Last fall, I was cleaning up some leaf litter and decaying wood after dark when I uncovered a tiny pair of green glowing eyes (or something) in the debris. These bioluminescent spots belonged to a multi-segmented, flattish critter approximately 2 cm long, black on its back and red underneath. The tail was forked, not unlike that of an earwig. Any ideas what it was?
–Len Aitchison, Hillsdale, Ont.
Entomologist Bob Anderson of the Canadian Museum of Nature tells us that what you uncovered was the larva of a firefly – likely the Pyralis Firefly (Photinus pennsylvanicus). The bioluminescence of these little creatures – which comes from a light organ near the tail – is caused by a mix of body chemicals that results in a cold, usually greenish-yellow light. The light organ of the larva you saw may have been divided, which may explain why you saw two lights on the same creature, Anderson says. Like other members of the beetle family, firefly larvae have two protrusions – tubular sensory structures called urogomphi – off the end of the body.
The larvae overwinter in the leaf litter in their pupal stage, then emerge in the spring a adult fireflies. Adults use their bioluminescence to attract mates. Each firefly species has it’s own light sequence – combinations of long and short flashes – which is how members of the same species recognize one another. Some species will purposely attract another species by mimicking its flash pattern. When a lovestruck member of the other species comes by, Anderson explains, “it’s dinner.”