The world is full of robber flies—approximately 7,000 species have been recognized worldwide and 1,000 are native to North America

A robber fly perched on a flat piece of sandstone in the red rock country of the Colorado Plateau. The choice of perch was not arbitrary, for here the robber had a view in all directions. It flew off, but returned in a few seconds with nothing. A few minutes later it flew again, intercepted an insect within six feet and returned with it impaled on its beak.

The world is full of robber flies—approximately 7,000 species have been recognized worldwide and 1,000 are native to North America. All robber flies are predators on insects and have two wings, prominent spikes on their legs, stout hairs on the body, a prominent proboscis fashioned into a sharp tube or beak and large, widely-spaced compound eyes. The mystax, composed of stout and stiff facial hair resembling either a beard or mustache, protects the robber’s face and eyes from a flailing, impaled prey. Robbers have three simple eyes in a pit between the compound eyes, near the top of the head. 

Robber’s modified mouthparts form a stiff, hollow beak that serves first as a dagger, then as a hypodermic needle and finally as a straw. When robbers impale prey they inject both neurotoxins and digestive enzymes. The neurotoxins quickly subdue and soon kill the prey. The digestive enzymes turn the prey’s internal tissues to soup, but do not damage the tough chitinous exoskeleton, which now serves as a watertight container full of thick broth. Fluids are sucked through the beak until the carcass until is a dry husk. 

Robber flies are flexible hunters. Most hunt by intercepting flying insects from one of several perches in their territory. Others snag prey from vegetation or pounce on prey on the ground. They usually return to a perch in their territory to consume the prey, which takes from 5 to 15 minutes for small prey but perhaps more than an hour for large prey. 

Robber flies usually take prey smaller than themselves, but they have been reported to take dragonflies, bees, wasps and butterflies that are larger. It appears that they eat anything that they can catch, for their prey species include flies, beetles, butterflies, moths, bees, wasps, ants, grasshoppers, crickets, lacewings, dragonflies, damselflies and mayflies. They also take other species of robber flies smaller than themselves.

Cannibalism is common in robber flies. This is a puzzle to me, for cannibals might take relatives, reducing their own fitness. In an amphibian species in which cannibalism has been studied, salamanders are able to discriminate between related and unrelated individuals of the same species—they will not eat close family members.  But I did not find any published report that robber flies could discriminate between kin and unrelated.

Robber flies can reach substantial population densities, so a range of 50 to 150 per acre would not be unusual. Such densities can have substantial impacts on insect communities. For example, during an epidemic of mountain pine beetles, robber flies can eat 1% of the flying beetles per day, and they can consume 2% of grasshoppers per day in grasslands in Nebraska. 

I have had the good fortune to see robber flies mating and I noticed that some species assume a position with the male on top of the female, while others use a tail to tail position, with adults facing opposite directions. I have not found any comment in the literature on this variation of amplectic position. Perhaps a species’ body shape, or the shape of male and female genitalia determines position. Shape of male claspers certainly varies strikingly among species. 

A study of robber fly mating behavior reported a case of thanatosis, or feigning death. In this species, males initiate mating by grasping a female. However, if the female perceives that he does not meet her hopes or expectations, she goes limp, pretending to be dead. A non-responsive body does not provide the stimuli that he needs to proceed with mating, so he literally drops her and flies away.       

Photos online show robber flies eating dragonflies and also dragonflies eating robber flies. Imagine the drama, strategy and action when these two ferocious predators and acrobatic fliers meeting in the air.