UTILITY OF BIRDS IN NATURE. 19 
them, feed first upon the fatty portions of the caterpillar 
in which they find themselves. The caterpillar thus unwill- 
ingly becomes their host, furnishing them with food and 
lodging from ‘and within its own substance. When they 
have made their growth, and it is nearly time for them to 
pupate, they attack the vitals of their host, killing it, and 
then pupating either within or upon its body. Soon they 
emerge as perfect flies, the females 
again seeking other caterpillars as 
hosts for their progeny. Often 
these parasites do not kill their 
bey Fig. 12.— Host caterpillar, with 
host until it has sought some place cocoons of the parasite upon its 
back. 
of safety and pupated. Every cat- 
erpillar or pupa thus destroyed nourishes one or many of 
these parasites, to emerge and attack surviving caterpillars. 
The parasites themselves, however, are often attacked in the 
same manner by a secondary parasite, which destroys them 
precisely as they destroyed the caterpillar. The larger pri- 
mary parasites may deposit a single egg or only a few in 
each caterpillar, while the smaller ones may deposit the 
entire brood in the body of a single caterpillar. 
Birds eat: both predaceous and parasitic insects. We have 
seen that they eat ground beetles, many of which are pro- 
vided with acrid secretions that are supposed to render them 
disagreeable and offensive to the taste, and so 
give them a certain immunity from their ene- 
mies. Evidently, however, it takes a very 
strong flavor to take the edge off a bird’s 
appetite, for birds eat bugs; and any child 
who has ever eaten berries from the bushes, 
and inadvertently put one of the berry-eating 
bugs in his mouth, knows how disgusting their 
Fig.13.—Tiger flavor is. There are some useful insects that 
form, “eaten are seldom eaten by birds. The very smallest 
by very few are beneath the notice of most birds. The 
om tiger beetles and some of the useful flies 
are so quick that birds find it difficult to catch them. 
Wasps and bees, though eaten by some birds, can protect 
themselves very well with their stings. Probably, however, 
