90 



THE YOUNG NATURALIST. 



from this procedure it will be plainly seen bow the balance of life is kept up, 

 the latter to prevent an undue number of white butterflies, and the former to 

 prevent glomeratus from exterminating P. brassicce and rapce. Dean Swift 

 very quaintly remarked"* " The vermin only tease and pinch, their foes superior 

 by the inch ; so Naturalists observe, a flea, hath smaller fleas that on him prey, 

 and these have smaller still to bite ; em, and so proceed ad infinitum,." This 

 does not sound so poetical as the new version " Big fleas have little fleas 

 upon their backs to bite em, and little fleas have lesser fleas, and so 

 ad infinitum." 



In this country very few people know the enormous amount of good these 

 parasites do for the community. Take for instance a species not indigenous 

 to a country, and see how they will increase, provided the climate be suitable, 

 and food abundant, until they become a pest and a nuisance. This can be 

 illustrated by an insect we all know so well, the small white butterfly P. rapce, 

 which was imported into America a few years ago, it is said by an enthusiastic 

 entomologist to enliven his surroundings, and make him think of " home 

 sweet home," little dreaming of the vast amount of injury it would do to his 

 turnip and cabbage crop, also to his neighbours and the country in general, 

 I will quote a sentence or two from the Canadian Entomologist, 1880 — 11 This 

 insect (Pieris rapce) was brought from Europe to Quebec a few years ago. 

 Trom Quebec it has spread over an immense area, extending now from 

 Alabama to the waters of Lake Superior, eastward to the Atlantic, and west- 

 ward many hundreds of miles, and over all this district it has done immense 

 damage to the cabbage crop." A parasite [Pteromalus puparum) has been 

 imported, and " Fortunately this little friend has also been introduced here 

 from Europe, and is rapidly spreading, following in the wake of its prey, and 

 where the parasite has fairly established itself this butterfly soon dwindles in 

 numbers, so materially as shortly to cease to be such an evil. The butterfly 

 spreads faster than its enemy, and is usually several years in advance of it," 

 (about 50 miles) we may confidently anticipate that this small fly will keep 

 this troublesome insect within due limits." 



The above named parasite is a much smaller fly than A. glomeratus ; they, 

 P. puparum make their cocoons within P, rapes after the latter has changed 

 into pupa. 



In the Entomologist, vol. xxi., p. 263, I recorded the unusual number of 

 A. glomeratus from P. brassicce, mentioned above, this brought a letter from 

 Washington, from Professor C. Y. Eiley, State Entomologist, requesting me 

 as a great favour to send him all the A. glomeratus 7 cocoons I could obtain, 

 for distribution in infested districts. 

 * Vol. vii. page 268, Edition 176b. 



