n6 THE POPULAR SCIENCE MONTHLY 



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 man- 

 ner 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 em- 

 braces 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 cer- 

 tain myriapods, worms and other occasional forms. In a very few 

 instances luminosity of more highly organized forms has been re- 

 ported, but for the most part these appear questionable at least. Of 

 the non-coleopterous insects, Diptera (Cliironomus) and Hemiptera 

 (Fulgoridas) are said to be luminous; the hills of the South American 

 termites (Neuropterse) have also been observed to be luminous. 



The majority of the insects commonly called fireflies belong to the 

 genus Lampyridas, 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 

 Elateridae,- 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, in- 

 tensity 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 



