INBREEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION 245 



ptera, which form galls on leaves, blossoms, buds, and roots, especially 

 of the oak, two generations occur annually, one in summer, the other 

 in early spring, or even in the middle of winter. The latter consists 

 of females only and reproduces parthenogenetically. We can readilj^ 

 understand this from the point of view of adaptation to particular 

 conditions, since the young wasps which emerge from their galls in 

 winter, or in the middle of a raw spring, are exposed to many 

 dangers and are terribly decimated before they can succeed in laying 

 their eggs in the proper place on the plant. Moreover, much precious 

 time would be lost by the mutual search of the sexes for each other, 

 — a search which would often be entirely without result. Thus the 

 wingless female of Biorhiza renwni (Fig. 124, A), which is not unlike 

 a plump ant, attempts, without taking food, and often interrupted 

 by a spell of cold or a snowstorm, to reach a neighbouring oak-shrub, 



Fig. 124. Alternation of generations in a Gall-wasp. A, winter genera- 

 tion {Biorhiza rmum). B and C, summer generations {Trigonaspis cntstalis). 



B, male. 



C, female. After Adler. 



creeps up on it, and lays its eggs in the heart of a winter bud, whose 

 hard protecting scales it laboriously perforates by means of its short, 

 thick, sharp ovipositor. 



After it has succeeded in sinking its ovipositor into the heart of 

 the bud, it goes on working for hours, piercing the delicate tissue 

 with a multitude of fine canals, one close beside the other, and then 

 deposits an egg in each of these. The whole detailed piece of w^ork 

 requires, according to Adler, uninterrupted active exertion for about 

 three days, even though in the end only two buds may be filled with 

 eo-o-s. If at everj^. egg-laying the arrival of a male had to be waited 

 for, an even larger number of females would fall victims to the un- 

 favourable weather and other dangers, while at the same time the 

 number of emerging females could be only half as large as it is. 

 It is obvious that in this case parthenogenesis is of very great 

 advantage. 



