o 



HISTOLOGY OF THE LIGHT ORGANS. 1 3 



fusing screen transmitting- almost as much as it reflects, and causing the 

 glow which is visible throughout the whole abdominal region when the flash 

 occurs. An examination of the dorsal side of the abdomen shows that only 

 along the median line, where the integument is dark, is the light absorbed. 

 Evidently the absorption is caused by the dark pigment. The rest of the 

 abdomen is all aglow during the flash. This could not occur if the "reflect- 

 ing layer" were efficient. 



The more radical views of Dubois* may be given equal weight. lie has 

 found the eggs of Pyrophorus noctilucus to be luminous even before they 

 were laid, so that the light appears to be transmitted in unbroken continuity 

 from one generation to the next. By following the development of the light 

 organs through all the different stages occurring from the beginning of seg- 

 mentation of the egg to the emerging of the adult insect, he has reached the 

 conclusion that the photogenic tissue in the light organs is derived directlv 

 from the underlying hypodermis, and that in the development of the light 

 organs a transformation takes place in the protoplasm of the cells, the older 

 cells toward the upper surface of the light organs becoming idled with 

 opaque, chalky granules forming the aforesaid "reflecting layer." 



Dubois has shown that the mollusk Pliolas dactylus produces photogenic 

 matter in the form of a liquid which remains luminous after filtering. The 

 photogenic liquid contains fine granulations that give it a cloudy appearance. 

 These granulations are of a colloidal nature which is lost in passing into the 

 crystalline, when the light emission ceases. The Orya barbarica of Algeria 

 secretes a luminous fluid which, under the microscope, shows photogenic 

 granulations that are said to become magnificent crystals during the light 

 emission. The theory of Dubois is that the light is produced as the result 

 of the action of an "oxidase" (oxidizing agent) upon another substance, 

 also of unknown composition. According to Townsend, the view generally 

 accepted is that the light results from the oxidation of a substance produced 

 by the metabolism of the light organs. The nature of the substance is 

 Unknown. That its photogenic property is independent of the life of the 

 cell is proven by the fact that when the organs are dried and reduced to a 

 powder the light reappears under the influence of air and moisture. t 



The thickness of the "reflecting layer" is aboul the same in both species, 

 but in the Photuris the layer of true photogenic tissue (Fig. 4, L) is much 

 thinner than in the Photinus. It will be shown presentlv that the light 

 from the Photinus is much richer in orange-red radiation than obtains in 

 the Photuris, which emits bluish light. Whether this is owing to a variation 

 in the thickness of the radiating layer is a question requiring further study. 



On a subsequent page it will be shown that after a firefly has done a 

 certain amount of flashing it suddenly ceases flashing and (if the photogenic 

 organs do not at once begin to emit a bright glow which soon decreases in 

 intensity, when the insect succumbs) it is useless afterwards. It would be 

 desirable to learn whether the insect stores a certain amount of energy or 

 "fuel," when in the larval state, which is consumed in flashing when in the 



*Dubois, Compt. rend./Ass'n franc, avane. Sci.,Sess. 22, p. 298; Lecons de Physiologie 

 P. generate et comparee, Paris, 1908. 



fin this connection see Bongardt, Zeitschr. f. Wissensch. Zool., 75, p. 1, 1903. Kastle 

 and McDermott, Amer. Jonr. Physiology, 27, p. 122, 1910. 



