254 WALTER N. HESS 



the light-organs are now well differentiated, although considera- 

 ble change takes place subsequently. 



The entire light-organ appears to be attached to the hypoder- 

 mis by a delicate, non-cellular membrane, 



The mature light-organ, in a larva one year old (fig. 9), shows 

 very distinctly the arrangement of the two layers, namely, the 

 reflector layer (R) and the photogenic layer (P). The cells of 

 the photogenic layer contain many rather small granules, yet 

 in the cells of this layer of the adult light-organ, except along 

 their margins, these granules are very large. The cells of the 

 reflector layer are much less granular. With the fixers and 

 stains used the cell walls of the photogenic cells are rather 

 indistinct, while those of the reflector cells are noticeable. 

 As will be observed by comparing the larval light organ with 

 the mature adult light-organ, the cells of the two structures 

 are similar in appearance, yet the reflector layer, in the larva, 

 is thinner than in the adult organ in proportion to the thick- 

 ness of the photogenic layer in the two stages. One large 

 trachea sends its branches to the cells of the larval hght-organ. 

 The hypodermal cells in the body wall are now much reduced 

 in size in comparison with those of the embryo. These cells are 

 especially small on the ventrolateral side of the light-organ. 

 Here they secrete a rather thick cuticula, but, due to its trans- 

 parency, the light from the organ easily penetrates it. 



ORIGIN AND DEVELOPMENT OF THE ADULT LIGHT-ORGANS 



Since the adult light-organs of insects are developed during 

 the pupal stage, a large number of larvae of Photurus pennsyl- 

 vanica were collected during April, for the purpose of obtaining 

 material for the study of the development of this organ. About 

 the 25th of May certain of these larvae built their pupal chambers, 

 and in about five days they transformed to pupae. 



Several of the active larvae were taken about a week or ten 

 days before their normal pupation period, and sections were made 

 of the sixth and seventh abdominal segments, to determine the 

 nature of the various histological structures. A transverse sec- 

 tion (fig. 11) shows a normal layer of hypodermal cells (H), 



