THE CANADIAN ENTOMOLOGIST. 193 



its larva in the stomach of any bird. In its native home in Europe it is 

 seldom so very destructive as here, for the reason that a small four-winged 

 fly, Pteromalus piiparum, an insignificant looking little creature, is a para- 

 site on the larva of this butterfly, and hunts its victims with the greatest 

 assiduity; alighting on their backs and thrusting its slender ovipositor 

 through the skin of the larva, it deposits a number of eggs there, which 

 hatch into tiny grubs, and these feed upon and eventually destroy the 

 caterpillar. By the constant efforts of this little parasite the cabbage 

 butterfly is prevented in Europe from becoming a very serious pest. 

 Fortunately this little friend has also been introduced here from Europe, 

 although in what manner is not known, and is rapidly spreading, following 

 in the wake of its prey, and where the parasite has fairly established itself 

 this butterfly, with its numerous progeny of green caterpillars, soon 

 dwindles in numbers so materially as shortly to cease to be so grievous an 

 evil. The butterfly spreads faster than its enemy and is usually several 

 years in advance of it, but we may confidently anticipate that sooner or 

 later this small fly will do for us what it has done for Europe — keep this 

 troublesome insect within due limits. Many other similar examples might 

 be given. 



Further, the help of friendly parasitic insects is so much more efficient 

 because it is in most instances discriminating. As far as is known, the 

 little parasite referred to attacks only the larva of the cabbage butterfly, 

 and in like manner many other parasitic species are restricted in their 

 operations to a single species, while in other instances they are confined 

 to a genus or a group of similar species. This is not so with insectivorous 

 birds ; they in most instances devour alike the useful and the injurious 

 species, and the question may well be raised in many instances whether 

 the good they do is not more than counterbalanced by the number of 

 useful insects they devour. Recent observations on the family of thrushes 

 by Mr. S. A. Forbes, of Illinois, seem to show that their insect food con- 

 sists largely of beetles belonging to the Carabidre, a family every member 

 of which is useful, since they feed both in the larval and beetle states 

 exclusively on other insects. 



The. field here open is a wide and inviting one, on which I trust some 

 of you will enter. I have but touched upon it ; as the results of more 

 extended observations are recorded the opinions here expressed may need 

 modifying. I desire to do justice to the birds. 



During the month of August last it was my privilege to visit the Great 



