The Flies That Ambush the Robber Flies

It is human nature to perceive flies as nasty things that should be killed, and I suppose this is only natural. After all, we are impacted pretty badly by the negative aspects of many kinds of flies, from the diseases they spread (most importantly) to the annoyance they cause when they land on us. The blood-feeding species of flies can cause pain when they bite or intense itching as the aftermath of their feeding. The presence of maggots in our garbage cans can be disgusting, and for all these reasons many flies have earned the title of “pesky fly”.

Let us now offer just a few of the reasons why “flies” should be loved and cherished, for it turns out that there are a great many kinds that are not disgusting, are not associated with filth, do not spread diseases, and should be welcomed into our backyard gardens. These kinds just don’t get the press coverage of the bad ones, making it that much harder for you to learn about them. While there are many large families of flies that are extremely beneficial to humans and our gardening activities we will focus on just one fascinating family in this particular article, and that family is called the Robber Flies. The scientific name for this family of flies is Asilidae, and the different species can be found throughout the world. Most are fairly large species, attesting to their ability to capture other insects and hold onto them while they get eaten. Some of the tropical varieties are, naturally, put together with beautiful colors, including metallic colors of blue or green.

Robber flies are generally pretty big flies, with some species approaching two inches in length! Both the larvae and the adults are predators on other kinds of insects, the larvae living in or on the soil and feeding primarily on the larvae of other insects. However, it is the fascinating adult flies that are most visible to us, and their name of “robber” fly is pretty descriptive as they rush out to assault other insects that pass by on the fly. Robber flies have huge eyes and excellent eyesight, and they easily detect nearby movements of other flying insects. Their legs are formed to point out in front, with strong muscles and rows of spines that help them to grab onto and hold the other insect. Their mouth is a strong, piercing beak, and this is shoved into the insect they have captured and the liquid insides of the other bug are withdrawn like a milkshake.

The robber fly likes to ambush its food. Unlike dragonflies, which spend much of their time in flight, soaring over ponds and landscapes looking for other flying insects, the robber flies prefer to sit on twigs that offer them a good view of the general area. No sense in wasting all that energy flying, when it’s just as easy to sit and wait. These flies have long legs, and the moment they detect a potential food going by they spring off the twig and rapidly capture the other insect while in flight. They then return to a safe perch to consume their prize. If there is any negative aspect to these predators it is that they can be somewhat destructive to honeybees, easily avoiding the stinger of the bee as it is captured. Perhaps we can balance that with the benefit they offer by also feeding on yellowjackets and other unwanted wasps.

The world of flies is a fantastic one, and we will discuss another important family of flies in another BugInfo article – the flower flies or Syphids. Many of the species of syrphids have evolved appearances remarkably like that of stinging wasps and bees, a phenomenon we call “mimicry”. By looking very much like an animal that could deliver a painful sting, it is assumed that the flies derive some amount of protection from predators that would otherwise eat THEM. Certainly, their deceptive appearance is so effective that we humans are likely to leave them alone, and some studies have shown that animals such as birds or reptiles also “learn” from a bad experience.

Robber flies also have perfected the art of mimicry, and many times you will see what for all the world looks like a large bumblebee, but in reality is a robber fly. They probably could deliver some amount of pain to your skin with their piercing mouth, but it’s much better for the robber fly never to be captured at all by a predator that might eat it. Biting mouths and pain only work so well. I once saw what I thought was an enormous bee fly past me, but when it landed on the end of an exposed twig, in a very robber-fly-like-manner, I took the chance to walk over and observe it more carefully. It did, indeed, turn out to be a large robber fly simply plopping down at the end of an exposed twig, where it could watch things nearby.

With over 7000 different species of robber flies now known throughout the world, and many of them residents of the United States, it is very likely that you will, on occasion, see one of these in your yard or on your walks in natural areas. For all intents and purposes these flies are completely harmless to people and our pets, although no doubt one would try to defend itself by biting if it were trapped in our clothing or captured in our hand. But, they do not sting and the bite would be more of a surprise than a painful experience. What should you do if you ever see one in your yard?…..I’d suggest stepping back and watching it for a few minutes to see what it does. You might get lucky and be able to observe it going after, capturing, and eating another insect. There is no reason to control or harm these beneficial flies.