Birds observed at Montreal. 419 
The Cabbage Butterfly, Pieris Rapae, was by some means 
brought to this country from Europe some twenty-five or 
thirty years ago, and as its principle food plant was plenti- 
ful, and the summer long and warm, it soon became exces- 
sively abundant.. Of late years, however, its numbers have 
been greatly reduced by a small hymenopterous insect, 
Pteromalus puparum. which, piercing the caterpillar with its 
ovipositor, deposits anumber of eggs in its body. The cater- 
pillar thus attacked, continues to feed, and in due time 
changes to a chrysalis, but never reaches the perfect or 
butterfly state. The parasites now finish their work, and 
transforming within the chrysalis, cut their way out, to de- 
stroy in their turn another brood of caterpillars, 
The May beetle is another instance. The larva of 
this insect passes its preparatory stages in the earth where 
it feeds on the roots of grasses and other plants, never 
‘appearing above ground until it emerges as a beetle, but 
even this concealment does not save it from its enemy, a 
large black ichneumon fly, Typhia inorata, which, by some 
wonderful instinct finds it and deposits an egg in it, after 
which its death is only a question of time. The thorough- 
ness of the work done by the parasitic insects is no doubt 
largely owing to the fact that as a rule they restrict their 
attacks to a single species, or to species belonging to the same 
genus. Moreover, the life of the perfect insect is generally 
brief and almost entirely occupied in providing for the con- 
tinuance of the species, hence these parasitic insects are con- 
stantly occupied in searching for the particular kind of 
larvee to which their instinct teaches them to commit their 
eggs. The bird might eat the caterpillar if it came in its 
way, the parasite must find and destroy it, or fail to accom- 
plish the chief end of its existence. But the question may 
he asked, how is it that with this army of parasitic insects 
to help us, we are ever troubled by injurious species ? Well, 
Nature’s plan is not to exterminate any species, but to keep 
all within proper bounds, we, however, are continually vio- 
lating her laws, covering acres of ground with wheat, 
cotton, or some other crop to the entire exclusion of all 
