Robber flies are conspicuous denizens of the desert southwest, but are common nearly everywhere. You might be seeing them yourself but simply not recognizing what they are. No wonder. Robber flies, also known as “assassin flies,” often resemble wasps or bees more than they do flies. They certainly bear little resemblance to house flies. They don’t carry diseases or bite people, either. Instead, they are swift predators of other insects.
There are nearly one thousand species of the family Asilidae in North America north of Mexico, but that doesn’t mean they have all been discovered yet. I helped discover a new species of Laphria myself, in a park in Cincinnati, Ohio. Prior to my specimens, the species (still awaiting formal description) was known from only one other specimen collected in Pennsylvania.
Robber flies vary greatly in size (3-50 mm), shape, and color pattern. Some are robust bumble bee mimics. Others, like those in the subfamily Leptogastrinae, are slender and nearly invisible as they navigate through tall grasses.
There are some things to look for that are common to all robber flies, however. They have a deep, concave area between the eyes at the top of the head. This helps set them apart from similar flies like mydas flies (Mydidae) and dance flies (Empididae). Robber flies also often have a “bearded” face, with long hairs over their mouthparts. Even with the setae, the stout beak-like mouthparts are often visible.
Look for robber flies in a variety of habitats, from deserts to grasslands to forest openings with dappled sunlight. Robber flies like to perch on the ground, rocks, logs, tree trunks, or foliage where they have a great vantage point to scan the landscape and the sky above them.
Watch one on the tip of a twig or a leaf as it cocks its head toward insects passing overhead. The fly may leave its perch abruptly, but you should stay put. It may well return with a victim to dine on. Asilids are able to intercept flying insects in mid-air, much like a flycatcher bird does. Some species seem to have their food preferences, but most are generalists. Very large robber flies can kill insects as large as adult grasshoppers, or even dragonflies.
Members of the genus Diogmites are known as “hanging thieves” for their habit of swinging from the front or middle pair of legs while manipulating prey with the remaining two pairs, as the one above is doing with a skipperling it caught.
The bite of robber flies is administered with that beak, driven into joints in the exoskeleton of the prey insect. The “necks” of insects are especially vulnerable, and flying beetles are impaled while their elytra are open, exposing chinks in their armor. Paralytic compounds and digestive enzymes are likely injected during the bite as victims cease struggling almost immediately. The fly then withdraws the liquefied internal tissues of its meal.
As easily as adult robber flies are observed, the corresponding larval stage remains quite a mystery. Those species that have been reared are known to be external parasites of beetle grubs, or other insect larvae.
Before you set out into the field to find robber flies, it might help to become familiar with all the different genera you are likely to encounter. Remember that they often resemble insects other than flies. A good place to start learning is the image gallery at BugGuide.net. Click the “images” or “browse” tab near the top of the page to get more images. Another outstanding internet resource is the robber fly site built by Herschel Raney, an extraordinary nature photographer and self-taught expert on asilids. Three individuals are webmasters for a global robber fly website.
As the popularity of these amazing insects continues to grow, no doubt more resources will be created. For those who prefer printed to electronic references, my Kaufman Field Guide to Insects of North America includes two plates devoted to robber flies, with complementary text on the opposite pages.
Stop, listen (for a loud, droning buzz that stops abruptly) and look (carefully, for despite their size robber flies can be rather cryptic) for these winged wonders of the insect world. You will not be disappointed that you did.