Black Horsefly

This is not a fly you want to see lighting on any part of your body, not that there are any flies that you’d really like to see light on yourself. But unlike most flies, this guy, the black horsefly, Tabanus atratus, has scissor-like jaws and a taste for blood.

And through its bite, it can transmit a variety of diseases, including anthrax.

Fortunately, we humans aren’t high on its menu, although they don’t completely eschew our blood. Blood is blood, after all, but their tastes run toward cattle, horses and other creatures.

Black horseflies are found over most of eastern North America from southeastern Canada to Florida and west to the Rocky Mountains.

They range in size from 2.1 centimeters long — slightly less than an inch — to 2.8 centimenters, slightly more than an inch. They are blackish in color, a mix of dark grays, blacks and even dark purples. The name, atratus, means clothed in black, which is the inspiration for another common name, the mourning fly.

Black horseflies have massive eyes that wrap around the head. The eyes in males actually touch each other; females have a space separating the eyes.

Black horseflies need wet conditions in order to breed, laying their eggs on vegetation along the edges of wetlands, either over water or wet soil. They’ll spend the first part of their lives there as larvae. As adults, they’ll stick close to home, seldom straying more than a mile or so from where they were born.

Black horseflies are like mosquitos in that it’s the females that have the taste for blood rather than the males. Both male and female horseflies will feed on the nectar of flowers, but in order to reproduce, females need protein and get it by feasting on blood. Males don’t have the mouth parts necessary to cut into skin, so they stick to the sweet stuff throughout their short adult lives.


Both horseflies and mosquitos also have anticoagulants in their respective salva to keep the blood of their victims flowing. Mosquitos also have anesthetic that makes their jabs painless to the victim and allows them to do their thing without being detected; black horseflies don’t, making their cuts painful. Cattle victimized by a swarm of horseflies have been seen dripping with blood.

But black horseflies and mosquitos differ in how they “enjoy” their meals. As noted, horseflies cut their victims, but then they lap up the blood using a spongy part of their mouths called labella. Mosquitos, on the other hand, have a syringe-like probiscus that they use to stick their victims and suck up their meal, straw-like.

And while black horseflies do carry and transmit infections, they’re nowhere near the disease vectors that mosquitos are.

Horseflies lie in wait under the shade of shrubs and trees waiting for a nice, blood-filled target to wander by. In large measure, they use those massive eyes to spot their prey; smell and carbon dioxide emissions play a role as well. Dark colored, moving animals are most likely to be assailed. Horseflies are most active in the hours after dawn and before sunset. Temperatures and sky conditions can affect their activity.

The larval stage in black horseflies lasts two years. Black horsefly larvae are aggressive hunters; their prey includes both invertebrates and vertebrates. They’ll even attack, and eat, each other.

The lifespan of adults, is short, however, males shorter than females.

Black horseflies are member of Tabanidae, the horsefly family.

Loxahatchee National Wildlife Refuge