On the question of fruit flies

Inquiring minds


Each week the staff at the Roseville Library answers more than 2,500 questions on every subject under the sun. Here is one of the most interesting ones they’ve gotten lately.


Editor’s note: With our regular library liaison away on a summer vacation, we’re reprinting some classic “Inquiring Minds” columns. The question and answers below originally ran in September 2006.



Q. My son is doing a sixth grade science project on where do fruit flies come from? Is it through spontaneous generation or what?


A. Well, your son will probably want to come up with a few alternatives to spontaneous generation. As an explanation for the origins of life, that idea was pretty thoroughly discredited several centuries ago in 1668 when an Italian named Francesco Redi proved that maggots cannot arise spontaneously from meat that is sealed away from flies. Redi, who must have been blessed with a strong stomach, devised an experiment to study the effects of putrefaction in meat. Half the meat he left open to the air and flies; the other half he covered with gauze.  After a short exposure to the warm Tuscan climate, both kinds of meat were equally disgusting to smell, but there were no maggots on the meat covered by gauze.  The other meat was crawling with maggots, which led Redi to conclude correctly that the flies were responsible for introducing the maggots into the meat.

But that still leaves unanswered the question of where the fruit flies come from. You bring home some beautifully ripe peaches, put them in a bowl, and the next day, there’s a cloud of small insects hovering over the dish. Were the fruit flies already present in larval state when you bought the fruit at the grocery store, or were they waiting at your house for the fruit to arrive? Either prospect is rather unappetizing. Happily, there’s a third alternative.  

Fruit flies are powerfully attracted by the yeast that ferments overripe fruit, and they’re perfectly capable of infiltrating your house from the outdoors when the delicious odor of decaying fruit arises. Of course, once they establish a foothold at your house, you won’t necessarily get rid of them by dumping out the fruit bowl. Fruit flies can live on the slime found in the kitchen drains, on a sour dishcloth or even in a crumb-laden crack in the kitchen floor. There they will lurk, patiently waiting for you to return from the grocery store with the next bag of fruit. 

(“Asimov’s Biographical Encyclopedia of Science & Technology” and internet resources.)


Do you have a question for the staff at the Roseville Library? You can call them at 651-724-6001 or ask your question in person at the Information Desk, Roseville Library, 2180 Hamline Ave. Library hours are 10 a.m. to 9 p.m. Monday through Thursday; 10 a.m. to 5 p.m. on Friday and Saturday; and noon to 5 p.m. on Sunday.

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