90 
INTRODUCTION. 
use of this instrument is to pierce the skin of the 
caterpillar, and to form a conduit for conveying the 
eggs into the hole thus prepared for their reception. 
When the fly has selected a caterpillar fitted for her 
purpose, she alights upon its back, and plunges her 
weapon into its body, chiefly at the incisures of the 
segments, depositing an egg at every insertion. This 
operation is repeated till no fewer than thirty or forty 
eggs are sometimes laid in the body' of a single ca- 
terpillar. These are soon hatched in their singular 
nidus, and the grubs which they produce imme- 
diately begin to feed on the substance of the living 
animal. They do not, however, devour every part 
indiscriminately, but are taught by a wonderful in- 
stinct to abstain from injuring any vital organ, as if 
aware that their own existence depended upon that 
of their unwilling foster-parent. In consequence of 
this, the caterpillars survive for a considerable time, 
and sometimes retain sufficient strength to assume 
the pupa state, in which, however, they invariably 
perish. But most frequently the grubs arrive at 
maturity before that change takes place, and in that 
case they escape from the body of the caterpillar by 
gnawing a passage through its sides. Having in 
this way effected their liberation, they arrange them- 
selves round the sides of the caterpillar, which is 
now so exhausted that it soon dies, and spin cocoons 
of a fine yellow colour, in which they are transformed 
into pupae. When the perfect fly is ready to emerge, 
it pushes open a small lid at one end of the cocoon, 
