956 



INSECTA. 



connecting nerves in the larva forming one por- 

 tion of the nerves for the future wing of the per- 

 fect insect. The nerves for both pairs of wings 

 are then derived separately from two portions of 

 cord and two distinct ganglia, and this is the 

 state in which they are also found soon after 

 the insect has changed to a pupa (jig. 414). 



Fig. 414. 



Pupa of Sphinx ligustri. (Newport, Phil Trans.) 



The connecting nerves are then derived from 

 the cord (e, h\ and being joined each to the 

 first nerve from the next ganglion assist to form 

 the future alary nerves (f, '). Now as the 

 change to the perfect insect proceeds, the second 

 ganglion (2) becomes approximated to the third 

 (5), which gradually disappears, and the cord 

 between it and the fourth becomes enlarged 

 and shortened, and passes on each side of the 

 insertion of the muscles in the centre of the 

 meso-thorax, the cord between the second and 

 third ganglion having also become obliterated, 

 so that there is then no ganglion intervening 

 between the origins of the two pairs of wings, 

 but only a portion of cord. The nerves for the 

 two pairs of wings then approach each other 

 diagonally, the anterior pair being directed 

 backwards, and the posterior forwards, until 

 they meet and form a plexus, their roots still 

 continuing distinct from each other; the root 

 of the anterior being derived from the cord 



posterior to the united second and third gan- 

 glia, and that of the posterior from the cord 

 connected with the united fourth and fifth gan- 

 glia. After forming the plexus, the nerves are 

 again separated and given to the anterior and 

 posterior wings. The reason for this curious 

 union and complexity in the distribution of the 

 nerves to the wings is not at first very evident, 

 but on a little reflexion it is found to be regu- 

 lated by one of those beautiful provisions in 

 the animal economy by which the most perfect 

 harmony in the exercise of all the functions of 

 the body is preserved. The wings, the most 

 powerful and most constantly employed organs, 

 are not merely required to act with energy, but 

 in the most perfect unison with each other, 

 more especially in insects of long-continued 

 or rapid flight, and hence must be supplied 

 with power from the same centre, not merely 

 that of voluntary motion but also of sensation. 

 That this is the reason for this curious union of 

 the nerves for the wings seems apparent from 

 the circumstance that it exists in very many 

 tetrapterous insects of rapid or powerful flight, 

 as in the Apida and Ichneumonidtf, while in 

 others, even of the same order, as in Athalia 

 centifolitf, which is well known to fly heavily 

 and but a short distance, there is no such com- 

 bination. In the Scorpion-fly also, Panorpa 

 communis, it is absent, and the alary nerves 

 originate by double roots without forming 

 a plexus as in the larva of the Sphinx, while 

 the flight of the insect is sluggish and but of 

 short duration. Besides this it may be re- 

 marked that in many Coleoptera in which the 

 anterior wings or elytra are merely elevated and 

 nearly motionless during flight, the nerves are 

 derived separately from the cord, and proceed 

 to their destination without being first com- 

 bined in a plexus. 



The cord and nerves of the abdomen, as before 

 stated, we regard merely as a cauda equina. 

 We have before explained the varieties in the 

 formation of the cord in different insects, and 

 need but further remark that in each instance 

 the cord in the abdomen, as in other parts, is 

 covered in by a strong fibrous membrane, which 

 separates it from the cavity of the abdomen. 

 In the Gryllida we have distinctly recognised 

 muscular fibres running transversely above the 

 cord from one side of the body to the other. 

 They have also been observed by Burmeister, who 

 supposes them to assist in the function of respi- 

 ration by contracting the segments, and thus 

 aiding in the act of expiration. We have seen 

 similar trans-muscles lying above the mem- 

 brane that binds down the nervous cord in the 

 abdomen of Bombus terrestris. The mem- 

 brane is continuous with that which covers the 

 cord in the thorax. A similar membrane was 

 formerly noticed by Lyonet in the Cossus,* 

 and subsequently by ourselves in the Sphinx.f 

 Between this membrane and the cord there is a 



* Recherches sur 1'Anatomie et les Metamor- 

 phoses des differentes Especes d'Insectes, ouvrage 

 posthume de Pierre Lyonet. Paris, 1832, fig. 18, 

 p. 52. 



t Phil. Trans. 1834, part ii. p. 395, pi. xiv. 

 fig. 9. 



