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.
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.
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.
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.
One generation, therefore, lays its eggs in the parenchyma of tender leaves, and has only to pierce through a thin layer of plant-tissue, while the other must penetrate deep down into the hard winter bud, to be able to deposit its eggs in the proper place, and we therefore find that in the two kinds of female the ovipositor differs in length, thickness, and general structure, and so also does the whole complex apparatus by which the ovipositor is moved. But these differences are associated with the form of the abdomen, in which the ovipositor lies, and with the strength and shape of the legs, which must be shorter and stronger when the boring has to be performed through a hard plant-tissue or to a considerable depth. We can readily understand how numerous must be the secondary variations which a transformation of the ovipositor brings in its train when we compare the ovipositor apparatus in the two generations of one of these species (Fig. 126).