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Do flies actually get on my food when they land on it? – Henry E., age 10, Somerville, Massachusetts
Imagine you’re at a picnic and just about to bite into your sandwich. Suddenly you see a fly moving towards its path, coming home to its food with the help of its compound eyes and antennae. It manages to survive your SWAT, lands on the sandwich and then starts to throw at it!
It may sound kind of gross, but the fly may just be gulping air on its digested food, or spitting on you.
Most of the more than 110,000 known fly species do not have teeth, so they cannot chew solid food. Their mouthpieces are like spongy straw. Once they land on your food, they need to release digestive juices to turn it into a pre-digested, thick soup that they can swallow. In short, some flies are on a liquid diet.
In order to fit more food into their stomachs, some flies try to reduce the amount of fluid they have already consumed. They turn the food into bubbles of vomit to dry it out a bit. Once some of the water has evaporated they can swallow this more concentrated food.
Humans don’t need all this spitting up and vomiting to get nutrients from our food. But you produce a digestive juice in your saliva, an enzyme called amylase, which pre-digests some sandwich breads when you chew. Amylase breaks down starch, which you can’t taste, into simple sugars like glucose, which you can taste. So the longer you chew it, the sweeter the bread becomes.
Did you know that flies can taste food even without their mouth? As they descend, they use receptors on their feet to decide whether they are eating something nutritious. You must have observed that a fly rubs its legs together, as if a hungry customer is getting ready to eat. This is called grooming – the fly is essentially cleaning itself, and may also clean the hairs of its legs and the taste sensors on the fine hairs, to detect what’s in the food.
Should you trash fly food?
When a fly touches your sandwich, it probably isn’t the only thing that landed that day. Flies often settle on bulky items, such as trash or rotting food, that are full of germs. Germs can hitch a ride and, if the fly lingers long enough, jump on its food. It is much more dangerous than their saliva as some germs can cause diseases like cholera and typhoid. But if the fly doesn’t live for more than a few seconds, the germs are less likely to be transferred, and your food is probably fine.
To prevent insects from landing on your food, you should always keep it covered. If you have an infestation of flies in your home, you can use simple traps to get rid of them. Carnivorous plants can also eat flies and help control their populations.
Are flies good for anything?
Spitting on food and spreading diseases sounds disgusting, but flies aren’t all bad.
Next time you’re out, look closely and you might be surprised how many flies visit flowers to get nectar. They are an important group of pollinators, and many plants require flies to help them reproduce.
Flies are also a good source of food for frogs, lizards, spiders and birds, so they are an important part of the ecosystem.
Some flies also have medical uses. For example, doctors use blow fly maggots — young, immature forms of flies — to remove decaying tissue in wounds. Maggots release antiviral and antimicrobial juices, and these have helped scientists create new treatments for infections.
More importantly, the fruit flies you may have seen flying around ripe bananas in your kitchen are invaluable in biological research. Biomedical scientists around the world study fruit flies to find causes and cures for diseases and genetic disorders. And in our lab, we study how insects see the world, and how they use their vision to fly. This knowledge could inspire engineers to build better robots.
So, although it’s a nuisance to drive flies away from your sandwich, maybe you should leave a few bits of your lunch?
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