Firefly, a name applied to many 'phosphorescent' insects, which are all included within the order Coleoptera, or beetles. Some of them (among the Elaters) give forth a steady light, and these may be distinguished as fireflies proper from the Glow-worms (q.v.) and 'lightning-bugs' (among the Lampyrids), which flash light intermittently. In the phosphorescent Elateridae, furthermore, the production of light has its seat more anteriorly than in the Lampyridæ, where it is abdominal. As the Lampyrids will be discussed under GLOW-WORM, only the Elaterid fireflies need be noticed here.
The most brilliant fireflies are species of Pyrophorus, most at home in tropical America. One form—Pyrophorus noctilucus—common in the West Indies and Brazil, attains a length of about an inch and a half, and has a dark rusty-brown colour. On the upper surface of the first ring of the thorax are two yellowish oval spots, which are brilliantly luminous during the nocturnal activity of the beetle, while on the first ring of the abdomen a still brighter organ is situated. Even the eggs are luminous, and excised portions placed in a damp chamber remain functional for two or three days. The pounded debris of the insect is also luminous. The luminous organs are special modifications of the epidermic (hypodermis) cells, which are disposed in two layers, of which the outer alone is luminous, while the inner contains masses of waste products, and is riddled by air-tubes. The luminosity depends on a process of oxidation; the oxygen is supplied by the tracheæ, and the brilliancy varies with the respiratory activity, being (according to Heinemann) only indirectly affected by the nervous system. What the substance oxidised really is remains undiscovered. On the sleeping or entirely passive insect a soft light may be observed; the real blaze is only exhibited during active respiration, and may be exaggerated experimentally by blowing in an extra supply of oxygen. Dubois's experiments seem to show that the fireflies utilise their phosphorescence to guide their steps.
The light of these living lamps, or cucujos, as the Mexicans call them, has frequently saved a traveller's life; the Indians 'travel in the night with fireflies fastened to their hands and feet, and spin, weave, paint, and dance by their light,' and they are often used for decorative purposes. A few of them in a glass vessel give light enough to read by, and when not wanted for any such purpose they may be set to catch mosquitoes in the house. In Vera Cruz, according to C. F. Holder, the Pyrophori are so commonly used as toilet ornaments that they form an important article of trade. The natives lure them by means of lights waved on the end of sticks, and catch them in nets. 'They are then placed in a box covered with a wire netting, bathed twice a day in tepid water, and at night fed with sugar-cane.'
See GLOW-WORM, PHOSPHORESCENCE; C. F. Holder, Living Lights (Lond. 1887); R. Dubois, Les Elatérâdes lumineux (Meulan, 1886); F. F. Moore, Fireflies and Mosquitoes (Lond. 1888); and numerous papers quoted by Holder. For a detailed account of the anatomy and physiology of Pyrophori, see C. Heinemann, Archiv. f. mikr. Anat. XXVII. (1886), pp. 296-382.