436 i Mr. Newport on the Anatomy and 
ence to a frequent submersion of the insect may fairly be inferred from facts 
supplied to us by comparative anatomy in the Vertebrata. The nostril of the 
common Seal has a form somewhat analogous to that of the spiracle of this 
insect, which the animal closes most effectually, at will, when he dives, and a 
similar power may be possessed by Pteronarcys and its affinities. 
Distribution of the Trachee.—The internal organs of respiration are almost 
as remarkable in their distribution and structure as the external. In most 
winged insects, more especially those of swift flight, or with large wings, the 
principal trachez of the body are either considerably enlarged in diameter, or 
are dilated in some parts of their course into vesicles or sacs. But although 
Pteronarcys has large and powerful wings, its flight is laboured, heavy and 
slow, while not a trace of vesicular structure exists in any part of its internal 
respiratory organs. Even in the principal tracheze of the thorax (fig. 102, k), 
which communicate directly with the spiracles by small chambers, from 
which go off the main trunks to the wings, to the dorsal and ventral surfaces, 
and to the alimentary canal, there is only a very slight enlargement. In other 
parts of the body the trachez are uniform in size, are narrow, slender, and 
ramose. "They are of a light brown colour, as in Perla, and differ in this re- 
spect from those of Sialis, which are of a jet-black. The principal trunks at 
the sides of the segments consist of two sets, which pass backwards, curved 
in opposite directions, from one segment to another, like the abdominal tra- 
chee in Melolontha, as shown by Strauss *, and as I have found also in Lam- 
pyris and some other genera. These trunks unite opposite to each spiracle, 
and then divide and pass on to the next and are again united. They are 
smaller in diameter, compared with the size of the insect, than the corre- 
sponding trachez in Sialis, or even in Perla. In each of these genera, as in 
Pteronarcys, the respiratory system is without vesicles, although in Sialis, 
which is slow and heavy in flight, the principal tracheze are somewhat en- 
larged. The largest trachez in each of tbese genera are the lateral and | 
ast veíeg trunks of the sides and pectoral surface of the thoracic segments. 
Besides these, there are in Pteronarcys and Perla a pair of longitudinal tra- 
chez, passing directly backwards on each side of the nervous cord and gan- 
* Considérations 
fig. 4 générales sur I’ Anatomie Comparée des Animaux Articules, &c., 1828, 4to, pl. 7- 
