The Nature Writers of Texas

The best nature writing from the newspaper, magazine, blog and book authors of the Lone Star State . . .

Thursday, June 04, 2009

Robber Flies Are Deadly Predators
by Ro Wauer

It’s been said that robber fly predation on other insects can be compared with peregrine falcon predation on birds. Both are dramatic predators that take a significant number of prey. Although dragonflies also take numerous insect prey, including other dragonflies, from what I have observed in recent weeks, robber flies are far more successful in catching prey. And a close-up look at one of our many robber flies is rather scary. I am surprised that one of the horror movies, so common today, has not been developed around a monster robber fly.

Although robber flies can vary considerably in size (some tropical species can be more than a foot in length) and bodies, they all possess extremely long spiny legs, bearded face, piercing mouthparts, large eyes, hollowed out area between the eyes, and a bristly humped thorax. Some are stout like bees while others are long and narrow with a very long abdomen. And the various species, of which there are about 1000 in North America, utilize a wide range of habitats, from our yards to the beach to desert scrub and even grasslands. They can be found perched on the ground, on branches and leaves, or on various structures.

Robber flies will eat almost any flying creatures they can catch. Some of their prey can be considerable larger than they are, and one’s daily diet may include everything from bees and wasps to grasshoppers, butterflies, beetles, and other flies. Most hunt from perches where they wait for passing prey, and some species actually establish a flight territory that they defend from other robber flies. They possess a mobile head that can move about in various directions. When seeing a passing prey, they will immediately give chase and catch it in mid-air. They may even fly ahead and intercept the prey from an angle. They will then fly to a shady perch, holding the prey in their long spiny legs, and consume their catch by sucking their prey dry with hypodermic-like mouthparts.

The robber fly life cycle is rather ordinary. The females lay eggs on or just beneath the soil surface. Upon hatching the larvae (tiny, slightly flattened worms) crawl about in the soil and feed on tiny arthropods. Some species may remain in the larval stage for a full year, but most at least stay as larvae through the winter months. By spring, the larvae feed on decaying vegetable matter and the larvae of other insects. And by late spring or summer they pupate and wriggle to the surface where the adults emerge.

Robber flies are members of the Order Diptera or flies, a huge group of insects that includes everything from mosquitoes, wasps and bees, to house flies. What separates robber flies from most of the other flies is their aggressing predatory behavior. During some periods of the year they seem to be especially common, and I have often wondered if their numbers actually control populations of some other flying species. During a recent visit to the Lower Rio Grande Valley, I found several robber flies that had captured butterflies. Although they often are difficult to photograph when perched, they seem to allow a much closer approach when feeding on prey. I realize that robber flies have every right to coexist with all the other native insects, but I must admit that I am bothered when I find one with a butterfly firmly gasped in its long spiny legs.

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