Answer

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Flies and Regurgitation

Watch the movie "The Fly". They regurgitate on everything they are going to eat.

The acids in their vomit soften the food so they can suck it up. What they do not suck up, they leave. *

Flies excreet digestive juices onto whatever they land on as a method to taste it before they start eating it. So I suppose the answer is yes.

They also regurgitate on everything they land on. It is how they can land on the same spot they where earlier. They follow their wastes smell. *Though it is so totally nasty, it is true. I heard it on the news. They vomit on our skin and everything else they land on.

And here is an opposing view:

Well, has anyone actually watched a fly land. It's not like their mouthparts always touch the landing area of their flight. I have watched a fly land many times on non-edible surfaces and,it certainly never looked like anything was excreted from them. A fly is not always in the process of eating. Also a fly has a very good sense of smell. It doesn't make any sense for a animal to try to eat something that it knows it can't. Plus a fly would dehydrate itself completely at some point. I mean, come on.

And is it vomit or excretion from the other end?

I'm not sure about this, but I always thought it was that a fly craps on everything it lands on? Seperate from the crapping and landing, I was also told that they DO vomit on whatever they eat, to help the process along. But this would be two different questions if they do crap where they land, AND vomit on what they eat. Hmmmmmmm...

I do not know if it is vomit or poop, but my question is what difference does it make? Whatever it is, is sickening, not to mention unhealthy. If you want to prove they leave either vomit or poop on everything, do an experiment. Go to a place where there are lots of flies. Lay down a piece of white paper and leave it for a time. I think 30 minutes would be long enough, but it might take longer. Return to the paper. Look at the little dots. Whatever it is, vomit or poop, it came from the flies.

Answer

Yes. I saw it on Animal Planet once

The fly is not regurgitating. Flies secrete saliva (drool) onto whatever they're eating. They're not eating your desk surface, your plate, your table, your hand - whatever else they land on. So no, they don't spit and drool EVERY TIME they land. They never vomit. Flies don't actually do that. They're spit is not what you need to worry about anyway. The reason a fly is unsanitary is because they land lots of places: On dung piles, bathroom floors, trash cans, toilets, animals.... THAT'S why flies are gross and dirty, and why you should not eat food they've landed on. Otherwise, you may be the one vomitting.

The answer is NO, flies do not throw up everytime they land. For those that say yes, with caveats, the answer is incorrect. The question asks if the flies throw up everytime they land, thus the answer is no.