246 



THE EVOLUTION THEORY 



In summer the climatic conditions are incomparably more 

 favourable for the gall-wasps, and accordingly we find that the 

 summer generation is bi-sexual, but, strangely enough, is so different 

 from the winter generation that the relationship of the two forms 

 was for a long time overlooked. The antennas, the legs, and particu- 

 larly the ovipositor, the whole shape of the animal, its size, the length 

 of the abdomen, the structure of the thorax, and manj^ other points 

 are so different that as long as the structural features afforded the 

 only criterion of relationship, the systematists quite naturally placed 

 the winter and summer forms in different genera. It was only when 

 Dr. H. Adler succeeded in breeding the one form from the other that 



people were convinced that such marked 

 differences in structure could be found 

 within the same life-cycle. 



But we see here quite clearly why the 

 two D-enerations had to become so different; 

 simply because the winter generation had 

 to adapt itself to different conditions from 

 the summer generation, above all as to 

 the laying of its eggs within the tissues 

 of a plant of a different constitution. In 

 our example, the winter form Blorhiza 

 renum pierces the terminal buds of the 

 oak, and lays in each of them a large 

 number of eggs, sometimes as many as 

 The two kinds of qoo, SO that a very large gall is formed. 



Galls formed bv the species. A, ? . . . / ^ n \ 



the many-chambered galls pro- m which a great many larvas can find 



food, and grow on to the pupa-stage. 



From this spongy gall, something like 



an inverted onion in shape, and about 



the size of a walnut (Fig. 125, A), there 



emerge in Jul}' the slender, delicatel}^ formed male and female 



gall-wasps which were long known as Trigonaspls crustalis. Both 



males and females are winged, and Hy rapidl}' about in the air 



(Fig. 1 25, B and C). The sexes pair, and the females lay their eggs 



in the cell-layers on the under side of an oak-leaf, on which arise 



small, wart-like, kidney-shaped galls (Fig. 125, B) which fall to the 



ground in autumn, and from which there emerge, in the middle of 



winter, the plump, wingless females, to which, as we have already seen, 



the name Blorhiza renum was given. 



One generation, therefore, la^-s its eggs in the parenchyma of 



tender leaves, and has onl}' to pierce through a thin layer of jjlant- 



FiG. 125. 



duced by the parthenogenetic 

 winter form, Biorhiza renum. B, 

 those produced on oak-leaves by 

 Trigonaspis crustalis, the bi-sexual 

 form. After Adler. 



