This group gathers Insects subcategory members belonging to the order Coleoptera, the beetles, the single largest order in the entire animal kingdom by number of described species, defined by a hardened, shell-like pair of forewings, the elytra, that meet in a straight line down the back and protect a functional pair of flight wings folded beneath. It spans the ground beetles, fast-running predators found under stones and leaf litter worldwide, the scarabs, including the dung beetles and the enormous rhinoceros and Hercules beetles, the ladybirds, familiar aphid-eating predators, the fireflies, whose bioluminescent courtship flashes are produced by a light-emitting chemical reaction, the weevils, recognized by an elongated snout, the click beetles, capable of snapping their body straight with an audible click to right themselves or escape a predator, and destructive pest species such as the boll weevil and the mountain pine beetle.