of sucrose alone, with a mean life span of 34 days. During the summer in temperate regions of the world, the entire life cycle (from egg to egg-laying adult) can be accomplished in 10 days to 2 weeks.
Flight and Dispersal
House flies are comparatively slow fliers, with a normal flight speed of about 2 ms_1 (or 7.2kmh~1). They have an innate tendency to disperse from their rearing site, even when conditions are favorable. Capture-release studies with marked house flies indicate that 8595% of the flies stay within a 3 km radius of their release point after 4 days, although a few individuals may travel over as much as 20 km.
House flies are one of very few species of flies that purposely enter human structures, such as houses and barns. This propensity for entering dark openings has implications on the dispersal of this species, because house flies readily enter cargo or passenger areas of trucks, trains, ships, and airplanes. By this means, gene flow between geographically distant populations is easily, although accidentally, promoted.
Ability to Land and Walk on Ceilings and Vertical Surfaces
A common question about house flies is, “How do the flies land on ceilings, and how do they walk up smooth vertical surfaces, such as glass windows?” In landing on ceilings, a house fly normally performs a “half-roll” and reaches its legs out to the ceiling. Contact of the tarsi with the ceiling inhibits flight and the fly comes to rest, generally facing the direction that it was flying. A close look at the structures found on the tips of the tarsi help to explain the fly’s ability to cling and walk on ceilings or smooth glass windows. The apical tarsal segment bears a pair of curved claws that are used to cling to rough surfaces. At the base of each claw is a padlike structure, called the pulvillus, which bears a large number of glandular setae. These setae are coated with secretions that make them sticky, allowing the fly to walk on vertical, or even inverted, smooth surfaces.
THE HOUSE FLY AS A VECTOR OF HUMAN AND
ANIMAL DISEASE
House flies, particularly in large numbers, are a nuisance to humans when they enter houses, land and feed on human food, and spot windows with their feces. Of greater importance to humans, however, is their ability to spread human and veterinary disease agents. House flies have been associated with over 100 pathogens that can cause disease in humans and animals. Unlike the patho gens responsible for many other insect-borne diseases, the pathogens spread by the house fly do not usually multiply within the fly, nor do they require association with the fly for part of their life cycle. Instead, the usual association between house flies and pathogenic organisms is one of physical transmission of pathogens the flies pick up on their bodies at one feeding site (e.g., a garbage can or manure pile) and transfer to human and/or animal food on which they land and feed. House flies have been associated with the transfer of a variety of viral and bacterial diseases, such as typhoid fever, cholera, dysentery, and infantile diarrhea, as well as a variety of parasitic worms.
