238 LIFE AND HER CHILDREN. 
simple eyes. They do not need to pause for breath 
since that is taken in through the holes in their 
sides (b, Fig. 81), and they eat so greedily that after 
a time their skin becomes too tight for the food they 
are packing into it. Then they pause and turn pale 
and remain still for a while, after which each one 
bends up his back, swells out his rings, and so splits 
the inconvenient coat along the back. Then drawing 
out first his head and then his tail, he comes out fresh 
in colour with every joint and hair in its place, and 
begins gorging once more. This they do as many 
as five separate times, and at the end of these 
changes their new form is already growing within 
them, for if you cut open a caterpillar just before it 
casts its last skin, you may see the outline of the 
wings and antennae of the future butterfly in a 
watery state, each in its transparent sheath. 
And now they must shut themselves up from the 
outer world, for each one has to make a sipping 
mouth instead of a biting one, and to gather up his 
muscles to make his shoulders strong to bear his 
wings ; and above all he has to draw together the 
line of nerve knots which in the caterpillar are 
stretched along his body as we saw them in the 
leech (p. 143), but which in the butterfly must be 
concentrated so as to make great central nerve 
stations in the head, for the use of the large eyes and 
sensitive antennas which are coming, and other sta- 
tions under the shoulders to supply his wings with 
energy. 
So each caterpillar again leaves off eating, and 
finding some firm spot on the trunk of a tree or a 
post, or a stem of a plant, makes there a little hillock 
