No. 533] PHOTOGENIC ORGANS OF LAMPTBIDJE 307 



species which had been studied previously. The two 

 species most common here (Washington, D. C.) are 

 Photinus pyralis Linn, and Photuris pvnnsyl runico 

 Deg., and the majority of our studies have been made on 

 them. The classification relationships between these in- 

 sects and the others that have been studied may be seen 

 from Oliver's recent catalogue (3). As already stated. 

 Photinus pyralis had been studied to some extent by 

 Seaman, and its near relative, P. marf/iurllus, by Towns- 

 end, but so far as we have been aide to find, no studies 

 have been made on any species of Photuris. 1 



A large number of slides have been made, containing 

 transverse, longitudinal and oblique sections of the two 

 insects above mentioned, and a few transverse sections 

 of Photinus consauyuincits. With these slides compara- 

 tive studies of the structure of the .photogenic organs 

 have been made. The most essential result of these 

 studies is that in these three species the structure of the 



ilar to that described 1 for some of the other species of 

 Lampyridae which have been examined. Many of the 

 drawings given by Townsend of the structures in Pho- 

 tinus marf/inrflHs may represent with equal faithfulness 



Photuris Pennsylvania; our slides of Photinus consan- 

 fiuiucns were not entirely satisfactory, but so far as could 

 be seen, the structures in this insect are identical with 

 those in its larger congener, pyralis. 



In all three insects the luminous organ is divided into 

 two distinct layers, the inner one being white and opaque, 

 and serving as a reflector, and the outer being yel- 

 lowish and translucent, and containing the actual photo- 

 genic mechanism. The photogenic organs, as brought 

 out by prior studies, are penetrated from the interior 

 of the insect outward, by innumerable tracheae, which 

 ramify and anastomose within the true photogenic 

 tissue, and unite within, above the reflecting layer, to 



