Alfred Charles Kinsey

“ The galls of two successive generations may be produced on different parts of the plant and may present entirely different forms, e. g., the gall of one generation of Andricus operator is mainly a large mass of wool on the young stems, leaves, or aments of the oak; the second generation gall is a small, seed-like, naked capsule between the cup and the acorn. The insects of the two generations of some species may be so entirely different as to warrant believing them to belong to different genera, if the relationships of the two forms has not been discovered. ”

“ Now, it is conspicuous that specialization in the choice of hosts is restricted to the species of particular tribes, and that practically every species in those tribes has adopted the special host. All but one of the species of the Cynipini are found regularly on oaks and all of the species of Rhoditini are found as regularly on roses, though rarely one or two of the species of each group will produce galls on a second plant as well as upon their own hosts. Finally, it is to be seen that all of the species which inhabit the plants of the many genera belong to the one tribe Aulacini. ”

“ Every known species of the genus occurs on roses. One species, R. rosæ, is believed to produce a gall, rarely, on plants of the related genus Rubus but, although this gall does resemble the mossy gall occurring on rose, I do not know that any one has bred the adult wasps from the blackberry gall and definitely proved the identity of the maker. Other records of species of Rhodites occurring on Rubus are undoubtedly errors in the determination of the host. There is no question that the insects are remarkably specialized in their choice of hosts. ”

“ Here and in the very closely related genera are the only instances of normal sexual reproduction found in the whole family, except such as occur in the sexual generation of heterogenous species. In Diastrophus the male sex is much less abundant than the female, constituting only about thirty per cent of the total number of individuals, and it is therefore very likely that parthenogenetic reproduction occurs frequently in the group. ”

“ It is hard to understand how this extreme specialization has come about. That the oak has advantages as a shelter for an insect is readily seen, but it belongs to a comparatively small family of plants, the genus has only a limited distribution over the world today, and, whatever the qualifications of the oaks, it is not apparent why other plants should be so completely abandoned. That these other plants are capable enough of producing galls is evident, for other gall insects of other families produce galls upon them in abundance. ”

“ The difference of colouring is unimportant, and is chiefly observable in a slight variation in the colour of the legs; nor is the size of the body very different, while the form and surface markings agree in many points.” And he then pointed out that the only important differences are those of form of abdomen and of ovipositor, due to the different modes of reproduction of the two forms. Among our American species the differences between the adults of the two generations are even less marked. ”

“ To anticipate the conclusions which will be drawn later, it may be said that all other considerations, without an exception, point to those insects which have the arcuate vein as being most primitive; those having the angulate vein are by all means the most specialized of the gall-wasps; and an arrangement of the groups of intermediate grades of specialization strictly parallels an arrangement of those groups on the basis of venation only. ”

“ Though it is still likely that the males do occasionally fertilize the females in these species, it is quite certain that, as Adler put it, “we can predict that they [the males] will probably become extinct in the course of time.” In Disholcaspis we have instances where apparently only females are ever produced, but it is still possible that there is a bisexual, an alternate generation yet to be discovered among those species. At any rate complete agamy has been attained by at least those five species of Andricus studied by Adler. ”

“ Timaspis and Aylax show conditions intermediate to Diastrophus, where the segment covers about one-half of the abdomen. In Rhodites the segment is larger that anywhere outside the group of the oak gall-wasps, and in this genus the peculiar production of the hypopygium also indicates an advanced degree of evolution. Of the oak gall-wasps, Neuroterus shows the most primitive second segment. In some of the species called “Andricus” the specialization reaches its extreme. ”

“ Still another thing which indicates decreased vitality in the oak gall-wasps is the short duration of the adult life of most species. Having observed some thousands of adults of scores of species on trees until the insects died natural deaths, I found that about two days was the average length of life for those species, while many species will die almost instantly after oviposition, even if that is only a few hours after the emergence from the gall. ”

“ We may allow details of relationships to be worked out by other lines of evidence, but we should attach some importance to the very primitive character of all the galls of Aulacidea (without a single exception) , to the less primitive but still simple qualities of the galls of Aylax, Diastrophus, and Neuroterus, and to the highly developed characters of the galls of Cynips (European sense) , of Disholcaspis, and of Amphibolips. ”

“ And such complete uniforinity is not very likely mere coincidence but must be truly significant of the genetic relations of the insect producing the gall.
Reproduction in Disholcaspis must be entirely agamic. No males have ever been bred from or found in the galls of this group.
Whether alternation of generations occurs in Disholcaspis, with possibly bisexual reproduction in an alternate generation, is not definitely known. The genus is confined to America, so European workers have not studied the life histories of any of the species. ”

“ It need hardly be argued that this specialization has very likely been the more recent development in the evolution of the Cynipidæ. It is not unreasonable to believe that the primitive gall-wasps inhabited a wide range of plants and that these polyphagous forms have become relatively few as the numbers of the more specialized species increased. That the small percentage of the insects which are polyphagous today, which still possess the more primitive habit, are to be considered actual remnants of the ancient forms, can be readily shown. ”

“ Again, agamic reproduction has evident advantages but it is inconceivable, from what we know of reproduction among animals, that such a method of propogation, especially where agamy has become the only method, can result in anything less than a loss of vitality and the ultimate extinction of the species. ”

“ On the basis of these considerations, the Cynipidæ may be divided into two miore or less distinct groups. One group, including mainly oak gall-wasps, has little vitality, possesses a mode of reproduction which would seem to guarantee a continuation of the lowering of vitality, and has a manner of living its larval life which invites a tremendous number of parasites and offers often insurmountable obstacles to the insect's growth to maturity. ”

“ Up to the present time the relationships of the genera or even species of the Cynipidæ (Hymenoptera) have been poorly understood. Species have been catalogued without much attempt to discover an arrangement which would indicate evolution, and the genera have been in large part unnatural groups containing large numbers of unassorted, diverse organisms. Moreover, few suggestions have hitherto been made as to the origins of the peculiar biologic phenomena so characteristic of the more specialized gall-wasps. ”

“ But this does not always apply to the progeny of any one individual; my first breeding of Rhodites rosæ gave six females and seven males, although subsequent breedings of the same species have only rarely given me any males at all. Gradually the male sex is disappearing from the genus and in consequence agamic reproduction is, likewise gradually, becoming the sole means of reproduction for these insects. ”

“ An enlarged abdominal plate may be the result of the fusion of two or more plates or the actual increase in size of that plate in the course of evolution. The primitive condition, somewhere in the more or less remote ancestry of any group of insects, showed the segments more or less equal in development, and any condition which shows a single segment especially developed is undoubtedly the result of later evolution. ”

“ Such a complete set of data, interpreted in the light of other evidence, should prove of great value for discovering an arrangement of the species of the family which will indicate the natural order of relationships. ”

“ It is evident that the specialized host was not adopted by species of the Cynipidæ severally and at different times, but at two distinct periods, by two groups of forms, at a time about coincident with the differentiation of the tribes which those two groups now constitute. And it is as clear that the Aulacini comprise forms most like the primitive polyphagous species from which the Cynipini and Rhoditini have arisen at different times. ”

“ It is likely that under natural conditions there is less of this sort of mortality than when the gall is gathered into breeding jars, but even this indicates an amount of vitality possessed by the insects which is none too sufficient.
On the contrary, species of Aulacidea are very easily bred in numbers and it is a rare thing to find adults left in the galls. The adults of these species will always live for many days. ”

“ It may be too much to suggest that in all the instances of extreme differences in the successive generations the differences are due primarily to the extreme differences of the parts of the plants affected, for we do not know enough about the factors influencing gall production to compare adequately the effects ”

“ In the case of other species, most likely for the majority of the oak-gall-producing Cynipidæ, only the alternate generations are siInilar, every agamic generation being succeeded by a bisexual generation, which in turn produces an agamic generation. Where this heterogeny occurs the origin and development of an agamic generation has been so involved with the origin and development of the alternation of dissimilar generations that the problem is more properly discussed in the section of this paper on the alternation of generations. ”

“ The wing venation of the Cynipidæ is remarkably uniform throughout the whole range of species. Only the genus Eschatocerus, with one known species, shows any considerable modification from the type; in that species the first and second cubital cells are closed by the extreme reduction of the second abscissa of the radius and of the second crossvein. The related Figitidæ show a wide range of variation from the complete cynipoid venation to a condition almost as specialized as that found in those Chalcididæ which have retained only a single vein. ”

“ The adults of the species brevipennata, centricola, maculipennis, and weldi have parapsidal grooves extending to the pronotum (not quite entire in weldi) , have the second abdominal segment “tongue-shaped,” i. e., produced dorsally, and in other ways are generically different from the species belonging in Disholcaspis, which genus was correctly limited by original definition to formns having the parapsidal grooves extending only to the middle of the mesothorax and the posterior edges of the second segment of the abdoinen perpendicular or nearly so. ”

“ From the table it will be seen that the character of the radial cell is of generic importance. Although it is true that Aulacidea is separated from Aylax primarily upon this character and that this is the best reason for considering Aulacidea a distinct group from and more primitive than Aylax, nevertheless, in many other instances where the genera are founded upon other morphological characters, it will be seen that the condition of the radial cell is as good a generic character. ”

“ Adler (1881) has definitely proved, by the breeding of successive generations, that Diastrophus rubi has no alternation of generations, and nothing known concerning emergence dates, dates of appearance of the galls, etc., indicates that we should expect heterogeny in the group. Herein these insects are more primitive, less specialized than the oak gall-wasps. ”

“ Whether the degeneration of a segment that has become thus specialized ever occurs is a question to be debated for each group studied.
The dorsal plate of the second segment of the abdomen of Cynipidæ is always more developed than any of the other dorsal plates and in many. instances the plate has become so enlarged as to cover almost the entire abdomen. That this enlargement in the true gall-wasps is not the result of fusion is evidenced by the continued existence of the other plates underneath the enlarged plate. ”

“ It is to be expected that, where such limited considerations are drawn upon, the interpretations may be made in diametrically opposite fashions and we need not be impressed with either set of interpretations until one of them is supported by other sorts of evidence. The important fact is that the vein considered does clearly show a path which was taken in the evolution of the insects. ”

“ The adults of most species of cynipids are very weak creatures, fatally injured by the slightest touch or by a sudden change of temperature or of humidity. One of the greatest obstacles in my experimental breeding has been the enormous amount of destruction caused if a rainy or even foggy day occurred at the time of emergence of a lot of adults. Although I kept the insects under cover, at such a time they would not oviposit or even make any attempt to climb over the trees ”

“ Nothing that I know of from observation of the American species would indicate that there is anything but normal reproduction in the group. Aulacidea nabali emerges late in May and the galls are mature in September, appearing however a couple of months before that. A. podagræ emerges in mid-June, and the galls are good-sized by August. A. tumida emerges in early June; A. annulata emerges in early June; and the galls must develop o, the plants before they have become mature, which date for Lactuca is late July or early August. ”

“ The first abscissa of the radial vein of the wings of the Cynipidæ varies from an arcuate to a sharply angulate condition, some species showing a pronounced, vein-like projection into the radial cell at the apex of the angle made by the vein. Every degree of gradation between the extreme forms may be found. ”

“ Still another indication of the reduced vitality of most of these cynipids is the number of adults which reach maturity but then do not succeed in making (chewing) their way through the walls of the gall. It is hard to determine what percentage of adults thus fail, at the last moment, to achieve the goal of their lives, but it is apparently large, for almost any collection of galls, cut open, will show many such individuals in galls which were evidently mature before they were gathered. ”