438 INTRODUCTION TO ZOOLOGY CHAP, xxvn 



or cells, in each of which a larva lies. It is in August and 

 September that these galls are most beautiful ; later they 

 become dry and brown, though they still contain the larvae, 

 which winter within the galls and pupate in the spring. 

 Both male and female flies are known to emerge, though the 

 former are rare, and it seems probable that parthenogenesis 

 is of general occurrence here also. 



Both the Gall Wasps described above have a simple life- 

 history with only a single generation in the year, the most 

 striking feature being the apparently continual partheno- 

 genetic reproduction in the marble gall on the oak. Most 

 of the Gall Wasps of the oak, however, go through a curious 

 alternation of two generations in their annual life-cycle an 

 alternation of a parthenogenetic generation with a normal 

 sexual one each of the two generations producing a character- 

 istic gall on some part of the tree which is unlike the gall 

 produced by the other generation. These two generations 

 when first described were thought to have no connection with 

 each other, and so each was given a separate genetic and 

 specific name. These names are still largely used, though 

 they are now known to refer to different stages, merely, in 

 the life -cycle of one individual ; the dual names are still 

 retained in this book, though perhaps the time has come to 

 simplify the nomenclature. 



Oak Galls ^e ^ a ^ Wasp which forms the common 



with " spangle gall " may be studied as an example 



Alternating O f a g a u with two alternating generations (Plate 



terations. y ^ ^ The gpangle gall (f orme d by Neuroterus 



lenticularis) is extremely common on the under side of oak 

 leaves in late summer, forming little disc-like plates which 

 are thicker in the centre than round the margin, reddish in 

 colour, and have brown stellate hairs over the surface. In 

 the autumn these galls fall with the leaves to the ground, 

 separating from them as the leaves decay. The Gall 

 Wasps emerge in the spring, and here, as in the marble gall, 

 all the emerging insects are females which can lay partheno- 

 genetic eggs. They fly off and pierce the young leaf and 

 flower buds which are just about to start growth. The result 

 is, not the formation of spangle galls again, but of the widely 

 different "currant galls" (Plate V., A^) which appear either 

 on the young leaves or on the staminate catkins. The insect 



