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 margincdis, and Hij- 
dr ophilus piceus b , the imago has only one ganglion less 
than the larva, but more generally it loses four or Jive. 
Dr. Heroic! has traced the gradual changes that take 
place in the spinal marrow of the common cabbage-but- 
terfly {Pontia Brassicce), 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- 
* Cuv. Anat. Camp. ii. 319. 337. 
b Ibid. ii. 322, 323- ; 338. 339—. 
c Plate XXX. Fig. 1. d Ibid. Fig. 2. 
