Science: Mosquito Psychology

What do mosquitoes think about?

Principally, sex and food. But the Army Quartermaster Corps, which has studied mosquitoes intimately in hopes of crossing soldiers off the mosquito menu, has discovered that the insects are complex psychologically, and react to a long list of stimuli. Last week it released its findings:

The Army’s Peeping Toms confirmed the theory (TIME, April 16, 1945) that the girl mosquito’s hum is sexual advertising. With tuning forks of the proper frequency, they posed as susceptible females, got chased by mosquito males.

Two species were studied: Aëdes aegypti (which carries yellow fever) and Anopheles quadrimaculatus (malaria). Aëdes males responded best to a mating call of 350-750 vibrations per second; Anopheles preferred a lower range (320-480). In spite of overlapping of ranges, no male made passes at a female of the wrong species. The Army’s conclusion: a female mosquito must have other attractions than hum.

Aëdes males become sex-conscious five hours after emerging from their pupal cases. But Anopheles males are moody. Sometimes the sweetest hum will not excite them. With antennae removed, both Aedes and Anopheles males are nonresponsive. If a female beats her wings at the wrong rate, she hums the wrong pitch, never acquires a mate.

Inner Calls. The study of hum appeal was pure science. More practical was the study of biting habits (female only). The Army learned that skin color makes little difference. A victim’s hand bathed in green light was bitten as readily as the same hand bathed in red. Mosquitoes, probably, can see only large masses in black-&-white. They are repelled somewhat by a black or a white shirt, prefer intermediate shades. A female mosquito with eyes removed can find a victim; but with antennae removed, she goes hungry.

Smell is probably more important than sight as a food-finder. Mosquitoes were attracted by air which had blown past a human being. They could also feel warmth at considerable distance. A human forearm cooled a few degrees did not interest them. If heated slightly it attracted them quickly. Moisture was important too. But on very damp days, a sweaty forearm was not as attractive as on dry days.

Army researchers (still continuing) had yet to produce the hoped-for result: a repellent based on mosquito psychology. But they had confirmed one popular theory: mosquitoes like some people better than others. The reason was still a mystery, except for the observation that mosquitoes dislike tough, hairy skins. Given a wide choice, they select a broad expanse of hairless human back.