Recently I’ve learned via Twitter that lots of cats (dogs, too) enjoy chasing after fireflies. But please don’t let your pets eat the fireflies! This is for their own sake, not for the fireflies.
Living in a world full of insect-eating predators, fireflies have evolved potent defenses to protect themselves. Firefly blood carries a bitter-tasting and toxic chemical called lucibufagin (lucifer = Latin for light bearer + Bufo = toads that produce similar chemicals).
Lucibufagin (often nicknamed LBG) makes a powerful poison because it is effective against almost any animal. It binds to and disables an enzyme, known as the sodium-potassium pump, that’s essential for all animal cells. This enzyme actively transports ions across cell membranes, generating an electrical potential that allows us to do really important things like think and move our muscles. To protect themselves, many plants and a few animals have converged on stocking similar toxins in their chemical arsenal.