A Staff Report from the Straight Dope Science Advisory Board

Do flies vomit every time they land?

January 2, 2003

Dear Straight Dope:

I heard that flies vomit every time they land. Is this a fact?

This is one of those oft-repeated "factoid" that floats around on long lists, so it's worth a Staff Report just to get the real story on record.

In short: No.

Three reasons:

(1) There are some 90,000 different species of flies in the world, and they have all sorts of different feeding strategies. It's like saying "birds have razor-sharp talons and beaks they use to tear their prey apart" instead of specifying that you're talking only about bald eagles. The particular factoid you've heard presumably refers to the house fly, Musca domestica, and has little or no relevance to the remaining 90,000 species.

(2) House flies secrete saliva whenever they attempt to feed, but they don't attempt to feed every time they land. They only do so when they're in need of nutrients, and the chemical sensors on their feet and/or antennae inform them that they may be standing on a source of nutrients.

(3) Strictly speaking, vomiting is the regurgitation of stomach contents, and house flies never regurgitate their stomach contents. What they do is technically spitting (or drooling), not vomiting.

By the way, house flies don't spread disease by spitting on your food, disgusting as that sounds. The disease comes from their feet when they land on your food after they've earlier been feasting on manure or viscera or other bacteria-laden goodies.

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