24 MODERN CLASSIFICATION OF INSECTS. 
intermediate between Isogenus and Nemoura), the larve are des- 
titute of these external organs of respiration. 
In Nemoura cinerea Pictet, Oliv., the under surface of the prothorax 
is furnished with six elongated filamentous sacs (jig. 60. 14.), similar to 
the sacs observed on the abdomen of the larve of Phryganez. These 
organs do not exist in the five other species of the genus, of which M. 
Pictet has described the larvae, thus proving the slight importance of 
these modifications of the respiratory apparatus in the Annulosa. The 
rudiments of the wing-cases are perceived in the enlarged posterior angles 
of the meso and metathorax of the larva, and the pupa state is only to be 
known by the increased size of these wing-cases, which “se développent 
peu-a-peu dansla nymphe.” In the genus Perla, as now restricted, these 
wing-cases are much less distinct than in the P. microcephala P%et. 
(which is, I apprehend, an Isogenus * ), Chloroperle (P. virescens Pict.), 
P. nigra, and the Nemoure ; in all which the wing-cases in the pupze are 
detached, and considerably elongated (jig. 60. 13.). The resemblance 
between the larve of the smaller species of caudated Perlidze, and the 
Nemoura is so complete, that M. Pictet could not discover any 
“ caractére constant pour les distinguer,” although in the perfect state 
the latter are destitute of the pair of anal filaments which exist in their 
larve. A species of this family, Semblis viridis (Chloroperla ?) has 
been made one of the subjects of Dr. Carus’s observations on the cir- 
culation of the blood in insects. (See Spence, in Mag. Nat. Hist. Jan. 
1830, p. 49. and Carus, Eintdeck. einfach. Herzen besch. Blutkreis, ec.) 
The family EpHEMERID& * Leach, comprises the well-known tribe 
of insects, ordinarily known under the name of May-flies, distinguished 
* M. Pictet informs me that he considers the Isogenus Nubecula Newm. to be 
the Perla bicaudata Linn. ; but this is doubtful, as the Linnzan§ description is too 
vague, and the Linnean collection affords no decisive information. 

* Bistioc. Rerer. To THE EpHeEMERID. 
Clutius. Opusculum de Hemerobio. 4to. Amsterd. 1634. 
Swammerdam. Historie viid het haft (Ephem.) Amsterd. 1675. — Ditto, in Book 
of Nature, pl. 13, 14, 15. 
De Geer. Obs. sur les Ephem. in Mém. Acad. Se. Paris, tom. ii. Say. Etr. 
Schiffer. Das fligende Uferaas. 4to. Regensb. 1757, and in Abh. von. Ins. 3 b. 
Williamson, on Ephemeron Leukon, in Trans. Soe. Philadelphia, vol. y. 1802. 
Collinson, in Phil. Trans. 1746. (Eph. vulgata. ) 
