16 
HALF IlOUftS WITH INSECTS. [Packard. 
(Fig. 5) bearing a close resemblance to the common house¬ 
fly, are devoted to this work of parasitism-extraordinary. 
The process is thus: the parent fly lays its eggs within or 
upon the body of its victim, most commonly a caterpillar 
which is full-grown and about to enter upon its transforma¬ 
tions. The young worm hatches and feeds upon the large 
stores of fatty tissues which surround the vital organs of its 
host. How very nice must be the adjustment of relations 
between the two animals, when in the case of the parasite, 
the slightest deviation from its path involving any injury 
to the neighboring nerves or vessels of its host will bring 
Fig. 5. 
Tachina and larva. 
eventual ruin to itself by hastening the death of the cater¬ 
pillar ! This delicate balance between the growing parasite 
and wasting victim may continue through the chrysalid 
state of both insects, until just as we think the butterfly 
will break its prison walls, lo! the trim, neat form of its 
unrelenting enemy steps forth from its body ready armed 
and equipped for active service. 
The late Mr. Herrick some years ago announced the dis¬ 
covery of the fact that the eggs of the canker worm were 
preyed upoh by a minute ichneumon-fly. Here is an anom¬ 
aly—an egg nourishing its inhabitant and immersed in the 
16 
