Entomology: Lifesaving Light

Light from a single, well-defined source holds such a strong attraction for night-flying moths that a flickering candle flame can lure them to a fiery death. But when light comes from all angles, as in a brightly lit room or outdoors during daylight, some moths cease all activity, as if they had been "turned off." Scientists have long wondered: What throws the switch?

In the current issue of Applied Optics, Entomologist Philip Callahan, of the Department of Agriculture, reports on delicate experiments with which he answered the question. Callahan caught some giant cecropia moths, which live in the woods, studied them under a binocular microscope and decided that it was tiny spikes at the base of their delicate, fernlike antennae that reacted to strong light. To check his theory, he blacked out the moths' eyes, painted each antenna black, except for the tips of the spikes, and ran minuscule wires into the main antennal nerves. Then he began subjecting them to light of varying intensity.

As Callahan had suspected would be the case, the spikes proved to be precisely tuned receivers highly sensitive to the wave lengths of visible light. At the base of the spikes, tiny sensors transformed the light into nerve impulses that sent electrical signals to the brain. Under strong light, those impulses automatically blanked out the sense of smell and responses to temperature and humidity on which a moth relies as it flies around in search of a mate or a place to lay eggs.

The moth's spikes, a scant 26 ten-thousandths of an inch long, provide it with an automatic lifesaving device. Without them, says Callahan, the slow and conspicuous insects would probably take wing during daylight. And if they did so, they would make themselves easy prey for birds.