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Ask a Scientist

Anthony Frew, of Ardgowan School, asks:-

"Why do flies put a bit of spit on you before licking you?"

Ruud Kleinpaste, the bugman on TV's Maggie Barry Garden Show and author of the book Scratching for a Living responds:

An adult fly is in a rotten position, really, since it has no chewing or sucking mouthparts. Its mandibles are more like a soft sponge and that means it won't be able to "chew up" all sorts of solid foods, such as sugar crystals, etc.

The fly literally "vomits" all over a possible food item in an attempt to dissolve the food with its own vomit (spit or stomach juices). When the food is dissolved, it just sponges the whole mixture up and departs!

In order to aid digestion, flies spend about 20% of their time regurgitating and re-swallowing dissolved food, even when they are sitting upside-down on the ceiling! (Don't try that at home!) The resulting brown spots are the remains of the dried liquid which have been regurgitated -- and therefore not fly poos, as most people erroneously think!

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