In a previous episode of Bug of the Week, we met industrious bumble bees (NEEDS LINK!! SEPT 10, 2007) as they foraged in a patch of pumpkins. As I watched their nectar gathering antics among the blossoms, I noticed a rather large, hairy fly lurking on a nearby leaf. This remarkable fly sported a large dense mustache of blond hairs and stout bristles on its body and legs. The space between its eyes was curiously dented as if some small tinker had whacked it in the head with a tiny hammer. Upon my approach, the fly lifted-off from its perch and disappeared into the field of leaves. A short time later and a small distance away, I spotted the fly again, but this time a hapless bumble bee was slung beneath its body in a macabre embrace of death.
This was the work of the robber fly. Robber flies are one of the most fascinating groups of predators in the insect world. More than 4,700 species of robber flies have been described worldwide. They consume a wide variety of prey, primarily other insects and spiders. Robber flies, like the one in my pumpkin patch, have large eyes, excellent vision, and capture their victims on the wing. Strong forelegs enable the robber to grab prey and a powerful beak is then inserted into the victim’s body. Potent saliva laced with neurotoxins and digestive enzymes are pumped into the victim. Neurotoxins rapidly immobilize the prey while the digestive enzymes turn solid tissues into liquids that are easily sucked up through the beak and down the gullet of the fly. After a short time, the victim’s remains are discarded and the robber hunts again. Robber flies do not fear a tussle and often capture stinging insects such as bees more than half their size. Immature stages of robber flies are legless maggots. Their time is spent in the soil where they hunt and eat many different types of insects including pests such as white grubs. Several species of robber flies closely resemble stinging bees and wasps found in the same habitats. This resemblance may provide protection from predators such as birds that learn to avoid stinging insects for dinner. By mimicking the appearance of bees and wasps, robber flies may also be able to dupe potential victims who mistakenly fail to identify them as foes rather than friends. For obvious reasons this clever form of deception is called aggressive mimicry.