152 F. X. WILLIAMS 



The fat-spheres which are situated in the ventral portion of 

 segments six and seven of the abdomen, and are one or two roWs 

 deep (A, figs. 24 and 25), are destined to give rise to the photo- 

 genic organ, and may therefore conveniently be called the pho- 

 togenic fat-spheres. The most ventral of these sometimes lie 

 directly upon the hypodermis. The longitudinal segmental mus- 

 cles span these and other fat-bodies. None of the photogenic 

 fat-bodies are among those of large size, such as commonly occur 

 in the mid-abdominal region. 



The first change observed in the photogenic fat-spheres takes 

 place a short time before the larva enters the pupal stage. In 

 the case especially studied (fig. 24) the larva was quite active 

 and bore no evidence that it was soon to pupate. (It may be 

 said here that pupating larvae and pupae were never found to 

 possess a general effulgence as is noticeable in Photuris penn- 

 sylvanica, where even freshly emerged adults glowed in the 

 thoracic region. I do not believe that this phenomenon is in- 

 volved in the formation of the light-organs.) Here the pho- 

 togenic were found to differ from the other abdominal fat- 

 bodies, first, in taking the stain a little more deeply, second, in 

 having more distinct walls, and third, in having larger nuclei. 

 These changes are in general, fairly evident in a sagittal section. 



The next stage observed was the partial envelopment of the 

 photogenic fat-bodies by a layer of yellowish-brown material, 

 staining much like plasma or dermis (figs. 17, 18, and 24). This 

 does not seem to be a secretion from the fat-body as it lies wholly 

 outside the latter. It is at first fairly homogenous, save that 

 there are small black particles along the inner border. It is 

 not improbable that this cap-like layer partially enveloping the 

 fat-body and diminishing in thickness along the edges, is formed 

 from material in the blood, which, together with certain haemo- 

 cytes, has been attracted to one side of the fat-body by some 

 substance, and has thus come partially to invest it. 



The fact that the blood-cells (figs. 17 and 18) are almost 

 always in contact with this investment and not applied else- 

 where to the fat-body, would seem to indicate that these bodies 

 exert a certain attraction. Here also coagulated plasma gathers 



