INSECTA. 329 



They all have two antennee and a distinct head. The nervous sys- 

 tem of most Insects — the Hexapoda — is generally composed of a 

 brain formed of two opposing ganglions, united at base, giving off 

 eight pairs of nerves and two single ones, and of twelve ganglions*, 

 all inferior. The two first are situated near the junction of the 

 head with the thorax, and are longitudinally contiguous ; the anterior 

 sends nerves to the lower lip and adjacent parts ; the second, third 

 and fourth belong to each of the three first segments, or those which 

 form the thorax in the Hexapoda ; the remaining ganglions belong 

 to the abdomen, so that the last or the twelfth corresponds to its 

 seventh ring, and is immediately followed by those which compose 

 the organs of generation ; each of these ganglions transmits nerves 

 to the parts of its respective segments. The two last, which are 

 closely approximated, also send some to the terminal annuli of the 

 body. The frontal region exhibits three particular ganglions called 

 frontal by Lyonet, from the first of which arises posteriorly a great 

 nerve with enlargements, the longest of all, that he denominates the 

 recurrent. The first ordinary or sub-cesophagean ganglion, gives 

 off, according to him, four pairs of nerves, and each of the following 

 ones, two ; so that by counting the eight pairs of the brain, and the 

 ten spinal bridles, which may also be considered as so many pairs of 

 nerves, we shall have in all forty-five pairs, exclusive of the two 

 solitaiy nerves above-mentioned, or from twelve to fourteen more 

 than are found in the human subject. The two nervous cords which 

 form the ganglions by their union, are tubular and composed of two 

 tunicks, in the exterior of which we observe tracheae ; a medullary 

 substance fills the central canal. The admirable work of M. Herold 

 on the anatomy of the larva of the great Papilio brassicce, L., 

 studied throughout its various degrees of developement, and to the 

 period of its transformation into a chrysalis, shows us that the ner- 

 vous system and that of the digestive organs experience remarkable 

 changes ; that in the beginning, the nervous cords are longer and 

 further apart, an observation which strengthens the opinion of one 

 of the greatest zootomists of the age, Doctor Serres, on the origin 

 and developement of the nervous system. In our general remarks 

 on points common to the three classes of articulated animals provided 

 with articulated feet, we mentioned the various opinions of physi- 



tra, there is none in the second segment or mesothorax ; but in the following one 

 or the metathorax, there are two pairs, one anterior, which being situated near 

 the articulation of this segment with the preceding, may be considered as belonging 

 to the latter, and the other smaller, and placed close to that of the first abdominal 

 segment. 



• Several of the Lamellicornes in a perfect state form exceptions. 



