PHOTOGENIC ORGANS OF LAMPYRIDS 159 



and this is a common occurrence in end-cells elsewhere than in 

 the light-organ. This can be shown in fresh preparations treated 

 with caustic potash. 



In a freshly formed pupa the tracheal end-cells have not yet 

 been differentiated from the thick tracheal epithelium, but cell 

 division seems to be more frequent in the portion of the epi- 

 thelium immersed in the photogenic layer than elsewhere along 

 the trunk, and it is likely that some of these dividing cells ulti- 

 mately migrate from their original position along the main trunk, 

 beyond the small branches containing taenidia and secrete the 

 branching tracheoles. The fact that the tracheal epithelium is 

 especially thick in the photogenic layer would seem to strengthen 

 this interpretation. (Geipel found some tracheal end-cells in 

 the urate or upper layer of the light-organ of Photinus mar- 

 ginellatus). Still the examination of numerous sections has 

 failed to reveal two sorts of tracheal cells such as occur in the 

 adult. 



The photogenic organ, when in the process of formation, fre- 

 quently suggests a hypodermal origin, for it is then rather struc- 

 tureless and closely applied to the body wall. Indeed some 

 sections show the hypodermis sending up extensions between 

 the cells of the organ. This is perhaps one of the reasons wh} r 

 Dubois ('98) believes that the photogenic organ, at least in 

 Pyrophorus noctiluca and in Lampyris noctiluca, is derived by 

 direct proliferation from the underlying hypodermis. It seems 

 to me more logical to suppose that, as in Photuris and Photinus 

 and Lampyris, the light-organ of all Lampyridae is derived from 

 the fat-body. But unless a complete series of the developmental 

 stages is at hand, it is unsafe to speculate on the origin of the 

 light-organ. A careful examination of the initial stages, I am 

 sure, will convince anyone that the hypodermis has nothing to 

 do with the formation of the light-organ and is distinctly sepa- 

 rated from it. The cells of the hypodermis contain no blue- 

 staining proteid granules as do the photogenic cells at first (figs. 

 25 and 27), and they also appear much too small to give rise 

 to these large cells. 



