From Deep Look:
Females of one firefly group, the genus Photuris, have learned to copy other fireflies' flashes to attract the males of those species. When one arrives, she pounces, first sucking his blood, then devouring his insides…Firefly light is biochemical. But fireflies like the Big Dippers do much more with chemistry than just make light. They can mix together an array of other compounds, including invisible pheromones for mating, and others called lucibufagins ("loosa-BOOF-ajins") that ward off predators like spiders and birds.
At some point, the Photuris "femme fatale" fireflies lost the ability to make their own lucibufagins. So instead of chemistry, these bigger, stronger fireflies became adept at imitation, and evolved to turn into insect vampires to take these valuable compounds from other fireflies to boost their own defenses.
