150 F. X. WILLIAMS 



whole of one or more ventral abdominal segments. Briefly- 

 stated, the light-organ is composed of two layers of cells, an 

 inner or non-photogenic (urate or reflector) layer, and an outer 

 luminous or photogenic layer. The inner layer functions as a 

 reflector and is opaque in life and rather translucent when fixed 

 and stained. From the contents of its cells it is also known as 

 the urate layer. The outer or photogenic layer is clear in living, 

 and deeply-staining in fixed material; it owes the latter property 

 largely to the numerous little photogenic granules which some- 

 times fill its cells. Outwardly the photogenic later is overlaid 

 by the thin hypodermis, which in this region usually secretes 

 non-pigmented cuticle. Tracheae and nerves penetrate both 

 layers inwardly, but in this respect the photogenic layer is the 

 more richly supplied. The vertical branches from the main 

 tracheal trunks give off many fine tracheae, which terminate as 

 branching capillaries or tracheoles among the photogenic cells. 

 These capillaries are surrounded, at least in part, by cytoplasm, 

 which, together with the basally situated nucleus, constitutes a 

 tracheal end-cell. 



DEVELOPMENT OF THE PHOTOGENIC ORGANS IN THE ADULT 

 PHOTINUS CONSANGUINEUS 



The development of the photogenic organ of the adult is best 

 studied in the male, for it here occupies the whole ventral por- 

 tion of segments six and seven of the abdomen. (It is a com- 

 mon error, as pointed out by Vogel, for students of the photo- 

 genic organs of the Lampyridae, to designate these organs as 

 occurring on the fifth and sixth segments, etc., when in reality 

 they are situated on the segments immediately following. This 

 can be readily seen in the larva, also by counting the dorsal 

 abdominal segments of the pupa and adult.) 



In sections of a larva a day or two after hatching, the fat- 

 body consists of large,* often free cells, each containing a single 

 nucleus, and with the protoplasm well vacuolated (fig. 20), 

 They measure about 8-12 micra in diameter, and all appear 

 histologically similar. A larva sixteen days old shows fat cells 

 (fig. 21) which are frequently more or less united into sheets or 



