356 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



4. THE PHOTOGENIC ORGANS. 



The luminous apparatus of the male of Photinus pyralis — the more 

 commonly seen of the sexes — occupies the entire ventral surfaces of 

 the two abdominal segments next to the last, and a portion of the 

 preceding segment. That of the female is a small rectangular area on 

 the third abdominal segment from the last ; both sexes have also two 

 very small points of luminous tissue on the last abdominal segment. 

 In general the luminous apparatus of other Lampyridae is confined to 

 a similar location on the body, though some species of PTiausis and 

 Phengodes show a wider distribution of the organs. 



The luminous organ of Photinus pyralis, in common with those of 

 the other Lampyridse which have been studied, consists of two layers 

 of cells, under the outer transparent chitin. These layers of cells are 

 penetrated by numberless tracheae, the ends of which are connected 

 by a network of very fine tracheoles, the whole system resembling the 

 finer veining of a leaf. On the inner surface of the organ these tracheae 

 unite to form larger passages, which unite near the spiracle with the 

 breathing tracheae. It is practically certain that during the life of 

 the insect these tracheae are filled with air. Of the two cell layers, the 

 outer consists of a mass of some special type of nucleated cell, of 

 unknown nature, penetrated by the aerophore cylinders, while the 

 inner layer is composed mainly of urates, and probably serves as a 

 sort of reflector. 



Several studies of the structures in different species of Lampyridae 

 have been made, which agree with each other in a general way. 

 McDermott and Crane ( 52 ) have shown that the structures in PJiotinus 

 pyralis, P. consanguineus, and Photuris pennsylvanica are quite 

 similar, and agree very well with those described by Townsend ( 66 ) for 

 PJiotinus marginellus . The organs of Photuris presented some slight 

 differences from those of the other species. Lund ( 42 ) has recently 

 examined the photogenic tissues in a number of Lampyrids, and 

 come to very similar conclusions. 



Bongardt ( 3 ) has studied the photogenic organs of PTiausis (Lam- 

 pyris) splendidula, Lampyris noctiluca, and Phosphaenus hemipterus, 

 three European Lampyrids, and apparently failed to find anastomosis 

 of the tracheoles. However, the author has recently examined some 

 sections of the luminous tissue of Lampyris noctiluca (female), and 

 had little difficulty in seeing the tracheolar anastomosis; the struc- 

 tures differed somewhat from the American Lampyridae, the distribu- 

 tion of the tracheal branches being less regular, and the "cylinders" 

 (as in Photuris also) less sharply defined than in Photinus. He has 

 also examined the tissues of Photinus scintillans and Lecontea lucifera 

 and found them to be practically identical with those in the insects 

 previously studied. 



