Encounter: Robber Fly, raptor of the insect world

With the mean looks and businesslike patience of an assassin, the Robber Fly captures its prey on the wing

Robber Fly. The first time I heard that, I thought, "Why does it have such a name?" I learned that they stay on a perch waiting for any unlucky victim to pass by and quickly catch them on the wing, just as highway robbers waylay and attack passing vehicles.

I saw my first Robber Fly quite recently. While birding, something caught my eye. A large insect flew low, carrying something equally big in its legs. As I watched, it landed on a log. I looked through my camera and zoomed for a good look without spooking the insect. The fly-like predator was holding onto a cicada. I wrote it off immediately as a scavenger feeding on dead insects. Only later did I learn the true identity of this mean-looking fly.

Robber fly with its dinner, a Cicada
Robber flies are also known as Assassin Flies -- makes them sound more vicious, doesn't it? Vicious or not, they are voracious eaters, willing to eat almost any insect or small vertebrate that they can overpower, including other Robber Flies! Their hunting strategy is to wait on a perch from where they can scan the surroundings. When some unfortunate insect flies past, the Robber Fly intercepts and attacks it. The victim is then carried off to be consumed at leisure.

Robber Flies, which include around 7,000 species distributed worldwide, are grouped under the family Asilidae in the order Diptera, comprising 'two-winged' flies. They have strong spiny legs which help them grab their prey. Their huge compound eyes help them detect movements quickly. These eyes are protected from the thrashing wings or legs of their prey by outwardly projecting hairs located between their eyes. After pinning down the prey, Robber Flies use their strong and sharp proboscis to inject neurotoxic venom and digestive juices, which immobilize the prey and start liquefying its body from the inside. This slurry is then sucked up by the fly using its proboscis.

Beej adds: Robber Flies deliver painful bites. While trekking in Brahmagiri, Coorg, I made the mistake of using deodorant and then wearing a sleeveless vest while walking the forest trail. For some reason this attracted Robber Flies. I deduced later that the deodorant had attracted a number of flies and other winged insects and the Robber Flies were in hot pursuit of easy targets. Occasionally, the Robber Flies bit me when I tried to fend them away with my hand. The bites turned into fluid-filled vesicles that hardened into bubble-shaped cysts on the skin between my knuckles and refused to go away for many weeks.

Text and photo by Arun Menon