1-8.
FLIES
FIies (order Diptera) are found allover the world, but they are most abundant in
warm, humid climates. Houseflies, which comprise the majority of all flies, found in
homes and food service faciIities are the most important of the nonbiting species in the
transmission of diseases. These and other fiIth flies spread disease "mechanically;"
that is; they land on a contaminated site (that is, pathogen-containing feces), get
pathogens stuck to various body parts, and then contaminate our food when they land
on it to feed. The medical history of past wars indicates that the health of troops has
been seriously affected by flies. They carry the organisms that cause dysentery and
may carry those which cause cholera, typhoid, and other diseases. In the tropics,
various skin and eye diseases may be spread by flies. These include such serious
diseases as yaws and trachoma. All flies have complete metamorphosis with four
distinct stages in their Iife cycle: eggs, larvae, pupae, and adults (see figure 1-16).
Figure 1-16. Life cycle of the common housefly.
a. The Housefly, Musca domestica. The housefly is known in all areas of the
world and is the most widely distributed insect of importance to humans. In general, the
housefly is gray in color and the thorax has four broad, dark stripes running
longitudinally (see figure 1-17). The mouthparts are the sponging type and are adapted
for taking up Iiquefied food (see figure 1-18). The adult female lays her eggs in
garbage, pit latrines, animal manure, spilled animal feed, and soiI contaminated with
organic matter. Houseflies are very prolific; the female lays several masses of eggs.
The eggs hatch in less than 24 hours under ideal environmental conditions. The
maggots are white and about one-half inch long when mature. This is the larval stage,
which lasts from 3 to 24 days depending on temperature. In warm weather, this stage
develops rapidly in 3 to 7 days. When the full-grown larvae are ready to pupate, they
move out of the breeding site into dry soiI. The pupal stage lasts from 3 to 5 days,
depending upon the temperature. When the pupal stage is completed, the adult
emerges from the pupal case, works its way to the surface of the ground, and after its
exoskeleton dries and hardens, flies away to feed. Mating may take place a day or two
following emergence from the pupaI case.
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