Even when flies land on and start to eat your food, they don't chew it. These insects don't have teeth -- not even the types of flies known to bite. Instead, they eat by partially liquifying it so they can siphon it with their mouthparts. The flies' specialized feet receptors allow the creatures to begin enjoying a meal the instant they land.
Would You Like Flies With That?
Because flies don't have teeth for chewing up food, they prefer softer meals, like rotting meats and fruits. Fruits are appealing to flies because of their high sugar content. One of their most typical meals is animal feces, which has a powerful odor that makes it easy to locate, and which is very soft, making it easy for them to absorb and eat. Flies eat the rotting flesh of carrion, or dead animals, for similar reasons. They lay their eggs in such food sources so that, when they hatch, the maggots can feed themselves where they lie as they develop into adult flies.
Savoring With Their Feet
Flies don't have to actually eat their food to taste it -- they can start savoring a meal as soon as they land. This is because flies have taste receptors in their feet, allowing them to crawl over potential meals and taste them before actually digging in. They have taste receptors in their mouths, too, so if they like the initial taste of what they've landed on, they can press their mouths against it for a better taste.
Digest First, Eat Second
Flies expel some of their digestive fluids so they can start breaking down food before it enters their mouths. They don't have teeth for breaking down food and therefore must subsist on soft foods. The fluid they spit or vomit onto their food partially liquifies it -- enough for them to press their absorbent, strawlike mouthparts against the food and suck it up.
Precision Incisions, Not Bites
Though some flies are known for biting, like horse flies, the painful sensation of the so-called bite is not actually caused by teeth. Flies like these have mouthparts with sharp edges, which work like knives to make small cuts in their prey's flesh. When blood pools in the hole, they are able to suck it up. These types of flies frequently target livestock like horses and cattle -- abundant food sources without the means to effectively swat them away.
Tom Ryan is a freelance writer, editor and English tutor. He graduated from the University of Pittsburgh with a degree in English writing, and has also worked as an arts and entertainment reporter with "The Pitt News" and a public relations and advertising copywriter with the Carnegie Library of Pittsburgh.