250 WALTER N. HESS 



Buchner ('14) and Pierantoni ('14) both consider the light- 

 organs a symbiotic structure in which there are found luminous 

 bacteria, or fungi, that cause the hght. Buchner showed a very- 

 close similarity between the granules in the photogenic layer of 

 the light-organs and the symbiotic bacteria of the homopteron, 

 Aphrophora spumaria. 



LOCATION AND STRUCTURE OF THE LIGHT-ORGANS 



The light-organs of all the luminous Lampyridae were found 

 lying next to the sternal side, in the eighth abdominal segments 

 of larvae, and in one or both of the sixth and seventh abdominal 

 segments of the adults. In the larvae the organs appear as two 

 small elliptical discs (fig. 1). In an adult male of Photurus 

 pennsylvanica the light-organs cover the entire sternites of 

 the sixth and seventh abdominal segments (fig. 2), while in the 

 female of this species the organs occupy only about two-thirds 

 of the corresponding sternites (fig. 3). During the first one or 

 two days of adult fife, the larval light-organs can be seen to emit 

 light from the eighth abdominal segment (fig. 3). The phagocy- 

 tosis of these organs however, is usually so far advanced by the 

 end of the second day of adult life that they no longer emit 

 light. 



In the species studied, the arrangement of these organs, in 

 the larva, is shown diagramatically by figure 4. The photo- 

 genic layer (P) lies next to the ventrolateral sternite of the eighth 

 abdominal segment. In the adult male (fig. 5) the photogenic 

 layer (P) extends entirely across the sternite next to the hypo- 

 dermis, while the reflector layer (R) completely covers this layer 

 on its dorsal side in both the larva and the adult. 



The mature fight-organs whether in the larva (fig. 9) or in 

 the adult (fig. 17), are of the same general structure. Both 

 are composed of two layers of cells, the inner reflector layer 

 (R) and the outer photogenic, or luminous layer (P) . The former 

 is composed of fairly regular polygonal cells in which are located 

 a large quantity of crystals of urate salts. This layer in life is 

 opaque and chalky in appearance. The ventral layer is com- 

 posed of two parts: the tracheal structures (T) and the photo- 



