The Case of the Headless Horse Fly
Horse flies are biting insects. They feed on the blood of their victims. The insect has a blade-like part of its mouth that it uses to slash the skin of an animal. The fly then sponges up the blood that emerges.
Horse flies feed on mammals, including horses, cattle and, occasionally, humans. The bite of the horse fly is painful; consequently, food targets of this insect are not willing participants. Horses and cows stomp and run. Humans swat and sometimes catch the offending insect.
When a horse fly is captured, the more sadistic among us have, at times, destroyed the insect by decapitation—an obvious effort to get even with an insect that would dare dine upon us.
When a horse fly is beheaded, a strange thing sometimes happens. The headless horse fly flies away! Now that is the stuff of horror movies or Halloween tales.
But how does a horse fly manage to fly without its head? Good question. Scientists who study such things really don't know either. However, there is much that is known about insect flight.
For instance, many insects fly when they sense danger. A horse fly, even a headless one, can be provoked to take flight by touching it. The touch displaces sensory hairs on the insect's body. A nerve impulse is transmitted from the hair, and the insect responds by taking flight. Obviously, that action does not require the presence of the insect brain in the case of the headless horse fly. The brain was, after all, removed with the head.
Some insects, including flies, take flight when their feet are no longer in contact with a surface. That response is a tarsal reflex action. It works in the same way as a reflex action in our knee when tapped with a mallet. The information is gathered by the feet, or tarsi, but is not transferred to the brain. The information goes directly to the wings. So if the headless fly is lifted off the surface, it begins flight activity, and away it goes.
How a headless insect maintains orientation in flight is another question that comes to mind. In general, insects make constant corrections for forces that tend to disrupt the stability during flight. These destabilizing forces are measured by hairs on the insect body and wingbeat patterns modified by the central nervous system.
Flies even have flight stabilizing organs. Called halteres, these organs replace the back pair of wings of flies and work like gyroscopes to monitor forces working on the insect during flight. The information appears to be processed in the central nervous system, not the brain.
What all of this means is that a headless horse fly does not need its brain to fly. And so for a few seconds, it can take off and fly—even approaching a horse fly's top speed of 22 miles per hour. But without a head, the horse fly cannot see where it is going and will crash into the first thing in its way. That is because the eyes of the insect are used to determine the flight path. Literally, the headless insect cannot see where it is going.
Or if nothing gets in the way, the headless flying fly does a dive into the earth when its systems begin to break down because of the loss of a brain. In the meantime, though, it is a sight to see a headless horse fly zooming off into the wild blue yonder. Especially if it has just taken a bite out of your ankle!