LIVING FORMS AND LIGHT 535 



tions of the year, of the month, and of certain hours of the early eve- 

 ning, during which the male is attracted from below to the luminous 

 female at the surface. Various species of earth worms have also been 

 reported to be photogenic, and it seems probable that the usefulness of 

 the luminosity in this case is somewhat the same as in the Odontosyllid 

 mentioned above. 



The bivalve PlioJas dactylus presents another anomaly, however, for 

 it lives a rather sedentary life, and is certainly not poisonous, at least 

 to man. Yet it possesses definite photogenic organs. Although it is 

 possible that in this case the luminosity is protective, we probably have 

 here one of the cases of the limited use of photogenicity, not yet dis- 

 covered. 



Among the Crustacea there are several interesting cases of photo- 

 genicity, and in regard to them, and indeed to the whole subject of the 

 use of " phosphorescence " in sea-forms, Alcock's interesting book " A 

 Naturalist in Indian Seas," is well worth reading. Certain shrimp- 

 like crustaceans throw out from glands, corresponding to kidneys, a 

 substance which in contact with sea-water produces clouds of bluish 

 light. There seems but little doubt that this is defensive in nature, 

 and acts in much the same manner as the " ink " of the cuttle-fish. 

 Some of these prawns are provided with enormous eyes, others with 

 only rudimentary ones, and some with none at all. Alcock also men- 

 tions a large spider-crab, which, although completely blind, " shone 

 like a star." Here we may readily conceive the light is alluring in 

 function, serving to attract the creatures on which the crab feeds. 



Among the insects we find the most widely known cases of photo- 

 genicity, and probably, also, the greatest field of usefulness. "With the 

 true fireflies, the Lampyridse, the evidence that has been collected tends 

 to show that the possession of the photogenic function is primarily 

 a secondary sexual character. It has long been known that if the fe- 

 male of the European glowworm (Lampyris noctiluca) were exposed 

 by night, a male would shortly come to it. The use of the photogenic 

 function as a secondary sexual character has also been shown for the 

 Italian luciola, and for certain of the fireflies common in the eastern 

 United States (e. g., Photinvs pyralis), and it appears probable that 

 the same thing applies to the entire family. Curiously enough, the 

 true " lightning bugs " show but little tendency to come to ordinary 

 lights, though in Phoiinus pyralis either sex will respond to a small 

 electric bulb operated in imitation of the light of the opposite sex. 1 



Among the Pyrophores, the Elaterid fireflies of the tropics, such as 



the cucuyo of Cuba, the luminosity very probably plays the same role 



as in the Lampyrids. These insects give a light which is continuous, 



though of varying intensity, instead of a flashing light as is emitted by 



1 McDermott, Canad, Entomol., 1911 (in press). 



