8 A PHYSICAL STUDY OF THE FIREFLY. 



In photogenic bacteria the color of the light* appears to depend upon the 

 environment and the culture medium ; and in any one medium the light is 

 described as silvery white, bluish, greenish, or tinged with orange, depending 

 upon the species. Among the mushrooms there are similar variations, one 

 species emitting a bluish, another a greenish, and still others a whitish light. 



In addition to the fireflies investigated, through the kindness of Mr. F. 

 A. McDermott (then connected with the Hygienic Laboratory of the U. S. 

 Marine Hospital Service), an opportunity was granted to examine a large 

 glow-worm (female of Phcngodes laticollis) which along its sides emitted 

 (in spots of about i mm. diameter) a rich greenish light; also an Elater 

 (Pyrophorus noctilucus) from Cuba, of which the eye-spots (i mm. diam- 

 eter) showed a weak greenish glow; and a culture of photogenic bacteria 

 [Pseudomonas lucifera Molisch), which appeared a rich green in color though 

 weak in intensity. Because of the weakness of the light, which would have 

 required an exposure of eight to ten hours, using the large spectrograph, 

 no attempts were made to photograph the light of these animals. From 

 the data to be discussed presently, it will be possible to give an estimate of 

 the quality of the light emitted by these animals without subjecting it to 

 rigid examination. 



Some insects are described as emitting a "red light." If this is the "car- 

 dinal" or the "scarlet" one usually thinks of when reading such a descrip- 

 tion, the color must be very remarkable and deserves investigation. First 

 of all, it would be desirable to know whether the description is correct and 

 whether the "red" may not in reality be the "orange red" observed in 

 Photinus scintillans. 



Tests have been madef to decide on the color of the light emitted by 

 fireflies by examining them in the light of a kerosene or an incandescent lamp. 

 The writer has tried the same experiment and concluded that such observa- 

 tions are meaningless, for the reason that the light observed is composed of 

 that emitted by the firefly and of that emitted by the lamp, which, by 

 internal reflection in the photogenic cells, is returned to the eye. The 

 resultant color of the combination can hardly be considered a criterion for 

 judging the color of the (unmixed) light emitted by the insect. 



The popular notion is that the maximum emission of the sun and the 

 maximum color sensitivity of the eye coincide ; and that this is the result of 

 evolution of the eye so as to utilize the sunlight to the best advantage. In 

 the same manner comments have been made on the supposed coincidence of 

 the luminosity curve of the eye and the luminosity of fireflies. In the 

 attempt to harmonize all phenomena, the fact is overlooked that the struc- 

 ture of the (compound) eye of an insect is entirely different from the human 

 eye. It remains to be shown that the insect eye is sensitive to color with a 

 maximum of sensibility such as obtains in the human eye. 



*Molisch, Lcuchtentle Pflanzen, Jena, 1904. 

 I'Knab, Canadian Entomologist, 37, p. 238, 1905. 



