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 Lampyride is confined to 
a similar location on the body, though some species of Phausis and 
Phengodes show a wider distribution of the organs. 
The luminous organ of Photinus pyralis, in common with those of 
the other Lampyridx which have been studied, consists of two layers 
of cells, under the outer transparent chitm. These layers of cells are 
penetrated by numberless trachee, the ends of which are connected 
by a network of very fine tracheoles, the whole system resembling the 
finer veming of aleaf. On the inner surface of the organ these trachez 
unite to form larger passages, which unite near the spiracle with the 
breathing trachee. It is practically certain that during the life of 
the insect these trachee 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 Lampyridz 
have been made, which agree with each other in a general way. 
McDermott and Crane (2) have shown that the structures in Photinus 
pyralis, P. consanguineus, and Photuris pennsylvanica are quite 
similar, and agree very well with those described by Townsend (°*) for 
Photinus marginellus. - The organs of Photuris presented some slight 
differences from those of the other species. Lund (”) has recently 
examined the photogenic tissues in a number of Lampyrids, and 
come to very similar conclusions. 
Bongardt (?) has studied the photogenic organs of Phausis (Lam- 
pyris) splendidula, Lampyris noctiluca, and Phosphaenus hemipterus, 
three European Lampyrids, and apparently failed to find anastomosis ey 
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 Lampyride, the distribu- 
tion of the tracheal branches being less regular, and the ‘‘cylinders”’ 
(as in Photuris also) less sharply defined than in Photyunus. 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. 
