84 Descriptions of Preparations. 



oephalo-thoracic ganglion of the Crustacea ; it is by no means 

 invariably the case in other parts of the insect^s organism^ that a 

 fusion or distinctness of external segments is reproduced internally 

 by a fusion or segmentation of the chain of nerve ganglia. The 

 sixth, seventh, eighth, ninth and tenth ganglia are more or less 

 spheroidal in form, the two first disappear, the three last are re- 

 tained in the adult insect. The eleventh ganglion is distinctly 

 divisible into two lobes by a bilaterally symmetrical construction, 

 representing thus, as also by the distribution of its nerves, the two 

 ganglia, by the fusion of which it is composed. In the larvae of 

 some insects, as, for example, Phalaena neustria, Borcus paral- 

 lelepipedus, and Corethra plumicornis, these ganglia retain as much 

 or more of their typical distinctness, and even in the larvae of 

 Muscidae, where the ventral cord is not itself segmented, eleven 

 pairs of nerves are given off from it exclusively of those in con- 

 nection with the jaws, indicating thus that the number of post-oral 

 ganglia is twelve. The tenth, and the bilobed eleventh ganglia, 

 appear to correspond to the six post-abdominal ganglia of Astacus 

 Jfumatilis ; and the five ganglia from the fifth to the ninth, with 

 the five ganglia in relation with the five pair of ambulatory 

 limbs in the Crustacean. A slip of blue paper has been passed 

 underneath the two diverging cords of commissure of the third and 

 fourth post-oral ganglia, and we see in the intercepted space one 

 of the systems of ' respiratory,^ ' accessory,' or ' transverse' nerves, 

 connecting itself with the larger ganglia of the ventral in much 

 the same way as the ' ganglion frontale' and the ' lateral nerve 

 ganglia* are connected with the cerebroid ganglia. The ' transverse 

 nerves' of each inter-ganglionic space are not only connected with 

 these larger ganglia, but also with each other, so as to form a 

 continuous chain overlying the ventral cord. From this chain, 

 nerves pass to the tracheae and spiracles, and also to the muscles 

 which act upon these organs and upon the wings. One of the 

 large longitudinal tracheae is seen on the right side of the future 

 thorax, in which part of the body the respiratory system attains 

 ultimately its greatest size and importance, and has the system 

 of transverse nerves similarly evolved in correlation with it. One 

 of the diagonal muscles is seen passing through the interval between 

 the cords connecting the second and third ventral ganglia. This 

 and the succeeding interspace are represented in the perfect insect 



