PHOTOGENIC ORGANS OF LAMPYRIDS 183 



the fat-body, disappear in the forming light-organ. The 2'eflector 

 layer forms a sort of pavement epithelium of smaller, more 

 clearly defined cells, while the photogenic layer is made up of 

 larger, more rounded cells with more obscure limits. Small urate 

 crystals occur in the upper, while minute photogenic granules 

 darken the lower layer. 



11. Tracheae, and nerves enter both layers. In the photo- 

 genic layer, tracheal end-cells are developed. These end-cells 

 are penetrated by nerves. The tracheal end-cells have slender 

 capillary processes which seem to be intercellular and frequently 

 anastomose. 



12. The photogenic organs of the larva develop as evident 

 structures, in late embryos. They are derived from neighboring- 

 fat cells, by migration and proliferation. As in the adult, the 

 organ differentiates into two layers but the development is more 

 obscure. The larval light-organ does not function in the adult. 

 It appears to be a more primitive and less highly modified struc- 

 ture than the adult organ, and to be independent of the latter. 

 In some species, as Ellychnia (infrequently), and Lampyris, it 

 may grow in the adult. 



13. The physiology of the photogenic organs has been much 

 studied. The general belief is that photogeny is a process of 

 oxidation of the photogenic granules, the oxygen being furnished 

 by the tracheal end-cells. The process is dependent upon the 

 nervous system, but photogeny may also be effected by local 

 stimulation. Dried organs may be made to shine upon contact 

 with water. 



14. The photogenic granules are not fat-like but are probably 

 an albuminoid which is insoluble in water and in fat-solvents. 

 They have been regarded by some as luminous symbiotic bac- 

 teria. 



15. The function of the larval photogenic organs is uncertain, 

 but in the adult their purpose is to bring the two sexes together. 



16. The germ-band of the embryo is of the immersed type, 

 i.e., with yolk interposed between the amnion and the serosa. 

 The amniotic cavity is formed before gastrulation occurs. 



