76 THE CLASS OF INSECTS. 



more or less tainted with melanism, and according to Knaggs, 

 there is a "frequent recurrence of individuals wanting a hind 

 wing, which may be noticed even at large in Macaria notata." 

 "Few species are liable to the same extent of variation, and 

 many apparently to none at all." Certain species vary "ac- 

 cording as they luay have reproduced, generation after gen- 

 eration, on a chalky, peaty, gravelly, or other soil." Food also 

 exerts an influence in inducing variation, according as cater- 

 pillars of the same species feed on different plants ; this occurs 

 most commonl}' in the Micro-lepidoptera. (Knaggs, in the 

 Entomologist's Monthly Magazine, London.) 



Introduced species of insects, like those of plants, often thrive 

 more vigorous!}^ than the native forms. This is instanced by 

 native insects which abound in unusual numbers in newly 

 cleared districts where the former presence of forests and 

 their natural foes kept them under. The Potato-beetle, Can- 

 ker-worm, and ClisiocamjJO must have lived formerly in mod- 

 erate numbers on our native plants, where now countless hosts 

 affect our introduced plants. Among species introduced from 

 a foreign country we have only to instance the Hessian J'ly, 

 the AVheat-midge, the Coddling-moth, the Clothes-moth, the 

 Apple Bark-louse, and the Grain-weevil. Mr. W. T. Brig- 

 ham informs ug that some of the most abundant insects in the 

 Hawaiian Islands are introduced species carried by vessels 

 from Europe. Vanessa Antiopa^ Pyrameis cardui, and P. 

 AtaJanta, so abundant in this country, are supposed to be intro- 

 duced butterflies. Aj^hodius Jimetarius, found by us living in 

 dung on Mt. Washington, is one of our most common beetles, 

 and the Asparagus-beetle, introduced from Europe a few years 

 since, is common in gardens in Eastern New York, while Mr. 

 Walsh has recorded the appearance of the European Gooseberry 

 Saw-Fly, which ravages the Gooseberry and Currant. Pieris 

 rapce, the Cabbage-butterfly, introduced from Europe into 

 Quebec about 1859, soon became abundant within a circle of 

 forty miles radius about that city, and has even spread into 

 Maine and Vermont along the railroads leading from Quebec. 



Insect Years. There are insect years as well as "apple 

 years," seasons when insects most abound. Every collector 

 knows that there are certain years when a particular species of 



