170 F. X. WILLIAMS 



defined flashes of light and would necessitate the control by a 

 well developed system of muscles of large passages* (which do not 

 exist) in the light-organ. Lund severed the nerve cord of Pho- 

 turis and found that the insect lost control of the photogenic 

 organ; at most a faint residual glow persisting after the opera- 

 tion. In a similar experiment on the larva of this insect I found 

 that the light would quickly lose its brilliancy and finally dis- 

 appear. 



That the luminous substance is at certain periods also in a 

 diffuse condition, i.e., not confined to a light-organ, is evident 

 if one examine eggs which are not far developed. This diffuse 

 condition is still better shown in pupating larvae and in pupae 

 of Photuris pennsylvanica, and it even prevails in freshly hatched 

 adults, where it is more clearly visible in non-pigmented areas, 

 as at the sides of the pronotum. Effulgent pupae were cut in 

 three pieces to make sure that the glow was general. Dissections 

 showed points of light (granules) such as occur in the true photo- 

 genic layer. This diffuse light was more pronounced in the 

 head and thorax, and did not supplant the true photogenic organs 

 at any time. The dorsal vessel had no effulgence and it seems 

 probable that the luminous granules are distributed at such 

 times also in the yellowish-white fat-body which pervades the 

 insect. Such a glow seems, so to speak, to be a necessary accom- 

 paniment of the rapid metabolism which takes place at this 

 stage, and subsides as the development becomes the more com- 

 plete. 



Dubois (1886, p. 100) says, that at the moment of meta- 

 morphosis in the larva of Pyrophorus and in the pupa of Lam- 

 pyris (when histolysis is much more rapid than at any other 

 time) a diffuse glow spreads over the body. No such luminous 

 phenomenon was observed in the larva and pupa of Photinus. 

 It does not seem probable that these diffused photogenic granules 

 finally collect in the light-organs, as do for example the bacteria 

 in the mycetoms of Homoptera, but that they are used up in 

 situ, the granules of the photogenic layer being formed in the 

 cells of that organ itself. 



