Summary: The robber fly, also known as an assassin fly, hunts down other insects in mid air and stabs them with its short, strong proboscis. Robber flies can often be found hunting at the edge of wooded areas at the beginning of summer.
Flies are at the very bottom of the food chain, right? Flies feed on organic waste, stuff most animals wouldn’t touch much less eat. Flies are simply food for spiders and frogs. They are the annoying, filthy insects that regularly get swatted with newspapers and magazines, right?
The fly certainly has a lowly station in life. However, the robber fly has adapted into a predator. Robber flies have turned the tables and feed on many flying insects like butterflies, moths, grasshoppers and beetles. Robber flies even attack other predatory bugs like dragonflies, ichneumon wasps, and even some spiders. They are sometimes called fly hawks because they swoop down on deer flies and horse flies in open meadows. The bearded robber fly is also known as a bee killer because, well, the name explains itself.
Flies in the Diptera family Asilidae are known as robber flies because they attack and feed on flying insects that are often larger than themselves. There are over 7000 different species worldwide and more than 1000 species in North America. Because there are so many different species of robber flies they vary widely in appearance, but they are all predators and have some other common traits. These traits help them to be more effective hunters.
Robber flies are usually covered in hair, especially around the mouth, which protects their head from struggling prey. They usually have a long, tapering abdomen that is often stripped or banded. One distinguishing feature is that robber flies have an indented space in their head between their two large compound eyes. They have long legs covered in spines that are that are very strong so they can carry paralyzed prey.
Robber flies have a short, sturdy proboscis instead of a mouth that can chew. The proboscis is used by the robber fly to stab its prey in mid air. It then injects saliva into the victim that paralyzes it and starts to liquefy its body organs. The robber fly can then enjoy sucking out the insect juices at its leisure in a shady, wooded area.
Robber flies are found all over the world. They prefer to hunt in the sunlight so they can see their prey better, but they usually stay close to wooded areas so they can feed in the shade. Robber flies often hang from a leaf or blade of grass and wait to spot a passing insect which it will then fly out and investigate, only attacking if it deems the subject a suitable food source.
Female robber flies lay their eggs in the soil, or in other organic material. The flies do not live longer than a year and adults do not survive over the winter. The eggs of a robber fly hatch during the winter and mature underground. The larvae feed on grubs, insect eggs, or other organic material. The adult robber fly emerges in the early summer and mates soon afterwards.
Some robber flies wear disguises. There are many kinds of robber flies that mimic the appearance of bees. These robber flies are covered in yellow fur that makes them look like bumble bees, presumably to help them capture and feed on bees fooled by the trick. Bearded robber flies have hair all around their face that looks like a mask, which undoubtedly contributed to the origin of their name. One noteworthy robber fly is the hanging thief which will feed on captured prey while hanging upside down from one leg. Another is the red footed cannibal fly, which is a giant robber fly that has an abdomen with alternating yellow and black stripes, giving the insect the cool nickname of Panther fly.
Robber flies are important to nature and to people because they limit the populations of insect pests like flies, beetles, and grasshoppers. However, they cannot be considered strictly beneficial because robber flies also feed on insects like bees and dragonflies, and spiders which are also beneficial bugs.
Robber flies are not considered pests so insecticides are not necessary to control them. Robber flies can “bite” people, but do not do so unless they are trapped or confused. Don’t try to capture a robber fly with your bare hands, in other words. Robber flies are interesting insects that can be fun to observe during the early summer in fields or meadows nearby a wooded area.