HISTOLOGY OF THE LIGHT ORGANS. 13 
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. He 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 directly 
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 filled with 
opaque, chalky granules forming the aforesaid ‘‘reflecting layer.” 
Dubois has shown that the mollusk Pholas dactylus produces photogenic 
matter in the form of a liquid which remains luminous after filtering. The 
photogenic liquid contains fine granulations that giveit a cloudy appearance. 
‘These granulations are of a colloidal nature which is lost in passing into the 
crystalline, when the light emission ceases. he 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. 
The thickness of the “reflecting layer’’ is about 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 presently 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, avanc. Sci., Sess. 22, p. 298; Lecons de Physiologie 
P. générale et comparée, Paris, 1908. 
fIn this connection see Bongardt, Zeitschr. f. Wissensch. Zoél., 75, p. 1, 1903. Kastle 
and McDermott, Amer, Jour. Physiology, 27, p. 122, 1910. 
