436 Mr. Newport on the Anatomy and 



ence to a frequent submei'sion 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 Trachece. — 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 tracheae 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 tracheae of the thorax (fig. 10 i, 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 tracheae 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 Sig,lis, 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- 

 cheae 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 tracheae 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 tracheae are somewhat en- 

 larged. The largest tracheae in each of these genera are the lateral and 

 transverse trunks of the sides and pectoral surface of the thoracic segments. 

 Besides these, there are in Pteronarcys and Perla a pair of longitudinal tra- 

 cheae, passing directly backwards on each side of the nervous cord and gan- 



* Considerations generales sur rAnatomie Compar6e des Animaux Articules, &c., 1828, 4to, pi. 7. 

 fig. 4. 



