110 BRITISH INSECTS 



Familiar examples are the " oak-apple," which origin- 

 ates in a bud, and the various "cherry," " spangle," 

 and " button " galls that are attached to the leaves. In 

 a general way it may be said that each species of 

 gall- wasp is bred from a gall which is perfectly distinct 

 in form and appearance from all the other kinds, but 

 the life-cycles of certain species involve a remarkable 

 " alternation of generation." For example, a genera- 

 tion of winged gall-wasps of both sexes emerges from 

 the oak-apple in early summer. After pairing, the 

 females penetrate the soil, and lay their eggs in the 

 roots of the oak. This causes a crop of subterrannean 

 galls to be produced, and the grubs that feed therein 

 become wingless wasps, all females endowed with the 

 faculty of parthenogenesis, or virgin reproduction. 

 During the winter these insects creep up the oak trunks, 

 and by laying their eggs in the buds, give rise to the 

 oak-apples of the succeeding spring. Thus two differ- 

 ent galls, and what at first seem to be two kinds of 

 insects, really all belong to one and the same species. 

 Gall-wasps are small, dark-coloured insects. Our 

 largest, 1 which is bred from the well-known marble- 

 gall of the oak, is slightly less than one-sixth of an 

 inch in length. It was not observed in England until 

 1834, but is now very abundant. Yet, notwithstand- 

 ing diligent search, no male has been discovered. 

 Apparently the species is perpetuating itself, year after 

 year, through successive generations of partheno- 

 genetic females ! Not all the Cynipidce are gall- 

 causers. Some are parasites, while others are in- 

 quilines i.e., they lay their eggs in galls that have 

 been induced by other species. 



Most of the ichneumons, of which we have very 

 1 Cynips kollari 



