INTRODUCTION 



A GALL is an abnormal growth of plant tissue produced 

 by animal agency acting from within. All the natural 

 orders of plants include species which are liable 

 to be made use of by insects in this way. Each is 

 visited by its own special gall-maker, which need not 

 necessarily belong to the Cynipidae, for gall-makers 

 are also found among the Coleoptera, Lepidoptera, 

 Diptera, Nematoda^ and in other classes. Any organ 

 of the plant may become the seat of this hyperplasia, 

 but the form which the gall ultimately assumes is 

 governed by the potentialities of growth in the part 

 attacked, and by the nature of the animal excitation 

 present. The rose and some Compositae produce well 

 known galls, but the oak is the favourite home of the 

 Cynipidae. In this monograph Dr. Adler has described 

 those oak-galls and gall-flies most commonly found in 

 this country, with the exception of Cynips Kollari, 

 the Devonshire marble-gall, which does not occur in 

 Germany north of the Elbe ; as it is however one of 

 the most familiar galls on English oaks, a description 

 of it has been added in the appendix. Before Dr. Adler 



^ H. Charlton Bastian, ' Monograph of the Anguillulidae,' Lin. Soc. 

 Trans, vol. xxv, 1866. 



