It is a curious fact that Cynipid galls are formed chiefly on oaks, this kind of tree supplying a surprising number and variety of galls. The plants that furnish Cynipid galls in Europe are not numerous. A list of them is given by Cameron.[[433]] Several species, of the genus Rhodites, attack rose-bushes. One of the best known of our British galls is the bedeguar, found in various parts of the country on both wild and cultivated rose-bushes (Fig. 348), and caused by Rhodites rosae (Fig. 349). This gall has the appearance of arising from a twig or stem, but it is really a leaf gall. Pazlavsky[[434]] has described the mode of formation of the bedeguar. The female Rhodites in the spring selects a rose-bud—not a flower-bud—that should produce a twig and leaves, and pricks this bud in a systematic manner in three places. The three spots of the bud pricked by the Insect are the three undeveloped leaves that correspond to a complete cycle in the phyllotaxis of the plant. The three rudiments do not develop into leaves, but by a changed mode of growth give rise to the bedeguar. Usually this gall, as shown in our figure, is of large size, and contains numerous cells; but abortive specimens are not infrequently met with; sometimes a small one is seated on a rose-leaf, and it is thought that these are due to a failure on the part of the Insect to complete the pricking operation. Cynipidae will not go through their gall-making operations except under natural conditions. Giraud[[435]] attempted to obtain oviposition, on gathered twigs of oak, from flies in confinement; but, although he experimented with thousands of specimens, they on no occasion laid their eggs in the fresh shoots placed at their disposal, but discharged their eggs in little heaps, without attention to the twigs. The same observer has also called attention to the fact that after being deposited in a bud the eggs of certain species of Cynips will remain dormant without producing, so far as can be seen, any effect on the tree for a period of fully ten months, but when the bud begins to develop and the egg hatches then the gall grows.
Fig. 349.—Rhodites rosae, female. Cambridge.
The exact mode in which the egg is brought to the requisite spot in the plant is still uncertain. The path traversed by the ovipositor in the plant is sometimes of considerable length, and far from straight; in some cases before it actually pierces the tissues, the organ is thrust between scales or through fissures, so that the terebra, or boring part of the ovipositor, when it reaches the minute seam of cambium, is variously curved and flexed. Now as the canal in its interior is of extreme tenuity, and frequently of great length, it must be a very difficult matter for the egg to reach the tissue where it should develop. The eggs of Cynipidae are very remarkable bodies; they are very ductile, and consist of a head, and of a stalk that in some cases is five or six times as long as the head, and is itself somewhat enlarged at the opposite end. Some other Hymenoptera have also stalked eggs of a similar kind (Fig. 357, A, egg of Leucospis). It has been thought that this remarkable shape permits of the contents of the egg being transferred for a time to the narrower parts, and thus allows the broader portion of the egg to be temporarily compressed, and the whole structure to be passed through a very narrow canal or orifice. It is, however, very doubtful whether the egg really passes along the canal of the borer. Hartig thought that it did so, and Riley supports this view to a limited extent. Adler, however, is of a different opinion, and considers that the egg travels in larger part outside the terebra. It should be remembered that the ovipositor is really composed of several appendages that are developed from the outside of the body; thus the external orifice of the body is morphologically at the base of the borer, the several parts of which are in longitudinal apposition. Hence there is nothing that would render the view of the egg leaving the ovipositor at the base improbable, and Adler supposes that it actually does so, the thin end being retained between the divisions of the terebra. Riley is of opinion that the act of oviposition in these Insects follows no uniform system. He has observed that in the case of Callirhytis clavula, ovipositing in the buds of Quercus alba, the eggs are inserted by the egg-stalk into the substance of the leaf, and that the egg-fluids are at first gathered in the posterior end, which is not inserted. "The fluids are then gradually absorbed from this exposed portion into the inserted portion of the egg, and by the time the young leaves have formed the exposed [parts of the] shells are empty, the thread-like stalk has disappeared, and the egg-contents are all contained within the leaf tissue." He has also observed that in Biorhiza nigra the pedicel, or stalk, only is inserted in the embryonic leaf-tissue, and that the enlarged portion or egg-body is at first external. The same naturalist also records that in the case of a small inquiline species, Ceroptres politus, the pedicel of the egg is very short, and in this case the egg is thrust down into the puncture made by the borer, so that the egg is entirely covered.
Some Cynipidae bore a large number of the channels for their eggs before depositing any of the latter, and it would appear that it is the rule that the boring of the channel is an act separate from that of actual oviposition. Adler distinguishes three stages: (1) boring of the canal; (2) the passage of the egg from the base of the ovipositor, where the egg-stalk is pinched between the two spiculae and the egg is pushed along the ovipositor; (3) after the point of the ovipositor is withdrawn, the egg-body enters the pierced canal, and is pushed forward by the ovipositor until it reaches the bottom.[[436]]
About fifty years ago Hartig reared large numbers of certain species of gall-flies from their galls, obtaining from 28,000 galls of Cynips disticha about 10,000 flies, and from galls of C. folii 3000 or 4000 examples of this species; he found that all the individuals were females. His observations were subsequently abundantly confirmed by other naturalists, among whom we may mention Frederick Smith in our own country, who made in vain repeated attempts to obtain males of the species of the genus Cynips. On one occasion he collected in the South of England 4410 galls of C. kollari (at that time called C. lignicola), and from these he obtained 1562 flies, all of which were females. A second effort was attended with similar results. Hartig, writing in 1843, after many years' experience, stated that though he was acquainted with twenty-eight species of the genus Cynips, he had not seen a male of any one of them. During the course of these futile attempts it was, however, seen that a possible source of fallacy existed in the fact that the Insects were reared from collected galls; and these being similar to one another, it was possible that the males might inhabit some different gall. Adler endeavoured to put the questions thus raised to the test by means of rearing females from galls, and then getting these females to produce, parthenogenetically, galls on small oaks planted in pots, and thus completely under control. He was quite successful in carrying out his project, and in doing so he made a most extraordinary discovery, viz. that the galls produced by these parthenogenetic females on his potted oaks, were quite different from the galls from which the flies themselves were reared, and were, in fact, galls that gave rise to a fly that had been previously considered a distinct species; and of this form both sexes were produced. Adler's observations have been confirmed by other naturalists, and thus the occurrence of alternation of generations, one of the two generations being parthenogenetic, has been thoroughly established in Cynipidae. We may mention one case as illustrative. A gall-fly called Chilaspis lowii is produced from galls on oak-leaves at Vienna at the end of April, both sexes occurring. The female thereafter lays eggs on the ribs of the leaves of the same kind of oak, and thus produces a different gall from that which nourished herself. These galls fall off with the leaves in the autumn, and in July or August of the following year a gall-fly is produced from them. It is a different creature from the mother, and was previously known to entomologists under the name of Chilaspis nitida. Only females of it occur, and these parthenogenetic individuals lay their eggs in the young buds of the oak that are already present in the autumn, and in the following spring, when the buds open and the leaves develop, those that have had an egg laid in them produce a gall from which Chilaspis lowii emerges in April or May. In this case therefore the cycle of the two generations extends over two years, the generation that takes the greater part of the time for its production consisting only of females. Adler's observations showed that, though in some species this alternation of generations was accompanied by parthenogenesis in one part of the cycle, yet in other species this was not the case. He found, for instance, that some gall-flies of the genus Aphilothrix produced a series of generations the individuals of which were similar to one another, and were all females and parthenogenetic. In some species of the old genus Cynips no males are even yet known to occur. A very curious observation was made by the American, Walsh, viz. that of galls gathered by him quite similar to one another, some produced speedily a number of both sexes of Cynips spongifica, while much later on in the season the remainder of the galls gave rise to females only of an Insect called Cynips aciculata. It is believed that the galls gathered by Walsh[[437]] were really all one species; so that parts of the same generation emerge at different times and in two distinct forms, one of them parthenogenetic, the other consisting of two sexes. It has, however, been suggested that Cynips spongifica and C. aciculata may be two distinct species, producing quite similar galls.
Turning now to the questions connected with inquiline or guest-flies, we may commence with drawing attention to the great practical difficulties that surround the investigation of this subject. If we open a number of specimens of any kind of gall it is probable that several kinds of larvae will be found. In Fig. 350 we represent four kinds of larvae that were taken out of a few bedeguar galls gathered on one day in a lane near Cambridge. It is pretty certain that No. 1 in this figure represents the larva of Rhodites rosae, and that Nos. 2 and 3 are larvae of inquilines, possibly of Synergus, or of a parasite; while No. 4, which was engaged in feeding on No. 3 in the position shown, is possibly a Chalcid of the genus Monodontomerus, or may be Callimome bedeguaris. It is clear that, as we cannot ascertain what is inside a gall without opening it, and thereby killing the tenants, it is a most difficult matter to identify the larvae; the only safe method is that of observation of the act of oviposition; this may be supplemented by rearing the flies from galls, so as to ascertain what variety of flies are associated with each kind of gall. This last point has been well attended to; but the number of cases in which oviposition of inquiline gall-flies in the galls formed by the Psenides has been ascertained by direct observation is still very small; they are, however, sufficient to show that the inquilines deposit their eggs only after the galls are formed.
Fig. 350.—Larvae inhabiting bedeguar gall at Cambridge. 1, Rhodites rosae in cell; 2 and 3, larvae of inquilines; 4, larva of a parasitic Hymenopteron.
Bassett recorded the first case of the kind in connexion with a North American species, Cynips (Ceroptres) quercus-arbos Fitch. He says: "On the first of June galls on Quercus ilicifolia had reached their full size, but were still tender, quite like the young shoots of which they formed part. Examining them on that day, I discovered on them two gall-flies, which I succeeded in taking. They were females, and the ovipositor of each was inserted into the gall so deeply that they could not readily free themselves, and they were removed by force."