Insect Masquerade

Naturalists have noticed for at least a century that insects have a way of mimicking each other. Butterflies of two species not closely related often show similar patterns of bright colors. Generations of entomologists have suspected that nature thus protects a butterfly that birds consider delicious by enabling it to resemble one that is distasteful to birds--but this theory has been widely debated and rarely tested experimentally. In Natural History, Biologists Lincoln P. and Jane Van Zandt Brower of Amherst College settle at least part of the argument about the survival value of nature's insect masquerade.

Instead of working with butterflies, the Drs. Brower selected two insects, the bumblebee and the robber fly, that are very distantly related but look very much alike. Both are covered with light and dark fur; both have hairy legs and buzz.

The robber fly even has bunches of light hair on its hind legs to resemble the baskets of pollen that the bumblebee usually carries. The big difference between the two: the bee can sting and the robber fly cannot. The two doctors reasoned that the robber fly's beelike appearance protects it from predators that fear the sting of real bumblebees.

To prove this theory, they put toads in cages and offered them live dragonflies, bumblebees and robber flies. Inexperienced toads accepted all three alike, but toads that grabbed the bees got stung. Once stung, they would eat neither bumblebees nor robber flies, though the flies, in spite of their appearance, could not sting them.

The Drs. Brower believe that this experiment proves the survival value of mimicry for insects that are able to make themselves sound, look or act like less attractive relatives. But they think that the robber fly may get a second advantage from its resemblance to the bumblebee.

Something of an ingrate, it enjoys eating its bumblebee benefactors, which it grabs from behind and stabs with paralyzing mandibles. The doctors suspect that bumblebees mistake their look-alike enemies for their own kind, thus make less effort to avoid them.

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