ANATOMY AND PHYSIOLOGY. 159 
stalks. Besides furnishing a lming to the abdominal 
scales, dorsal as well as ventral, this woolly-looking 
substance occurs in the form of detached masses, 
which are gathered into a kind of omentum over- 
lying the intestinal canal, and filling up all the inter- 
spaces of the coils. In the spring the fat masses are 
comparatively few and small, the nutritive matter 
having been consumed during hybernation. But in 
the autumn the young females are loaded with fat, 
which appears in this form. : 
The use of these fat masses is obvious. They are 
an internal supply of nourishment, which the larva 
prepares and lays up for the pupa or the perfect 
insect, against such times as they are unable to pro- 
cure nourishment for themselves from extraneous 
sources. The pupa lives on this fat, and finds ma- 
terials in it for the structures which she is developing 
during her sleep. And the perfect female wasp finds 
here the sustenance which she cannot go to look for, 
and she would scarcely find if she did, during her 
long hybernation. The larva does not need the 
fat for the purposes of her own existence, for it 
seems to make no difference to her growth whether 
the fat be laid up in store or be consumed, as fast as 
she makes it, by parasites. The common cabbage 
caterpillar supplies a familiar instance of this robbery 
of trust-property, so to say. As soon as the cater- 
pillar ceases to make nourishment for the unbidden 
guests which the ichneumon-fly has inserted beneath 
her skin, just in fact at the time when she is about 
to change into a chrysalis, the grubs make their way 
out of the empty house in all directions, and the 
“host,” previously undistinguishable from other cab- 
