This kind of alternation of generations, that is, the alternation between unisexual (female) and bi-sexual generations, has been called heterogony. In the Daphnids, certainly, a difference in form between the parthenogenetic and the bi-sexual generation does not exist, for the same females which produce eggs requiring fertilization can also produce parthenogenetic ova, although these are very different from each other, as we have already seen. The difference between generations, therefore, does not lie in their structure, but in their tendency to parthenogenetic or to amphigonous reproduction, and in the absence or presence of male individuals. There are, however, other cases of alternation of generations in which the different generations diverge from each other in structure. One of the most remarkable of these is that of the gall-wasps (Cynipidæ). In many of these little Hymenoptera, 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 readily 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 renum_ (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, 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. [Illustration: FIG. 124. Alternation of generations in a Gall-wasp. _A_, winter generation (_Biorhiza renum_). _B_ and _C_, summer generations (_Trigonaspis crustalis_). _B_, male. _C_, female. After Adler.] 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 work requires, according to Adler, uninterrupted active exertion for about three days, even though in the end only two buds may be filled with eggs. If at every egg-laying the arrival of a male had to be waited for, an even larger number of females would fall victims to the unfavourable 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.