24? INTERNAL ANATOMY OF INSECTS. 



beetle, in the larva of which there is only one ganglion, 

 while in the imago there are four a . But as this one 

 ganglion occupies the whole spinal marrow, it is really 

 of greater extent than the four of the imago ; so that 

 even in this case there is a concentration of the cerebral 

 pulp. In some cases, as in Dytiscus marginalis, and Hy- 

 drophilus piceus b , the im#go has only one ganglion less 

 than the larva, but more generally it loses four or Jive. 

 Dr. Herold has traced the gradual changes that take 

 place in the spinal marrow of the common cabbage-but- 

 terfly (Pontia Brassica), from the time that it has attained 

 its full size to its assumption of the imago. Of these I 

 shall now give you some account. 



In the full-grown caterpillar, besides the brain there 

 are eleven ganglions, the chords of the four first inter- 

 nodes being double, and the rest single: from each gan- 

 glion proceed two pairs of nerves, one from each side. 

 In this the lobes of the brain form an angle with each 

 other c . In two days the double chords mutually recede, 

 so as to diminish the interval between the ganglions, and 

 the single ones have become curved : thus the length of 

 the spinal marrow is shortened about a fourth, and the 

 fourth and fifth ganglions have made an approach to 

 each other d . On the eighth day, when the insect has 

 assumed the pupa but remains still in the skin of the ca- 

 terpillar, the flexure of the internodes is much increased; 

 the first ganglion is now united to the brain, and the 

 fourth and fifth have joined each other, though they are 

 still distinct ; the spinal marrow has now lost consider- 



a Cuv. Anat. Comp. ii. 319. 337. 



b Ibid. ii. 322, 323 ; 338. 339. 



c PLATE XXX. FIG. 1. ri Ibid. FIG. 2. 



