Photo: Long-legged Fly with prey, Judy Gallagher
Judy Gallagher
October 2025
When I was growing up, my mother cautioned me to never let flies touch my food because they ate poop and their feet were likely covered in poop. I assumed that all flies ate feces, and I guess it's not surprising that I grew up disliking them.
Dance Fly with sponging mouthparts. I don't know what substance it is sponging off my fingernail. Judy Gallagher
To understand what adult flies can eat, it is helpful to know about their mouthparts. Some adult flies have piercing-sucking mouthparts, while others have sponging mouthparts. In either case, food must be liquid or very finely grained in order to be sucked or sponged up. If a fly lands on or preys on solid food, it can regurgitate a drop containing digestive enzymes, which dissolves the food so that it can be eaten by the fly.
Golden Dung Fly with fly prey, Judy Gallagher
One would think that Dung Flies would eat dung. Larvae of some species do, but the adults mostly eat other insects, especially other flies, injecting them with digestive enzymes to liquefy them, and occasionally eating dung or nectar to get extra nutrition.
There is another fly family, Lesser Dung Flies, whose larvae also feed on dung and rotting seaweed. The larvae feed on other things too, as suggested by an alternate name, Lesser Corpse Fly. I was unable to find studies detailing what the adult flies eat.
At least three different species of Blow Flies on dead catfish, Judy Gallagher
Adult Blow Flies like Bluebottle and Greenbottle Flies sometimes eat dung. They also dine on liquids from rotting garbage or carrion. It's easy to be disgusted by these flies' eating habits, but the flies play an important role in the ecosystem. They consume rotting organic matter and poop, which recycles nutrients back into the environment. They also help prevent the accumulation of rotting vegetation, dead bodies and waste.
Eastern Treehole Mosquito showing signs of a recent blood meal, Judy Gallagher
As I'm sure you're all aware, some adult flies, especially mosquitoes, drink blood. In most cases, only the female bites, and she needs the proteins in blood to develop her eggs. But there are a few species, including the Stable Fly, where both males and females feed on blood. Scientists are studying anticoagulants secreted by blood-feeding flies, looking for medical applications in preventing blood clots, so some human good may come out of this annoying feeding strategy.
Elephant Mosquito male feeding on nectar, Judy Gallagher
Male mosquitoes mostly feed on nectar, and don't feed on blood. But a recent study on male mosquitoes preserved in amber suggests that male mosquitoes also may have fed on blood in the distant past. Hover Flies and many other adult flies feed on nectar, and often pollinate flowers while they're looking for a meal. Fruit Flies love the yeast and bacteria in overripe or fermenting fruits, and also feed on decaying vegetables and other sugary waste.
Robber Fly with Eastern Amberwing dragonfly, Judy Gallagher
I previously mentioned the Golden Dung Fly, a predator on other flies. There are several fly families that specialize in eating other invertebrates. The most famous are Robber Flies, who catch their prey in the air. They usually fly back to a perch with their prey, inject digestive enzymes to liquefy it, and eat while perched. This Robber Fly took down an Eastern Amberwing dragonfly, prey that was bigger and heavier than the Robber Fly, and the weight of the prey caused them to crash on the beach at Leesylvania State Park. After a short while, presumably sated, the Robber Fly extricated itself and flew off.
Long-legged Fly with prey, Judy Gallagher
Long-legged Flies also are predators. Some species in this family specialize in mosquito larvae and are useful as a control of pests.
Flies and their food choices play an important role in our environment. It's ironic that I was brought up to think of flies as being dirty, yet without flies, the world would be a much dirtier place. Let's hear it for the flies!
View more of Judy’s articles on A Closer Look: Nature All Around Us (formerly Observations from Meadowood).