by Panceri, Quatrefages and other scientific men. But perhaps the most remarkable luminous marine organism is the bivalve, Pholas dactylus, known to the French as the "Pholade," and to the Germans as the "Bohrmuschel." This creature has definite luminous organs, whose tissue and secretions are strongly photogenic. It has been the subject of interesting researches by Dubois, and has been shown to react in a manner similar to that of other luminous forms. More recently, certain peculiar organs possessed by deep-sea fish have been determined to be light organs, and thus it appears that in the depths of the sea they need "artificial" light, when the sun's light fails to penetrate, just as on land when the sun is hid.
By far the most brilliant and most commonly known form of physiologic light is that given by the so-called fire-flies; this term embraces a large number of species of insects, mostly Coleoptera (beetles) of two or three genera. Besides these Coleoptera, there are a few luminous forms distributed among the other insects, together with certain myriapods, worms and other occasional forms. In a very few instances luminosity of more highly organized forms has been reported, but for the most part these appear questionable at least. Of the non-coleopterous insects, Diptera (Chironomus) and Hemiptera (Fulgoridæ) are said to be luminous; the hills of the South American termites (Neuropteræ) have also been observed to be luminous.
The majority of the insects commonly called fireflies belong to the genus Lampyridæ, including the Italian luciole (Luciola italica), the English and continental glow-worm (Lampyris noctiluca), the continental firefly (Lampyris splendidula), the American fireflies and "lightning-bugs" (Photinus pyralis, Photuris pennsylvanica, etc.), and a vast number of other luminous insects. Further south, as in Cuba, Mexico and Brazil, the more brilliant insects belong to the genus Elateridæ,-and embrace the cucuyo (Pyrophorus noctilucus and the cucuyana (Pyrophorus physoderus). In India there is said to be a luminous buprestid beetle.
Thus it will be seen that, so far from being a rare phenomenon, the emission of physiologic light is one of well-nigh universal distribution, and appears to be an important function in the life of those organisms possessing photogenic activity.
While most of the facts here given apply primarily to the fireflies, they may, in great part, be taken as true for the entire phenomenon of physiologic light. Different forms may show variations in color, intensity and mode of emission of the light, but basically it all seems to revert to the same cause—a cause as yet, however, unknown.
The light given by luminous insects is usually stated by authors to be greenish or yellowish; a few have claimed to observe insects to emit a reddish or bluish light, and marine forms have been reported to emit
