Firefly Larva

I found this creature on decaying wood, hidden among a crop of jelly ear mushrooms I was collecting.  Although it looks like a dinosaur or a flattened crocodile, the tee-shirt fabric it is crawling on gives away its size, only 3/8" long.

I initially thought it was a sow bug, one of the flattened cousins of the familiar pill bug, both members of the wood louse family.  When disturbed it curled into a defensive ball, a poor imitation of a "roly-poly bug" from my childhood.  After wasting time trying to identify it, I sent it to Bugguide and within minutes received this response, identifying it as a Photuris larva.  Photuris is a genus in the Lampyridae or firefly family.

The common perception of fireflies is romantic, the flickering lights in the dark summer night, as the blinking male looks for muted signals from lady fireflies perched on the grass below.  That evening is just the dramatic finale to a hard life as both predator and prey.

After emerging from an egg, all lamprid (firefly) larvae are predatory, primarily eating juicy slugs, snails and earthworms as well as small insects.  They have mandibles with small channels that inject chemicals that will paralyze and kill their prey, then liquify them for digestion.  What the final adult eats aside from tree sap is unknown and some adult species probably don't eat at all. Meanwhile spiders, ants, birds and even some bats are trying to eat them.

All lamprids have light organs in the egg, larval and pupal stage.  This is true of even the day-flying (diurinal) species discussed in the recent blog, which lose their bioluminescence in the adult stage when they would seem to need it most.  What purpose the light serves in the larvae is unknown, although some experts theorize that the light is an aposematic warning, telling predators "don't eat me, I taste bad and could make you sick."

This warning is not an empty threat.  Most firefly species have steroidal compounds that makes them distasteful or even toxic to predators, and the bioluminescence may be acting as a warning to potential predators.  These compounds are chemically related to the toxins on the skin of poisonous toads, called lucibufagins, a word that combines the Latin words for light and toad. 

Photuris eating another firefly - Asknature.org- CC

Our larva above is in the genus Photuris, and one thing the adult females eat is known - other male fireflies.  These "femme fatale" fireflies have light flashing patterns designed to lure males of a different genus (Photinus), and it is not for sex.  These Photinus males have lucibufagins in their blood and when disturbed they can use "reflex bleeding" to release them as protection from jumping spiders and other predators.  Photuris females are aggressive predatory beetles and they are obtaining the blood of Photinus males, absorbing the toxic lucibufagins for their own defense.

It was early afternoon when I released my specimen on a dead log.  It crawled under the rotting bark looking for something to eat.  Thus ends my romantic story of fireflies.

Chris Barnhart had this to add:

"We should be seeing the adults soon. You can tell the Photuris males readily from their display - they have a greenish light and a faster pulse than the Photinus (remember Photuris as 'fast green'  and Photinus as 'slow yellow'."