Tour de Fly

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Published on: May. 2, 2003

Last revision: Nov. 15, 2010

When you are shooing house flies off your picnic plate, or swatting mosquitoes at the fishing hole, it is hard to imagine that flies of any kind could be good for anything.

There are, however, some species of flies (Diptera) that help us. Pomace flies, for example, are vital research subjects in genetics. Hover flies and bee flies pollinate flowers. Robber flies kill many harmful insects, while tachinid flies are parasites of other pests. Even blow flies help crime scene investigators solve homicides.

You don't even have to leave home to encounter flies. We all know the pesky house fly, Musca domestica, in the family Muscidae. They are known to spread a variety of diseases, but ironically are fastidious groomers. Watch one and you will see it rub its feet, draw a leg over each wing and wipe its eyes. Those eyes are better than ours at perceiving motion, making the fastest fly swatter look like slo-mo. Pads on their feet let house flies cling to slick surfaces, or alight on the ceiling.

All flies go through the same life cycle, beginning life as an egg, which hatches into a larva, or "maggot." The larva feeds and grows, shedding its outer cuticle (exoskeleton) several times. Eventually, the larva molts a final time into a pupa. Inside this capsule-like stage, the insect is re-programmed, and its cellular structure rearranged into that of an adult fly. The time it takes to complete this metamorphosis depends on variables like temperature and humidity.

Adult houseflies live only two or three weeks, but there can be ten to twelve generations in a summer! The cycle from egg to adult can take as little as seven to ten days in warm weather. Horse dung is the favorite breeding medium for house fly larvae.

Besides house flies, you may confront "fruit flies" hovering over your bananas. These are actually pomace flies of the family Drosophilidae. Attracted to fermenting substances, they have an extraordinarily high tolerance for alcohol. Larvae eat mostly the yeasts that infect bruised produce.

Amazingly, these tiny flies have giant chromosomes in their salivary glands, allowing researchers to easily pinpoint specific genes. This unique feature, coupled with the flies' rapid life cycle, makes them ideal subjects for studying inherited characteristics. Drosophila melanogaster is the common species in the lab and in the wild.

Humpbacked flies (family Phoridae) look like pomace flies, but run more than fly,

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