432 Mr. Newport on the Anatomy and 
cellular spaces," *or bounded by distinct vessels," as I formerly pointed out 
in my article * INsEcTA*," and as since further exemplified by M. Blanchard}. 
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 branchize 
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 branchiz of 
some of the Ephemeride 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 branchiz exist as normal struc- 
tures in the imago Pferonarcys, 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 t, which does not possess the thoracic branchize, 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., 
ELS, who also has captured the insect in its native haunts, at Trenton F alls, 
. 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 
* 1250 E 
" pba of Anatomy and Physiology, part xviii. vol. ii. p. 981, 1839. 
omptes Rendus de l'Institut, May 1847, 1 See Perla abnormis, in the Postscript. 
