PLANT-EATING INSECTS 203 



menon which has been termed " alternation of generation." 

 It may be conveniently studied in the life-history of 

 the "spangle gall- wasp." Spangle galls appear in enormous 

 numbers on the underside of oak leaves in July. They 

 are small, button-like objects, usually pale yellow, though 

 sometimes brownish-red in colour, and are covered densely 

 with radiating stellate hairs. In September, or early 

 October, they fall to the ground, where they lie literally 

 by millions among the grass stems, in wheel ruts through 

 the woodlands, and in holes and crannies of the earth. 

 Here they remain throughout the winter, and it is 

 remarkable that they do not suffer loss in their severance 

 from the leaves. Indeed, each spangle gall may now 

 be regarded as an independent atom of vegetable life. 

 The cells of which it is formed remain capable of absorbing 

 moisture. Thus, during the winter, the galls swell up, 

 become more strongly obtuse, and increase considerably 

 in bulk. The larva lies snugly in its chambers at the 

 centre of the gall, secure from frosts and biting winds, 

 and surrounded by a plentiful supply of food. In these 

 happy circumstances it continues to feed throughout the 

 winter, and changes to the pupa in March. Towards the 

 end of this month the mature insect cuts its way out 

 of the gall. 



And now we come to a very remarkable fact, namely, 

 that no one has ever reared a male insect from a spangle 

 gall. If males ever existed, they must have become 

 extinct long ages before naturalists began to unravel the 

 mysterious life-histories of gall-insects in general. How 

 and why this state of things came about it is impossible 

 to conjecture ; but the fact remains that the gall-wasps 

 which issue from spangle galls are all " bachelor women " 

 and never go courting, for the sufficient reason that their 

 world contains no beaux. Nevertheless — and herein lies 



