BEHAVIOUR III 



in the accepted meaning of the term, may undergo marked 

 changes of habit on account of some new factor in their sur- 

 roundings. Such new factors are often due to man's uncon- 

 scious intervention, and the resulting changes of behaviour 

 among insects may therefore be noticed by those naturalists 

 who pay especial attention to those insects that feed on 

 plants cultivated as farm or garden crops, or that live as 

 parasites on domestic animals. There is a beetle, a small 

 weevil, Orchestes fagi^ that is very common on beech trees 

 in this country, the adults eating the leaves into holes, and 

 their grubs mining between the upper and lower leaf-skins. 

 During recent years both in England (F. V. Theobald, 

 191 2) and in Ireland (G. H. Carpenter, 1920) it has been 

 observed that large numbers of these beetles, blown off 

 beech trees into apple orchards, take to feeding on the 

 growing fruitlets — a new kind of tree and a different part 

 of the plant as compared with the normal feeding place of 

 their kind. Also a sucking insect, a plant bug Plesio- 

 coris rtigkolliSy whose normal food is the sap of willow 

 leaves, has been noticed repeatedly since the observations 

 of F. R. Fetherbridge and M. A. Husain (19 18) piercing 

 the skin of young apples to suck their juice. It is highly 

 probable that these are new modes of behaviour adopted 

 by members of these and other species that have been 

 accidentally brought into touch with a new plant on which 

 they are able to feed. 



Similar changes in habit are undoubtedly new departures 

 when they result from a newly introduced factor in the 

 surroundings of the insects that display them. Since the 

 beginning of this century tobacco has been grown in Ireland 

 on a small scale, and a crop in Co. Kilkenny was found to be 

 severely injured by multitudes of springtails feeding on the 

 leaves. These on examination proved to belong to a north 

 European species Isotoma tenella, never before recognised 

 in these countries. The minute insects had certainly not 

 been introduced with the tobacco, which was raised from 

 seed, and taking all the facts into consideration, no doubt can 

 remain that the presence of a large number of plants of a 



