24 INTERNAL ANATOMY OF INSECTS. 
of the cerebral pulp. In some cases, as in Dytiscus mar- 
ginalis, and Hydrophilus piceus*; the imago has only one 
ganglion less than the larva, but more generally it loses 
four or five. Dr. Herold has traced the gradual changes 
that take place in the spinal marrow of the common cab- 
bage-butterfly (Pzeris 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’. 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*. On the eighth day, when the insect has assumed 
the pupa but remains still in the skin. of the caterpillar, 
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 considerably more than 
a third of its length*. On the fourteenth day, the in- 
ternodes, except the double ones, have become nearly 
straight again; the fourth and fifth ganglions have coa- 
lesced so as to form one, and the sixth and seventh have 
each lost their pairs of nerves ©. Shortly after this, these 
* Cuv. Anat. Comp. ii, 322, 323— ; 338, 339—. 
> Prate XXX, Fic. 1. © Ibid. Fie. 2. 
4 Thid. Fic. 3. © Herold Schmett. ¢. i. f. 6. 
