4 A PHYSICAIv STUDY OF THE FIREFLY. 



of their lighting systems is no higher than our own (but their fuel supply is 

 evidently different from ours) and to learn that much of their secret may be 

 of value. 



Aside from this utilitarian question, there are others of equal importance. 

 For example, we know that closely related species of fireflies emit differently 

 colored light, variously described as blue, green, orange, etc. Is this color-'' 

 variation a subjective phenomenon, resulting from the variation in color 

 sensibility of the eye with variation in intensity,* or is it an objective 

 reality ? If the latter, then what are the laws governing the light emission ? 

 It is almost too much to hope to solve this problem in the near future, but 

 it seems possible to settle definitely the question whether or not the light 

 differs in composition and hence is "blue," "green," or "orange," as 

 observed. This knowledge is obtained from the spectral energy curves of 

 the light from various fireflies, to be described presently. 



Experimenters have become so accustomed to thinking of the artificial 

 production of light as being accompanied by a large amount of invisible 

 radiation that no experiments would be complete without a repetition of 

 I^angley's search for infra-red radiation. Accordingly this question has 

 also been given attention in this paper. 



In view of the doubt in many investigators' minds as to whether the color 

 of the light from the firefly is due to a variation in intensity rather than to 

 an actual variation in composition, it seemed worth while to attempt to settle 

 the question by making a thorough study of the light emitted by various 

 species of this insect. Accordingly, the photographic work was continued 

 for about four weeks, during which time 152 photographic exposures were 

 made, on two spectrometers differing widely in dispersion, the time of 

 exposure varying from 30 seconds to 5 hours. The time consumed in actual 

 photography, in holding the insects on the spectrometer slit, was over 56 

 hours. 



The histological data is introduced for clearness in discussing the present 

 work, as well as its bearing upon the whole subject. It permits also a 

 discussion of the plausibility of the functions assigned to certain parts of 

 the photogenic organs as viewed from a physical standpoint and more 

 especially from the optical properties of materials. 



While much has already been done on the light of the firefly, much more 

 remains undone, all of which no doubt will have direct bearing on the manner 

 of light production in animals. Among the important problems awaiting 

 solution are (i) whether, in animals having luminous organs emitting light 

 of different colors, the maxima of emission are different; and (2) whether 

 the maximum emission in any one photogenic cell is the same when the 

 light is emitted from the living animal and when the photogenic material is 

 excited to light emission by an oxidizing agent after the luminous organs 

 have been dried. 



Heretofore we have been led to think that "light" must be accompanied 

 by infra-red radiations ; and we seem utterly unable to turn away from this 

 idea. The various investigations of the firefly have without doubt opened 

 up a new line of thought in regard to the emission of heat and light. 



*Knab., Canadian Entomologist, 37, p. 238, 1905. Molisch, l^euchtende Pflanzen, 

 Jena, 1904. 



