Affinities of Pteronarcys regalis, ' 437 
* 
glia, which are somewhat larger than corresponding tracheæ in other insects 
in which the lateral abdominal trunks are dilated into sacs. A curious ter- 
mination of the lateral abdominal trunks occurs in the posterior segments. 
These trunks end abruptly immediately after their junction (fig. 11) in a kind 
of caecum, from which a small branch extends backwards to the caudal styles, 
analogous to the mode in which the branchial filaments are supplied from the 
main trachece. One of the most curious distributions of traches in Ptero- 
narcys is of those which are supplied to the alimentary canal. Tracheæ which 
supply this organ are rarely or ever dilated in any insect, not even in the Hy- 
menoptera, in which the sacs of the main trunks are the largest. They pass 
off as slender branches, either from the lateral sacs or from the main trunks 
in the immediate vicinity of the spiracles, and decrease in size as they are 
distributed on the canal, as I have figured and described* in the male of 
Bombus terrestris. A slender branch passes longitudinally backwards in that 
insect from the main trachea, behind the metathoracic spiracle on each side 
of the esophagus, to the anterior portion of the stomach on which it is distri- 
buted, and a similar origin and distribution of the gastric tracheæ exists in 
all insects with but little variation. In Pteronarcys a long, slender branch 
(fig. 10 i, Æ) passes off from the slightly enlarged trunk behind the mesotho- 
racic spiracle, and another (J) from behind the metathoracie, which are ex- 
tended longitudinally backwards, slightly reduced in size, as far as the middle 
of the abdomen, where they divide into branches which are distributed on the 
sides and anterior of the stomach. ‘This is the general character of the tra- 
chez in the whole of the Perlide and in Sialis. These exceptions to the law 
which I have heretofore endeavoured to exemplify by facts, that a vesicular 
form of the respiratory organs in insects has reference chiefly to power of 
flight, and enables the insect to alter the specific gravity of its body at the 
moment it takes wing, and thus diminish the amount of muscular exertion 
required in its movements, tend in reality to confirm the previous conclu- 
 Sions. The retention in the imago of the simple setaceous trachece of the 
larva is accompanied, as in Sialis, Perla, and Pteronarcys, with a - er 
of flight, although the species of each of these genera have ample wings, and 
might have been expected to be extremely active. Pteronarcys thus resembles 
* Philosophical Transactions, 1836, part ii, pl. 26. fig. 2. p. 564. 
