CHAP, xxvn INSECTA : HYMENOPTERA 



441 



within these galls is distinguished as Spathogaster baccarum. 

 From the currant galls, in June, emerge both male and 

 female gall insects ; the latter lay fertilised eggs in tender 

 young oak-leaves, causing once more the production of 

 spangle galls, and so the cycle is completed. 



Similar alternation of two generations one sexual and 

 one parthenogenetic and of the alternation of two corre- 

 sponding different forms of galls, are known in a great many 

 other cases. Some of these are given in the following Table, 

 and are illustrated in Plates V. and VI. Those mentioned 

 are only a few of the many that are know r n ; they are chosen 

 for mention here either because they are well-marked, easily 

 recognisable forms, or because they are very frequently found 

 in the south of England. 



OAK GALLS WITH ALTERNATING GENERATIONS 1 



* This form should perhaps be removed from the genus Neuroterus ; see Adler. 



1 The nomenclature is based on that given in Adler's Alternating Genera- 

 tions, A Study of Oak Galls and Gall Flies, translated into English and 

 edited by C. R. Straton (Clarendon Press, 1894). 



