FIREFLY, a term popularly used for certain tropical American click-beetles (Pyrophorus), on account of their power of emitting light. The insects belong to the family Elateridae, whose characters are described under Coleoptera (q.v.). The genus Pyrophorus contains about ninety species, and is entirely confined to America and the West Indies, ranging from the southern United States to Argentina and Chile. Its species are locally known as cucujos. 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, consisting 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 illumination, or make use of the insects for personal adornment.
The name “firefly” is often applied also to luminous beetles of the family Lampyridae, to which the well-known glow-worm belongs.