A PHYSICAL STUDY OF THE FIREFLY. 



CONTENTS. 



Page. Page. 



I. Introduction 3 



IT. Habitsof the Various Species Investigated. . 5 



III. Composition of the Light of Fireflies G 



IV. The Function of the Light emitted \>y Living 



Organisms 9 



V. Histology of the Light Organs 10 



VI. Methods of Investigation of the Light 



emitted 14 



VII. Spectral Energy Curves of the Light 



emitted by Various Species of Fireflies. . IS 

 VIII. Luminous Efficiency and Candle-power 



Measurements 26 



IX. Radiation and Temperature Measurements. 29 



X. The Fluorescent Substance in Fireflies. ... 37 



XL Infra-red Absorption Measurements 40 



XII. Nature of the Light emitted 43 



I. INTRODUCTION. 



During the latter part of the summer of 1908 the first attempt was made 

 to measure the radiation from the local species of firefly. The season was 

 late and but little was accomplished beyond the acquisition of some knowl- 

 edge of the habits of these interesting insects. The following summer, in 

 collaboration with Dr. H. E. Ives,* the spectral energy distribution of the 

 light from the species Photinus pyralis was obtained by photographic pho- 

 tometry. Last year an attempt was made to obtain photographically the 

 spectral energy distribution of the species Photurls pennsylvanica, which 

 comes earlier in the year. As mentioned elsewhere, f the work was begun 

 too late in the season, and in the course of only a few days this species had 

 disappeared for the year. However, in spite of the lack of sensitiveness of 

 the photographic plates in the red, there was considerable evidence that 

 the light from the Photurls, which to the eye appears a greenish blue, is quite 

 monochromatic, being deficient in the yellow and red rays which are promi- 

 nent in the light of the Photinus. Whether the maximum emission is very 

 different in these two genera remained undetermined until this year. The 

 work requires an intimate knowledge of the habits of the various species, 

 the lack of which has been the cause of most of the delays in reaching some 

 conclusion in regard to the various questions under discussion. 



One of the foremost problems which now occupies the attention of investi- 

 gators is the improvement of our methods of illumination. It is recognized 

 that our present methods of light production are of the most primitive type, 

 and yet we seem helpless in the matter. Recognizing our helplessness, we 

 should not hesitate to turn to other animals having photogenic systems 

 which apparently are far more efficient than our own, and try to learn their 

 methods of operation. It may be that in the ultimate analysis the efficiency 



*BulI. Bur. Standards, vol. 6, p. 321, 1909. 

 tRleetrieal World, 56, p. 1012, Oct. 27, 1910. 



