Robber flies (Asilidae family) are formidable aerial predators, often called “assassin flies” due to their aggressive hunting style against other arthropods. They focus solely on insect prey, not warm-blooded animals like humans. Robber flies do not bite or sting people to feed, but they use a specialized mouthpart defensively if they perceive a threat.
The Defensive Nature of the Stab
The action a robber fly takes against a person is never predation, as they are not blood feeders and are not attracted to human skin. They are docile around people and actively avoid contact. The only circumstance in which a robber fly uses its piercing mouthpart is when it feels physically threatened or is accidentally restrained. This defensive action occurs if a fly is carelessly handled, trapped between clothing and skin, or if a person attempts to swat or crush the insect.
The Anatomy and Immediate Effects
The robber fly does not have a stinger like a bee or wasp. Instead, it possesses a distinct, rigid, beak-like mouthpart called a proboscis. This structure is a pointed, hypodermic-needle-like tool designed for piercing the hard exoskeletons of insect prey. When defending itself, the fly uses its proboscis to deliver a sharp, penetrating “stab.” During the stab, the robber fly injects specialized saliva containing neurotoxic and proteolytic enzymes.
This potent cocktail paralyzes and liquefies the internal tissues of its prey. When delivered into human skin, the sensation is immediately sharp and painful, often compared to a bee or wasp sting. The effects are temporary and resolve within a few hours. The defensive stab is not medically dangerous and the fly is not known to transmit diseases.
Identification and Predatory Role
Robber flies are a diverse family with thousands of species worldwide, varying greatly in size and appearance; some mimic bees or wasps. They share several distinguishing physical characteristics. The first is a noticeably sunken area on the top of the head between their two large compound eyes. Many species also have a unique, dense patch of bristles around their mouthparts, known as the mystax, which resembles a thick mustache. Their body shape is often robust or humpbacked, and their strong, spiny legs help them grasp prey in mid-flight. These insects are generalist predators, attacking and consuming almost any insect they can overpower, including pest species such as grasshoppers, beetles, and other flies. Their predatory nature makes them beneficial components of the ecosystem, controlling the populations of many other insects.