The forewings of Coleoptera are highly modified into hardened, thickened, protective, shield-like structures, the elytra (singular, elytron). The primary wing venation is totally obscured, but longitudinal rows of pits or narrow striae are often present. The membranous hind wings are folded under the elytra in a manner unique to beetles. Flight is powered by the hind wings alone, although the outstretched forewings help glide and stabilize flight; in some flightless species the hind wings have atrophied and the elytra have fused together. In most beetles, the elytra cover the entire meso- and metathorax and abdomen, but in a few families, notably the Staphylinidae, the elytra are short and the abdomen is exposed.