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 antennæ, the legs, and particularly 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. [Illustration: FIG. 125. The two kinds of Galls formed by the species. _A_, the many-chambered galls produced by the parthenogenetic winter form, _Biorhiza renum_. _B_, those produced on oak-leaves by _Trigonaspis crustalis_, the bi-sexual form. After Adler.] 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 larvæ 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 crustalis_. Both males and females are winged, and fly rapidly about in the air (Fig. 124, _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 _Biorhiza renum_ was given.