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 antennae, 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 many 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 generations 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 Biorhiza 

 renum pierces the terminal buds of the 

 oak, and lays in each of them a large 

 number of eggs, sometimes as many as 

 300, so that a very large gall is formed, 

 in which a great many larvae 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 July the slender, delicately formed male and female 

 gall-wasps which were long known as Trigonaspis cruttcdis. Both 

 males and females are winged, and fly rapidly 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, 

 t-he name Biorhiza renum was given. 



One generation, therefore, lays its eggs in the parenchyma of 

 tender leaves, and has only to pierce through a thin layer of plant- 



F16. 125. The two kinds of 

 Galls formed by the species. A, 

 the many-chambered galls pro- 

 duced by the parthenogenetic 

 winter form, Biorhiza renum. B, 

 those produced on oak-leaves by 

 Trigonaspis crusialis, the bi-sexual 

 form. After Adler. 



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