12 A PHYSICAL STUDY OF THE FIREFLY. 



searchlight, which projects the light in a fixed direction. The pocket shape 

 of the photogenic cell might be brought about by gravity. The refractive 

 index of the material composing the so-called " reflecting layer" cannot differ 

 greatly from the water and other constituents in the photogenic cell and 

 hence there can not be a distinct reflecting surface. As stated elsewhere, 

 a careful examination of a firefly, Photinus pyralis, shows that the whole 

 dorsal region of the abdominal segments containing the photogenic material, 

 except the narrow median line P, Fig. 4, II, which is filled with dark pigment, 

 is aglow (in fact flashes strongly) when the insect emits its flash. This 

 could not occur if the so-called "reflecting layer" performed, to any appre- 

 ciable extent, the function attributed to it. 



Although from the earliest times the phenomena of light emission by 

 animals, "biological light," has attracted the attention of observers, only 

 within the past century has any serious investigation been made of the 

 photogenic organs. According to Townsend (1. c.) there now seems to be 

 a general agreement among experimenters that in the male lampyrids the 

 photogenic organs are composed of two well-defined layers: the dorsal, 

 chalky, opaque layer, R, and the ventral or true photogenic layer, L (see 

 Figs. 1 to 4). The latter is composed of two distinct elements: the tracheal 

 structures and the intermediate areas of parenchyma. The main tracheae 

 (see illustration, Fig. 4, III) of the photogenic segments send vertical 

 branches down through the light organs. These tracheae ( T, Figs. 3 and 4) 

 appear to connect with the main breathing system ; and from this evidence 

 some hold to the belief that the light is produced by oxidation. " The entire 

 system suggests that the air is drawn in through the breathing tracheae, and 

 forced through the line passages in the true photogenic tissue, where the 

 oxygen of the air is consumed in a biologic oxidation."* 



While this seems a highly plausible explanation, the details as to the 

 manner in which the light production is regulated do not appear so simple. 

 For, if the data on the temperature of the luminous segments, presented on 

 a subsequent page, has any significance, the oxidation seems a continuous 

 process, while the light production (the intense flashes) is controlled at will. 

 One peculiarity in the control of the light emission is the manner in which 

 the flashes may be emitted in one segment or in one small part of the lumi- 

 nous segment while all the rest of the luminous organs are emitting a bright 

 glow. A slight pressure on the luminous segment apparently paralyzes that 

 region and it begins to emit a strong glow, while at intervals the usual 

 flashes are emitted from the uninjured, non-glowing parts. 



The dorsal layer (R, Fig. 3) is composed of fairly regular polygonal cells, 

 Filled with white crystalline material, which is supposed to be urate salts 

 (either guanine or ammonium urate, or both ) . This is the so-called "reflect- 

 ing layer," the idea to be conveyed being that this material is present to 

 shield the central nervous system, lying directly above it, from the light 

 produced in the layer below it, and more especially to act as a reflector to 

 increase the illumination in the direction desired. From an optical stand- 

 point this does not appear very convincing; for the optical properties of a 

 granular mass of material such as described, and of the thickness indicated, 

 would have a low reflecting power and would be but little more than a dif- 



*McDennott and Crane, Amcr. Naturalist, 45, p. 306, 191 1. 



