FIREFLY, a term popularly used for certain beetles (Py rophorus) on account of their power of emitting light. The in sects belong to the family Elateridae (see COLEOPTERA). The genus Pyrophorus contains about 90 species, and is confined to America and the West Indies, ranging from the southern United States to Argentina and Chile. Except for a few species in the New Hebrides, New Caledonia, and Fiji, the luminous Elateridae are unknown in the eastern hemisphere. The light proceeds from a pair of conspicuous, smooth, ovoid spots on the pronotum and from an area beneath the base of the abdomen. Beneath the cuticle of these regions are situated the luminous organs, con sisting of layers of cells which may be regarded as a specialized portion of the fat-body. Both the male and female fireflies emit light, as well as their larvae and eggs, the egg being luminous even while still in the ovary. The inhabitants of tropical America sometimes keep fireflies in small cages for purposes of illumi nation, or make use of the insects for personal adornment. Some cases have been observed of large numbers of fireflies flashing in unison. The function of the flashing is probably to facilitate the meeting of the sexes. The name "firefly" is often applied also to luminous beetles of the family Lampyridae, to which the glow worm (q.v.) belongs.