432 Mr. Newport on the Anatomy and 



cellular spaces," " or bounded by distinct vessels," as 1 formerly pointed out 

 in my article "Insecta*," and as since further exemplified by M. Blanchardf. 

 It enters the branchia at the root of the main trachea (fig. 3 a) on the anterior 

 or external surface, and, bathing the whole of the branches, penetrates into 

 each filament, passing — as I have repeatedly witnessed in the simple branchiae 

 of Sialis, and I doubt not that the course is precisely similar in Pteronarcys 

 (fig. 4) — outwards along the anterior {d) and inwards along the posterior sur- 

 face (e), absorbing oxygen, by the principle of endosmose, from the air that is 

 mechanically mixed with the water, and giving out carbonic acid by the same 

 means. This takes place in every form of branchia, so that the function itself, 

 in every structural variety of the organ, is identical, although the conditions 

 under which it is exercised may vary. Thus the broad flattened branchiae of 

 some of the Ephemeridce may have reference to the occasional detention of 

 blood in those organs under particular circumstances, and a like explanation 

 may account for differences in the form, and in the situation of others. 



We have thus established the fact that true branchiae exist as normal struc- 

 tures in the imago Pteronarcys, as well as in the larva and pupa. Now Mr. 

 Barnston has informed me that in both the latter states the insect constantly 

 resides in the water, "at the bottom of streams and rivers;" while the larva 

 of Perla abnormis ;}:, which does not possess the thoracic branchiae, is always 

 found hidden in the clefts of water-logged timber on the surface, or even in 

 the trunks of trees and other places on the banks, — a difference in habit which 

 corresponds to the difference in structure. The imago Pteronarcys is a noc- 

 turnal insect ; the imago Perla abnormis is crepuscular, and perhaps diurnal, 

 although, as Mr. Barnston remarks, " it prefers the shade in the heat of the 

 day." The Pteronarcys comes abroad only at dew-fall, or in the night, and 

 Mr. Barnston has observed it, when on the wing, " constantly dipping on the 

 surface of the water." Another able naturalist, Edward Doubleday, Esq., 

 F.L.S., who also has captured the insect in its native haunts, at Trenton Falls, 

 in North America, has informed me, that he has taken it chiefly "on wet 

 evenings," and that it hides itself by day in crevices of rocks which are con- 

 stantly wetted by the spray of falling water, under stones, and in other damp 



* CyclopBedia of Anatomy and Physiology, part xviii. vol. ii. p. 981, 1839. 



t Comptes Rendus de I'lnstitut, May 1847. X See Perla abnormis, in the Postscript. 



