Affinities of Pteronarcys regalis. 429 
widely from the posterior angles. The mandibles are short, strong and cor- 
neous, agreeing in structure with the carnivorous habits of the species. The 
palpi are much shorter than in the imago. The prothorax is quadrangular, 
much wider than long, with free elevated margins, and with the angles elon- 
gated and slightly curved. The rudiments of the wings are broad and flat- 
tened, the posterior pair being directed transversely outwards. The abdomen 
is cylindrical, slightly tapering, and formed of ten segments, with obsolete 
marks of spiracles at the sides. The ninth segment is exceedingly short on 
the ventral surface, but is much elongated, and is united with the terminal 
portion of the tenth, on the dorsal, projecting over, and completely covering 
the ventral portion of the tenth segment as a triangular process. "The ventral 
portion of the tenth or anal segment is very short, and is divided longitudi- 
nally into two plates, which give origin to the caudal styles. The eighth seg- 
ment in the male pupa has its posterior margin on the ventral surface elon- 
gated, as in the male imago. The legs are strong and powerful, and the 
meso- and metathoracic pairs have the tibie densely ciliated for swimming. 
The branchiz on the pectoral surface of the thoracic segments correspond 
precisely, both in the situation and number of the tufts, with those of the 
imago, excepting only that the filaments are larger and more elongated, and 
one tuft exists in the pupa in the place of the future prothoracic and meso- 
thoracic spiracles, which is wanting in the imago, in which the spiracles 
exist, 
Branchie.— In the perfect insect (fig. 5) there are eight sets of branchial 
. Sacs, or tufts (b, b, b), distributed over the pectoral surface of the thoracic 
segments, and first two segments of the abdomen. The first of these is situ- 
ated in the soft tegument that connects the head and prosternum. It con- 
sists of three pairs of sacs arranged transversely, and partially encircling the 
neck like a collar or ruff. The second set consists of two pairs of sacs, one 
of which is behind the coxe of the first pair of legs, and the other imme- 
diately behind the ante-furcal orifices (f) in the prosternum. The third set 
consists also of two pairs of sacs situated on the anterior of the mans 
num. The fourth is a single pair of sacs between the coxe of the second pair 
9f legs and the mesofircaberifives (g). The fifth set, like the third, is formed 
of two pairs of sacs, one of which is in the tegument that. connects the meso- 
