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This very mystery -- vermin from nowhere -- gave rise to a charming and durable belief called "spontaneous generation." . |
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Until about 100 years ago, people were innocent of the concept of a fly's life-cycle, wherein it
is obliged to pass through various stages from egg to larva to adult. Rather, they observed
that if you left a piece of meat lying around, flies miraculously emerged from it.
This enchanting bit of unnatural history was first attacked by a 17th-century Italian doctor who hit on the grand notion of covering up the meat: No more flies. But I think fruit flies still make a fair case for spontaneous generation. The first time I had an opportunity to ask a fruit-fly expert where the fruit flies in my kitchen came from, he got very defensive and insisted that, despite what his neighbors claimed, they didn't come from his genetics laboratory. I later saw his laboratory, and while I would have sided with his neighbors, it turns out fruit flies are wild animals. "They're native to most areas of the country," says Colorado State University entomologist Whitney Cranshaw. "They come from outdoors." |
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They come when they smell your peaches rolling across the ripeness line. . |
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Fruit flies prefer a diet of yeast, that marvelous microbe that eats fruit and spits out
alcohol. Since flies disdain unripe fruit, and since that is precisely the sort of colorful
paperweight that supermarkets tend to distribute, it's more likely that your Drosophila
melanogaster came from the great outdoors than from the grocery store.
The obvious next question is, if the peaches came home "clean," and there are flies on them now, how long will it be before there are eggs or larvae (AKA "maggots," of course) inside the peaches? Do you eat the peaches, or not? The answer is there could be eggs on the fruit instantly, and 30 hours later, larvae -- perhaps 500 of them. If you don't disturb their happy community, the tiny, white larvae will spend five or six days gobbling the yeast and alcohol-rich products of fermentation, then crawl out of the sludge, hatch into red-eyed monsters and start the cycle again. And once they're established in your house, they can sustain themselves on an impressive range of nutrients. They can live on the slime inside a sink drain. They can flourish on a sour mop. They'll eat damp flour or food fermenting quietly in a crack in the floor. They've even proven capable of existing on a diet of alcohol fumes, their bodies deploying a special chemical that converts the alcohol to nourishment before it can poison them. |
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And in their eight-day life cycle, they breed prolifically. . |
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Some thoughtful soul has gone to the trouble of calculating that one pair of flies, in one year,
can produce a dynasty that, packed in a ball, would fill the void between the Earth and the Sun.
And that was at just 100 eggs per female. Some sources say they lay 1,000.
This sort of fecundity, however, is often a hallmark of small brains and a high death rate. Because they're slow to anticipate treachery, for instance, fruit flies are easy to trap. A wine bottle with a bit of fruit or wine in the bottom and a cone of paper set funnel-style in the top works brilliantly, though Cranshaw notes that the trap may also attract your victims' country cousins. All things considered, prevention seems key: Just eat the peach. |
 Vocabulary scientific method, n. When the Italian doctor, Francesco Redi, wrapped some pieces of meat in gauze, sealed some in flasks and left others in the open air, he embraced a new system of ruling out variables. Not everyone played. A popular "experiment" in spontaneous generation ran thus: Put sweaty underwear in an open bottle with wheat, and after 21 or 22 days the wheat will turn to mice.
Check out more of "The Skinny On ..." stories:
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Hannah Holmes usually chases bugs in Portland, Maine, but this month she's chasing
dinosaurs in Mongolia's Gobi Desert for Discovery
Online's Dinosaurs in the Dunes special. She
also writes for Escape, Outside, Sierra, Backpacker, Eco Traveler and Women's Sports and
Fitness. Write her at skinny@online.discovery.com.
Main Picture: Brian Frick | Copyright © 1997 Discovery Communications, Inc. |