While other fireflies light up warm summer nights, males and females blinking in ardent semaphore, females of the genus Photuris aren’t always in the mood for love. These flickering femmes fatales often fake the response of females of another firefly genus--Photinus--to lure in eager Photinus males. Then they kill and eat them. It’s not my idea of romance, says Tom Eisner, an entomologist at Cornell. Nonetheless, these deadly females have excellent reasons for their treacherous behavior, he has found. They eat Photinus males to protect themselves from predators.
A few years ago, Eisner and some colleagues discovered that many fireflies are distasteful to birds. The insects carry chemicals in their blood that he calls lucibufagins (from Latin meaning light toad, since similar toxins are found in some toads), a new class of toxic steroids. When fireflies are disturbed--attacked by a spider, say, or picked up by a pair of forceps--they reflexively emit droplets of blood. Predators that taste the stuff quickly let go of their prey.
Eisner decided to examine every firefly species he could to see which ones carry the toxins. (Local children collected his specimens and received a penny for each firefly bagged.) Photinus fireflies always had lucibufagins in their blood, as did adult Photuris females. But Photuris females newly emerged from their pupae did not. Since we knew that one ate the other, we put two and two together, he says.
In his lab, Eisner found that female Photuris fireflies that had never consumed a Photinus were attacked and eaten by jumping spiders. But when spiders picked up a female Photuris that had eaten a male Photinus, or one that had been fed lucibufagins, they immediately dropped it. Lucibufagins are so potent, he estimates, that a human would die by eating about 20 Photinus fireflies. He doesn’t yet know how the female Photuris fireflies resist the toxins but points out that many animals can eat substances that would kill other species.
One firefly, however, is quite enough for a female Photuris to ward off spiders during her life span, and it provides her with a nutritious meal that’s nearly two-thirds her size. It’s a hell of a big meal, says Eisner, like me eating a Labrador retriever.