140 F. X. WILLIAMS 



hain's iron haematoxylin with orange G, eosin or Congo red as 

 a counter stain, the first of these proving best. Sections were 

 cut from 3.5 to 15 micra thick. Osmic acid was much used 

 for the study of the 'tracheal end cells/ while caustic potash 

 brings out the tracheal capillaries very satisfactorily. 



I wish to thank all who have aided me in my work and espe- 

 cially Dr. W. M. Wheeler and Professor C. T. Brues, under whose 

 supervision my studies were made, for many helpful suggestions 

 and criticisms. 



ORIGIN AND DEVELOPMENT OF THE PHOTOGENIC ORGANS 



There are, in general, two views as to the origin of the photo- 

 genic organs in the Lampyridse; according to one they are 

 developed from the ectoderm; according to the other they are 

 related to the fat-body and are therefore mesodermal. These 

 two beliefs rest upon a diversity of observations and facts. 

 The development of the photogenic organs of the European 

 Lampyris noctiluca has been studied by Vogel ('13), who pub- 

 lished a rather brief paper without illustrations on the subject. 

 There does not appear to be any fundamental difference between 

 his observations and mine. As far as I know, VogePs studies 

 are the only ones of their kind. 



Lindemann ('63) considers the photogenic body to be a nerv- 

 ous organ. This view is not remarkable when we find that he 

 figures a part of the central nervous system as the light-organ. 



Owsjannikow ('68) believes that the light-organ of Lampyris 

 consists of a group of epithelial cells of a glandular nature. 

 Presumably, like the majority of glands in insects, he considers 

 these cells to be ectodermal. 



Gegenbaur ('74), in speaking of the light-organs of fireflies, 

 advances the view that the non-luminous or urate layer is derived 

 from the fat-body, whereas the photogenic layer is a modifica- 

 tion of the hypodermis. 



Heinemann ('86) in speaking of the elaterid, Pyrophorus noc- 

 tiluca, p. 298, says: "Die Leuchtorgane der Cucujo's gehoren 

 wie diejenigen den Lampyriden dem Hautsystem an." 



