Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 576   October 17, 1959
Forest Preserve District of Cook County
Daniel Ryan, President
Roberts Mann, Conservation Editor
David H. Thompson, Senior Naturalist

Have you noticed any tiny flies in your kitchen or around the fruit bowl? They are so small that they come through ordinary screens into homes, stores and restaurants. From midsummer until the killing frosts of autumn, swarms of them cluster wherever ripe or fermenting fruit is exposed -- outdoor markets, tomato canneries, garbage cans, melon patches, vineyards, and apple, pear or peach orchards. At this season, hordes of them are found around cider mills where they breed in the cakes of pressed apple pulp or pomace. A few adults and young survive the winter in basement drains and other protected places with food and warmth.

These Fruit Flies are strongly attracted by the sour or yeasty odors that come from rotting fruit and other fermenting substances. Yeast is the main food of both adults and young. Because of these preferences they are also called Vinegar Gnats and Pomace Flies. Dozens of species are known, all much alike in habits and appearance. By far the most common and best known is Drosophila melanogaster, the one reared in bottles on a large scale for school use, research, and as food for small aquarium fish, captive spiders and other insect-eaters.

Fifty years ago, the fruit fly was discovered to be an almost ideal laboratory animal for the study of heredity -- usually called genetics. Among its advantages is the fact that they breed the year around, and twenty or more generations a year are easily obtained. Each pair of parents can produce a hundred to several hundred offspring in a small milk bottle with some fermenting banana or specially prepared food. They are clean, inexpensive to rear, require little care, and a hundred of these bottles can be kept in the schoolroom or laboratory.

Over the years, geneticists have found hundreds of different heritable variations or mutations in Drosophila and have traced in detail their methods of inheritance. Breeding experiments between pure-bred pedigreed races of fruit flies with different eye colors, body colors, wings, and dozens of other characteristics, are widely used by students in schools and colleges for exciting studies of heredity Leaflets describing the methods for rearing fruit flies and for carrying out some of these experiments may be had free of charge from the General Biological Supply House, 8200 South Hoyne Avenue, Chicago 20, Illinois.

The adult wild fruit fly, seen through a hand lens or low-power microscope, has large bright red eyes and a tan-colored head and thorax. The abdomen of the female is crossed by dark lines. That of the male has a black tip. In addition, each of his front legs bears a conspicuous jet-black "sex comb" having several teeth. By their second day of adult life, these flies are sexually mature. Then the male begins his "courtship dance". Facing the female, he shifts from side to side, flirts his wings up and down, and does his best to show off.

Against a dark background the elongate white eggs are barely visible to the naked eye. Within two days, at room temperature, tiny white glistening larvae hatch. These feed constantly and reach full size in about six days. Then they form into straw-colored pupae, shaped like little wheat grains, which rest for five more days before the adults emerge.

Consequently, students can repeatedly observe the entire life history of fruit flies, learning about insect biology, as well as heredity, from them.

That mote in your eye may be a fruit fly.

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