Giant Black Robber Fly

About the Giant Black Robber Fly

The Giant Black Robber Fly is the largest robber fly in North America, and one of the most imposing insects in Texas. Big females can top two inches. The body is dull black with a faint gray bloom, the wings are long and smoky, and the legs are black and heavily armed with spines. The face carries a thick white beard of bristles, called the mystax, framed by huge dark eyes. Pale hairs often show at the sides of the thorax and at the base of the abdomen.

The species is a southern Great Plains specialty, found mainly in Texas and Oklahoma, where it hunts open grassland, prairie remnants, and brushy fields. Like other robber flies it hunts by ambush. It watches from a stem, twig, or fence wire, launches at a passing insect, grabs it with its spiny legs, and stabs it with a short, stiff beak. Saliva injected through the beak paralyzes the prey and liquefies its insides, and the fly carries the meal back to a perch to feed. Its size lets it take large, tough prey that smaller robber flies cannot handle, including grasshoppers, beetles, bees, and wasps.

Adults fly from late spring through summer. Females lay eggs in the soil, and the larvae develop underground, where they prey on the grubs and larvae of other insects before emerging as adults.

These photos come from Williamson County, including Lake Creek Trail, in August 2025 and May 2026.