STRUCTURE OF LUMINOUS ORGANS 77 



The insects possess the simplest types of intracellular 

 light organs, a mass of photogenic cells, which, in the 

 common firefly {a lampyrid beetle) of Eastern North 

 America, has probahly been developed from the fat body, 

 while in the New Zealand glowworm, the larva of a tipulid 

 fly (Bolitophila luminosa), part of the Malpighian tubule 

 cells have acquired photogenic power (Wheeler and 

 Williams, 1915). This is illustrated in Fig. 24. 



The photogenic organ of the firefly is made up of two 

 kinds of cells, a dorsal mass of small cells several layers 

 deep, the reflector layer, and a ventral mass of large cells 

 with indistinct boundaries, the photogenic layer (Fig. 25). 

 The photogenic cells contain a mass of granules, spherical 

 in the male and short rods in the female. The photogenic 

 cells are divided into groups by large tracheal trunks 

 which pass into the light organ and branch to form trache- 

 oles connected with tracheal end cells. The exact dis- 

 tribution varies in different species, but in all the arrange- 

 ment is such as to give a very abundant oxygen supply. 

 Each group of photogenic cells is surrounded by a clear 

 ectoplasm containing no granules. The tracheoles pass 

 through this and either end openly within the photogenic 

 cells or anastomose with tracheoles from neighboring 

 trachese. Nerves, but no blood-vessels — which are absent 

 in insects — enter the organ. It is difficult to determine 

 if the nerves supply the tracheal end cells or the pho- 

 togenic cells. 



The dorsal reflecting layer is made up of cells contain- 

 ing numerous minute crystals of some purin base, either 

 xanthin or urates, or both. They have a white milky 

 appearance and while they are certainly not good reflec- 

 tors in the optical sense, they do act as a white back- 



