Affinities of Pteronarcys regalis. 439 



conclusion similar to that which we arrive at from the anatomy of the di- 

 gestive organs. The number of segments to the body in Pteronarcys and 

 Perla is the same, fourteen in each, but that of the ganglia of the nervous 

 cord is different. In Pteronarcys the nervous system is composed of the 

 brain and cord with twelve suboesophageal ganglia. The first of these, the 

 analogue of the medulla oblongata of Vertebrata, is situated, as in other in- 

 sects, in the head, immediately below the brain, or cephalic ganglia, and sup- 

 plies the organs of manducation, the mouth and pharynx. . The second, third 

 and fourth, of larger size, are in the three segments of the thorax, one in each, 

 supplying the organs of locomotion, the legs and wings ; and eight smaller 

 ganglia, the first of which is in the metathorax, at a short distance behind the 

 great ganglion of that segment, while the remaining seven are in the abdomen. 

 But the nervous system in Perla consists of the brain and only ten ganglia in 

 the cord. Of these the medulla oblongata and thoracic ganglia are in their 

 usual situation, but the meso- and meta-thoracic are larger than in Ptero- 

 narcys, more especially the latter, owing to the fifth, or first of the smaller 

 ganglia of the cord, having united with the metathoracic in Perla, during the 

 changes of the larva and pupa, as I have elsewhere shown * takes place also 

 in the metamorphoses of the Lepidoptera, together with a shortening of the 

 cord in one or more of the basal segments of the abdomen. Owing also to a 

 similar cause, the shortening of the interganglionic portion of cord, the ana- 

 logue of the sixth ganglion in Pteronarcys occupies the position of the fifth 

 of that genus in Perla, the sulcus of the metafnrca [w) ; while the seventh 

 of Pteronarcys is situated in Perla at the anterior of the basal segment of the 

 abdomen, and is separated from the preceding ganglion only by a very short 

 portion of cord. Similar alteration in position, with coalescence of ganglia, 

 seems to have taken place at the termination of the cord in Perla, in which 

 the eleventh and twelfth ganglia of Pteronarcys seem to have become united. 

 This may account for the remarkable difference in the number of ganglia in 

 these two genera. Perla is thus as much in advance of Pteronarcys in the 

 general structure of its nervous system, as in that of its digestive and respi- 

 ratory organs. Approaching as these two genera do in their entire organiza- 

 tion to the Orthoptera, they seem to represent some of the lower forms of the 



* Philosophical Transactions, 1834, part ii. 

 VOL. XX. 3 M 



