On Halloween I found this little monster inside a rotting log. It’s a firefly larva (family Lampyridae), and it’s really only monstrous if you’re an organism on this predator’s dinner menu. These beetle larvae feed on land snails, slugs, and the larvae of other insects, and each species often has some specificity for its prey of choice.
Take a look at the pale terminal abdominal segment at the left end of the individual above. Like adult fireflies, the larvae use organs located here to produce bioluminescence. While adults use it primarily as a mating signal, larvae probably use it as a warning signal to potential predators. Fireflies contain chemicals that are distasteful or toxic. Anything that may try to eat one may make an association between the flashing and the sickening taste. After that, the predator may avoid flashing prey.
This individual must have been getting cozy for winter. They overwinter in the larval stage by hibernating underground, under bark, or in rotting wood. In the spring they continue feeding, pupate, and then by early summer they emerge as adults.