This spring, there was a nice ﬂight of newly-emerged variegated fritillaries (Euptoieta claudia) in the Wichita Mountains National Wildlife Refuge in southwestern Oklahoma. They never seemed to stop ﬂying. Butterﬂy after butterﬂy went ﬂitting by. When one appeared to slow down, something went whizzing a couple inches past my left ear, making a loud, helicopter-like sound. As I ducked, my eyes caught the shine of the wings of a large ﬂying insect. It made a straight beeline, so to speak, for the butterﬂy, colliding with a smack. The butterﬂy went limp and ﬂoated to the ground, like a leaf dropping in the fall, while the large insect held on and buzzed its wings on the descent. The pair landed on a large, lichen-covered boulder.Only then was I able to see that the large predator was a robber ﬂy.
Robber ﬂies and their cousins (house ﬂies, fruit ﬂies, horse ﬂies, mosquitoes, and crane ﬂies, among others) belong to the insect order Diptera, which in Greek means “two wings,” although—except for a few wingless species—all members in the order have one pair of wings and a pair of halters. Halters are club-like appendages that are essentially modi-ﬁed hind wings, which act as highly sophisticated balance organs. They oscillate while the insect ﬂies and provide signals about its orientation in space, allowing it to maneuver quickly.
Robber ﬂies are further divided into the taxonomic family Asilidae, which contains about 7,000 species worldwide, of which 1,000 are found in North America. All adults are predators of other insects and usually cap-ture their prey in midair. They range in size from 3 to 50 millimeters (0.12 to 2 inches) long. Some are remarkable mimics of bumblebees. Others have thinner bodies and resemble damselﬂies, and still others have a body shape resembling a miniature helicopter, with a bulky front end that tapers down into a thinner tail end.
Robber ﬂies have a few anatomic features that differentiate them from other ﬂies, such as compound eyes that are set apart with a divot-like gap between them. In this depression, three simple eyes, or ocelli, are found. Their compound eyes, made up of hundreds of individual pho-toreceptor units, called ommatidia, give them excellent eyesight and allow them to visually track even the fastest prey. A thick mask of bristly hairs covers each ﬂy’s face and protects it when dealing with potentially dangerous prey, especially those with stingers, kicking legs, sharp claws, and biting mouthparts. They also have long, stout legs that are covered with more bristly hairs, which are used to snatch up prey and keep struggling victims at bay.
Robber ﬂies have a beak-like proboscis, or mouthpart, within which they sheath hair-like stylets that pierce deep into prey. When attacking, a robber ﬂy aims its proboscis at a soft spot on its victim, such as the crease of the neck or the prey’s eye. Once the prey has been pierced, the robber ﬂy injects saliva through the stylets, which contains neurotoxins that quickly subdue victims. Other chemicals in the saliva, called proteolytic enzymes, start to break down and liquefy the victim’s soft internal organs so the robber ﬂy is able to suck out the nutrient broth.
Within each species, there is very little sexual dimorphism. Females are usually a little larger and in some species, they will have a pointed tip at the end of the abdomen, which is the ovipositor, or egg-laying apparatus. Males will typically have conspicuous bulbous claspers at the back end of their abdomen. Sometimes, these are marked with silvery bands, which make them even more noticeable. These shiny bands may be used in courtship, but very little has been documented about this behavior.
Males have been observed displaying to perched females by hovering in front of them and opening and closing their legs in a kind of semaphore. On some occasions, females, apparently not impressed with this courtship approach, have attacked and killed the performing males. Other males have grabbed females, as they would grab prey, and mated with them.
Documentation of ovipositing behavior is also limited. Apparently, depending on species, some females insert their ovipositors into sand, soil, rotting wood, or crevices in plants.
Robber ﬂies exhibit complete metamorphosis, passing through egg, larva, pupa, and adult stages. After eggs hatch into larvae, very little is known about what they eat. Apparently, robber ﬂies have four larval stages, or instars. In one study, larvae ate organic material such as decaying matter during the ﬁrst instar; in the second instar, they fed on beetle secretions; and during the third and fourth instars, larvae preyed on soft-bodied insects, mostly beetle grubs.
For adults, meal choice is remarkably diverse: bees, wasps, hornets, assassin bugs, stink bugs, huge grass-hoppers, dragonﬂies, heavily armored beetles, and even giant cicadas. Many prey items, such as stink bugs and blister beetles, have chemical protection against most predators, but robber ﬂies seem immune. Apiarists have reported ﬁnding piles of bee carcasses below robber ﬂy perches. Some robber ﬂies have even been observed snatching spiders from webs. And there have been recorded cases of robber ﬂies capturing and eating hummingbirds!
My interest in robber ﬂies was fervent when I found myself back in the Wichita Mountains National Wildlife Refuge. Though I was there conducting a butterﬂy survey, what really caught my eye were all of the ﬂies. For example, I was fascinated to see two different species of robber ﬂy, one appearing to embrace the other, but with its proboscis deep into the other’s neck. I kept getting distracted and found myself photographing robber ﬂies instead of counting butterﬂies. There were several perched around the trail and it was almost like walking through a gauntlet. I realized then how dangerous the world is for other insects. In fact, if robber ﬂies were the size of pit bulls, I doubt we humans would ever venture outside. --BER