Fig. 346.—Neuroterus lenticularis. Britain.
The Cynipidae are always small, frequently minute, Insects; usually black or pitchy in colour. The simple structure of the antennae and the number of their joints are of importance as an aid in identifying a Cynipid. The mesonotum is usually remarkably convex, and has, behind, a prominent scutellum, which more or less overhangs the small metanotum and the median segment; these are perpendicular in their direction; the sculpture of these posterior parts of the alitrunk is usually deep and remarkable. The abdomen has usually only a short petiole, so as to be pseudo-sessile; but there are some genera in which this part is rather long. The abdomen is generally so very much changed in outer form that its structure is not easily understood. The visible portion is frequently in larger part made up of the greatly enlarged dorsal plate of the second or third segment, or of both. These large plates are really chiefly composed of free flaps, and on lifting them up the large ventral plates are disclosed, although these appeared previously to be nearly or quite absent. In the female there is a very slender ovipositor, of which only a small part protrudes, although the organ is really elongate; it is drawn into the abdomen by means of a peculiar series of structures, the modified terminal segments to which it is attached being folded over into the interior of the body in such a way that the posterior part becomes situated anteriorly. In conformity with this arrangement, the ovipositor is bent double on itself, the anterior and the middle portions of the borer being carried into the body, leaving only a small part projecting beyond the extremity. The Cynipid ovipositor is an instrument of much delicacy, and is capable of a great deal of movement; it is usually serrate just at the tip, and although it looks so very different from the cutting apparatus of the sawflies (Fig. 344), it seems that it is really composed of pieces similar in their origin to those of the Tenthredinidae.
Fig. 347.—Ovipositor of Neuroterus laeviusculus. (After Adler.) a, a, The ovipositor partially coiled; b, extremity of posterior plate; c, c, muscles.
The wings frequently bear fine hairs; the paucity of nervures and the absence of the "stigma" are of importance in the definition of the family. The most important of the cells is one called the radial cell, situate just beyond the middle of the front part of the wing.
We cannot enter into a consideration of the classification of the family, as authorities are not agreed on the subject.[[429]] As regards their habits Cynipidae are, however, of three different kinds: (1) the true gall-flies, or Psenides, which lay an egg or eggs in the tissues of a growing plant, in the interior of which the larva lives after it is hatched; this mode of life may or may not, according to the species, be accompanied by formation of a peculiar growth called a gall: (2) Inquilines,[[430]] or guest-flies; these lay their eggs in the galls formed by the gall-makers subsequent to the growth of the galls, of which they obtain the benefit: (3) Parasites; these live, like most Ichneumon-flies, in the interior of the bodies of other living Insects; they prey on a considerable variety of Insects, but chiefly, it is believed, on Aphididae, or on Dipterous larvae. These parasitic flies belong to the sub-family Figitides.
A great deal of discussion has occurred relative to the nature and origin of galls, and many points still remain obscure. Considerable light has been thrown on the subject by the direct observations of modern naturalists. Previous to Malpighi, who wrote on the subject two hundred years ago, it was supposed that galls were entirely vegetable productions, and that the maggots found in them were due to spontaneous generation, it having been an article of belief in the Middle Ages that maggots in general arose from the various organic substances in which they were found, by means of the hypothetical process called, as we have said, spontaneous generation. Malpighi was aware of the unsatisfactory nature of such a belief, and having found by observation that galls arose from the punctures of Insects, he came to the further conclusion that the growth of the gall was due to the injection by the Insect into the plant of a fluid he termed Ichor, which had, he considered, the effect of producing a swelling in the plant, something in the same way as the sting of a bee or wasp produces a swelling in an animal. Réaumur also made observations on the gall-Insects, and came to the conclusion that the latter part of Malpighi's views was erroneous, and that the swelling was not due to any fluid, but simply to irritation caused by the prick; this irritation being kept up by the egg that was deposited and by the subsequent development of the larva. Observations since the time of Réaumur have shown that the matter is not quite so simple as he supposed, for though in the case of some galls the development of the gall commences immediately after the introduction of the egg, yet in other cases, as in the Cynipidae, it does not occur till some time thereafter, being delayed even until after the hatching of the egg and the commencement of the development of the larva. Galls are originated by a great variety of Insects, as well as by mites, on many plants; and it must not be concluded that a gall has been formed by Hymenoptera even when these Insects are reared from one. Extremely curious galls are formed by scale-Insects of the sub-family Brachyscelides on Eucalyptus trees in Australia; they are much inhabited by parasitic Hymenoptera, and Froggatt has obtained 100 specimens of a small black Chalcid from a single dead Brachyscelid.[[431]] The exact manner in which many of these galls originate is not yet sufficiently ascertained; but the subject of the galls resulting from the actions of Cynipidae has received special attention, and we are now able to form a conception of their nature. They are produced by the meristematic or dividing tissue of plants, and frequently in the cambium zone, which is caused to develop to an unusual extent, and in a more or less abnormal manner, by the presence of the Insect. The exact way in which a Cynipid affects the plant is perhaps not conclusively settled, and may be found to differ in the cases of different Cynipidae, but the view advocated by Adler and others, and recently stated by Riley,[[432]] seems satisfactory; it is to the effect that the activity of the larva probably affects the meristem, by means of a secretion exuded by the larva. The mere presence of the egg does not suffice to give rise to the gall, for the egg may be deposited months before the gall begins to form. It is for the same reason improbable that a fluid injected by the parent fly determines the gall's growth. It is true that the parent fly does exude a liquid during the act of oviposition, but this is believed to be merely of a lubricant nature, and not to influence the development. It is said that the gall begins to form in some cases before the larva is actually hatched, but the eggs of some Hymenoptera exhibit remarkable phenomena of growth, so that the egg, even during development of the embryo in it, may in these cases, exert an influence on the meristem. It is to reactions between the physiological processes of the meristem and the growing Insect that the gall and its form are due.
The investigations of several recent naturalists lend support to the view that only the meristematic cells of the plant can give rise to a gall. Riley says that the rate of growth of the gall is dependent on the activity of the meristem, galls on catkins developing the most quickly; those forming on young leaves also grow with rapidity, while galls formed on bark or roots may take months to attain their full size.
Fig. 348.—Bedeguar on rose, cut across to show the cells of the larvae; in some of the cells larvae are seen.