The all-too-familiar American cockroach almost seems to know where you're going to strike. What's the tip-off that sends the cockroach running?
Entomologists who have investigated this have found that cockroaches detect the puff of wind generated by a nearby moving object. They run away from the wind.
And it has to be a puff of wind, not a steady breeze. To make the roach run, the wind speed has to increase sharply over a small fraction of a second.
Cockroaches have special organs for detecting puffs of wind. Extending from the rear end of a cockroach are two tapered appendages, the so-called cerci.
The underside of each cercus has about 220 delicate hairs connected to the roach's nervous system. When these hairs are struck by a puff of air, they cause nerve impulses to be sent to the insect's leg muscles.
Each hair can flex a little more easily in one direction than in other directions, so each hair is especially sensitive to wind from a particular direction. Nerve impulses from the hairs are sent to the roach's legs in just the right pattern to cause the roach to turn away from the direction of the wind. A puff of air from the left causes impulses to be sent to the cockroach's left legs; as a result, the roach turns right, away from the wind.
A cockroach with one cercus damaged or removed makes wrong turns. A roach with both cerci damaged or removed doesn't try to escape at all.
So cockroaches get away because they can detect the puff of air that arrives ahead of a threatening object. The sensors are delicate hairs on the undersides of the cerci, the tapered appendages extending from the insect's rear end. Within a twentieth of a second after a sharp puff of wind strikes those hairs, the roach begins running the other way.