Aerial Assassins

Robber flies are top predators in the insect world.

A robber fly of the species Triorla interrupta has captured and is feeding on another robber fly species, Saropogon sp.

Photographs by author

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.

A few feet away and a few seconds before the event pictured above, a Saropogon sp. has captured and is consuming a robber fly of the species Promachus sp.

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!

On a boulder-strewn trail in the Wichita Mountains National Wildlife Refuge, Oklahoma, a robber fly, Efferia sp., has captured a variegated fritillary, Euptoieta claudia.

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