One evening in 1966 near Lake Placid, Florida, one of North America’s most beautiful moths flew into the web of a great orb-weaving spider. Trapped moths usually struggle desperately to escape, but this magenta-hued beauty, its wings boldly spotted with black-in-white bull’s eyes, lay unperturbed as the spider crept ever nearer. When it reached the moth, instead of delivering a killing bite, the spider hesitated for a moment, and then gently cut by turn each of the silken threads holding the insect, until it fluttered free.
We might be tempted to read into this story a Beauty and the Beast–like allegory. And in a way it is, for the moth’s beauty provides very real protection from the many beasts it encounters. Yet as Thomas Eisner, who witnessed this event, would discover, that beauty acts more by way of warning than heart-softening appeal, for it informs potential predators that beneath its skin-deep veneer flows poisonous blood.
Eisner is a professor of chemical ecology at Cornell University. One of the world’s eminent natural historians, he is also an award-winning filmmaker, the author of numerous landmark scientific publications, and a recipient of the US National Medal of Science. His latest book, For Love of Insects, tells the story of his research, along the way revealing how hard-won his acclaim is. His single observation of the spider and the beautiful moth, for example, led to thirty years of painstaking research on the chemical defenses of the ornate moth (Utetheisa ornatrix), revealing a life of extraordinary intricacy and complexity.
The moth’s chemical protection is acquired from its sole source of food, the rattlepod bush (Crotalaria spp.). Plants of the rattlepod genus produce alkaloids in order to defend their leaves and seeds from herbivores. The toxin can kill most animals, but the larva of the ornate moth is immune—indeed it prefers to feed on the plant’s most poisonous tissues, such as its seeds. The alkaloids thus acquired are stored in the insect’s blood, and after the caterpillar has transformed itself into a glorious moth, the toxins are deployed in frothy blood that is extruded from near its wingbases.
Understanding how the moth stores and uses its toxin for defense was only the beginning of Eisner’s discoveries. Through a series of ingenious experiments that involved rearing some caterpillars on alkaloid-free food, he discovered that the toxin is essential to the moth’s sex life. Males reared on a toxin-free diet seemed normal in every respect, yet they received a brusque brushoff from females whenever they attempted copulation. High-speed photography revealed that, when intent on copulation, male moths extend two large brush-like structures from a cavity called the cloaca, which are used to stroke the female for a few milliseconds prior to copulation. High-resolution microscopy showed that the brushes consist of soft, hollow scales that are filled with a derivative of the alkaloid toxin. Only those males that tar their mates with a toxic brush, so to speak …