438 Mr. Newport on the Anatomy and 



Perla and Sialis in the general character of its respiratory organs, but it dif- 

 fers from them in other parts of its structure. 



Alimentary Canal. — The alimentary canal (fig. 10) consists in chief part of 

 an enormously elongated oesophagus (m), which, instead of joining with the 

 stomach (/») in the metathorax, passes backwards, accompanied by the tra- 

 cheae {k, I), as far as the fourth segment of the abdomen, and then terminates 

 in the digestive organ, which does not exhibit even the smallest rudiments of 

 gastric glands, which exist of large size in Perla (fig. 12 o). This proves as 

 marked a difference in the habits of these two genera as exists between these and 

 Sialis (fig. 13), in which the oesophagus is extended backwards in the form of 

 a pouch {n), as in the Diurnal Lepidoptera. The stomach (p) of Pteronarcys 

 differs as much from that of Perla as the oesophagus. It is capacious, mus- 

 cular, and of considerable length, forming two or three convolutions before it 

 terminates in a pylorus, around which the Malpighian vessels are inserted ; 

 whilst that of Perla ahnormis is surrounded at its cardiac origin by six large 

 csecal glands (o), is short, somewhat funnel-shaped, straight, and has no con- 

 volution. In these respects the stomach of Perla resembles that of the Blat- 

 tidce. In the number, appearance, size, and place of insertion of the Malpi- 

 ghian vessels (r), Pteronarcys resembles Perla, but both differ from Sialis, in 

 which there are only six of these vessels, while in the others there are from 

 forty to fifty. These three genera also differ as regards the ilium (q), and the 

 form and size of the colon (*). In Pteronarcys and Sialis the ilium is short 

 and slender, but it is more than twice as long in Perla. The colon in the 

 latter is very muscular, is almost as large as the stomach itself, and has a 

 caecum projecting from it. In Sialis the colon is larger than the stomach, 

 and has a small lobulated csecum at its commencement, as in Lepidopterous 

 larvae ; but in Pteronarcys it is shorter and much smaller than in either of 

 these genera. These differences in organization indicate differences in kind of 

 food, or in habit and oeconomy, and establish the distinctness of the genera. 

 The existence of gastric glands in Perla shows that this genus stands at the 

 head of its family, and approaches the Orthoptera ; while the absence of these 

 in Pteronarcys, and in the smaller Perlidce, Nemoura, shows also that these 

 genera are of a type inferior to Perla. 



Nervous System. — The nervous system of Pteronarcys (fig. 14) leads us to a 



