Science: Love Song

How a male moth can find a female—even at night, through fog, and as far as seven miles away—is a favorite puzzle of entomologists. The male moth flies unerringly downwind, which rules out the possibility that the female sends out odorous particles. In the latest Interchemical Review (research house organ of Interchemical Corp.), John P. Duane and John E. Tyler, both of Interchemical, suggest a solution of the puzzle.

Duane (an amateur entomologist) and Tyler (a spectroscopist) teamed up to test the possibility that female moths send—and the males receive—mating calls in infrared (heat) waves. The researchers first took the temperature of the female night-mating moth with a tiny thermocouple buried in the fur of her thorax. They found that it might be as much as 11° above the temperature of the surroundings. Since all warm objects radiate in the infrared, the conclusion was that a hot-blooded female moth “must literally ‘shine’ against a background of cool forest objects.”

Next step was to study the character of the female’s radiation. Tyler knew that many organic chemicals, including some contained in moths, send out characteristic patterns of radiation with peaks on, certain frequencies. So he examined a virgin she-moth with an infrared-recording spectrophotometer. Sure enough. She did not “send” evenly on all wave lengths. Her radiation curve showed a pattern with peaks and valleys. A male equipped to receive infrared might recognize this coded signal as a she-moth’s love song.

How does the male receive the signal? With his antennae. Night-mating male moths have magnificent, feathery antennae. If they are cut off, the amputee has trouble finding a female.

Duane & Tyler examined microscopically the tiny hairs on the side plumes of a male’s antennae. Their typical length varied from 40 to 80 microns (.00156 to .00312 inch). A more significant finding: “All variations in the length of the hairs appeared to be close to four microns or multiples thereof. It is noteworthy that four microns is one-half the wave length of eight microns, which is well within the emission band of the female.” Duane & Tyler suggest: “The male . . . moth has a tuned antenna array which is his receptor for locating the female.”