OAK GALLS. Very noticeable, because of their size, the peculiarities of their forms and colours, and the distortions and alterations that they occasionally produce on various parts of Oaks, are the Galls, so frequently seen by even the least observant spectator who passes an Oak-tree in autumn. The development of the Galls, and of their makers, is now believed to be very peculiar in certain respects in the case of most of them; but only a few of them are here selected as examples. Galls are new structures budded out from stems, leaves, or other parts of plants, under the influence of a stimulus applied either by lower plants (Fungi), or by animals, which produce the Galls either for the protection of their young and immature larvae, or for their own protection at all stages of development. The makers of Oak Galls belong almost entirely to the great group of Hymenoptera, known as Cynipidae, or True Gall Flies, almost the only exceptions being a few Midges, which merely fold down the lobes of leaves, making them become fleshy. The Cynipidae are all of small size, seldom exceeding 1/5in. in length, with a rather greater spread of wing. They vary a good deal in colour--from reddish-brown to black--and frequently show metallic reflections. The wings are supported by a few nerves. The females possess, at the end of the abdomen, an ovipositor, and, by means of this organ, which is frequently of considerable length, they bore into some parts of the tree, and thereby give rise to the "Gall." Galls are found growing from every part of Oak-trees, from roots and branches, to leaves, stamens, and ovaries, or fruits; and, in harmony with such diversity of situation, they vary exceedingly in their appearance, consistency, and internal structure; but all those of the Cynipidae, or Gall Flies, in the strict sense, to which almost all the Gall-makers in the Oak belong, agree in this, that there is a central space, with walls in which the cells immediately around the space usually contain starch; while outside this layer there is generally another, for defence, formed of thick-walled, compact cells. In the central cavity lies the single, footless, pale grub of the Gall Fly; or, if the intruders have destroyed the host larva, two or more larvae of the parasites may be found in each Gall, generally inclosed in separate chambers. In many of the rounded Galls there is also a considerable layer of beautifully arranged cells (spongy tissue) in the walls. Galls on the roots require to be specially sought for, because of their being concealed under the soil. The Gall of Biorhiza aptera is one of the commonest. The others do not call for further notice here. Galls on the main stems are not very numerous. Far more conspicuous and important are those formed on the branches. Many of these are modified buds; numerous others are conspicuous Galls formed upon the leaves, usually on their lower surface; and still others are stamens or ovaries, changed in structure by the punctures of the parent insects. On the Continent of Europe, nearly 100 different forms of Galls have been found on Oaks; and in Britain, the number of forms already observed reaches nearly forty. Among the more widely-known Galls may be mentioned the Oak Apple found, in May and June, on the twigs, as an oblong mass, from 1in. to 2in. long, covered with a smooth, green and red skin, above a layer of soft tissue, in which are imbedded numerous larvae, each one in a hard-walled cell or chamber in the centre of the Gall. The Cotton Gall resembles a ball of white cotton wadding surrounding a number of minute thin-walled chambers, each occupied by one larva of small size. The whole mass may reach a size of 1in. or more across, and is very conspicuous, as it hangs attached to the male catkins in May and June. The Devonshire, or Marble, Gall of the Oak is very easy to recognise, as it is about the size and form of a boy's marble, green and soft when young, but brown and hard when mature. Two or more of the Galls often join together during growth, and form irregular masses. In the centre is a small cavity, inhabited by the larva. It is said that this Gall was introduced from the Continent, and was first observed in Devonshire; but, whatever its origin, it is now abundant in many parts of Britain. Experiments have not succeeded in rearing males, but have yielded many female Gall insects from these Galls, and, along with them, seventy or more kinds of insects that live in the Galls, either as guests (inquilines) or as true parasites. The name of the Gall-maker is Cynips Kollari. The Artichoke Gall, formed by Aphilothrix gemmae, very much resembles a miniature Artichoke, about 1in. long, formed of scales, with a central chamber like a small acorn. Various kinds of bud Galls are too small to be found without careful search, and are of interest chiefly because of the relation of some of them to other conspicuous forms that disfigure young Oak-trees in pleasure grounds. The leaves bear many curious forms of Galls, some of them so conspicuous that they cannot have escaped the notice of even the least observant. One that causes much damage is the work of Andricus curvator It is produced on the midrib, or leafstalk, and causes a marked bend, or twist, in the leaf, or at its base in the branch; the Gall forms a green swelling, about the size of a pea. Inside this lies a small, brown, kidney-shaped inner Gall, which, when ripe, is quite free from the inside of the wall of the large chamber. The Currant Galls, resembling translucent currants, pale, with purple or red mottlings, and very juicy, are common, in May and June, on the lower surfaces of leaves, and on catkins, where they look like bunches of Currants. The insects (Spathegaster baccarum) are easily reared from these Galls, if the latter are collected when fully mature, and prevented from becoming either too dry or mouldy. Dryophanta folii produces a globular Gall on the backs of leaves. It may reach 3/4in. in diameter, but the central chamber is small, the walls being thick, though soft and spongy. The surface becomes yellow and red. Dryophanta divisa also forms Galls on the lower surface of leaves, but the Galls are generally flattened, oval bodies, much smaller than the last-named, with harder, but thinner, walls. They are also more abundant throughout the country. Oak Spangles are among the most curious of Oak Galls. They occur on leaves, almost always on the lower surface. Three kinds have been distinguished in Britain, agreeing in being circular, about 1/5in. across, and, while on the leaf, quite thin, though bearing a very slight prominence in the middle. The commonest is covered with rusty brown hairs; it often nearly covers leaves. Of the other two, one bears similar scattered hairs, and has the margin turned up; while the third is green or purple-red, and smooth. All are the work of insects belonging to the group Neuroterus, and which are named N. lenticularis, N. laeviusculus, and N. fumipennis. Another species of this genus (N. numismatis) makes Silky Button Galls, so called from their resemblance to tiny, round, flattened buttons, covered with brown silk threads. These Galls are often formed in myriads on the lower surface of the leaves, so as, in some cases, to cover it almost entirely in autumn. Yet another Gall, of very frequent occurrence, is so inconspicuous as to be easily overlooked in the leaves, in which, during June, it looks like a mere low blister, about 1/8in. across. The insects reared from these Galls have been named Spathegaster vesicatrix. This may be called the Oak Blister Gall. A, Galls of Neuroterus lenticularis (Oak Spangle Galls)--1, On Lower Surface of Leaf; 2, Removed from Leaf; 3, Flat Section, showing round Chamber in centre where the Larva lives; 4, Transverse Section, in autumn, showing Form while still on the Leaf, with very small Chamber; 5, Transverse Section of Gall in following spring, showing changed Form and much larger Chamber. (Nos. 4 and 5 are about four times enlarged.) B, Galls of Spathegaster baccarum (Currant Galls of Oak)--1, Upper Surface of Leaf, showing Galls projecting through; 2, Lower Surface of Leaf, showing three Galls, of which the Upper is cut through to show the Larval Chamber; 3, Male Catkin bearing Galls. (All the figures are a little smaller than natural size.) A, 1, Galls of Dryophanta folii on Lower Surface of Oak Leaf, slightly smaller than natural size; 2, Transverse Section, showing Larval Chamber. B, 1, Galls of Spathegaster Taschenbergi on a Young Twig, natural size; 2, Gall enlarged. These Galls are violet, with a velvety surface, are soft, and are eaten by the larva till the wall becomes very thin. They appear in May and June; and, though not recorded from Britain, are believed to be the Galls that complete the cycle with those of Dryophanta folii. A, Galls of Neuroterus numismatis (Silky Button Galls of Oak)--1, Lower Surface of Leaf, bearing many, about natural size; 2, Gall showing the depressed Surface, enlarged; 3, Gall seen from the side; 4, Gall in Section, showing small central Chamber for Larva, hollow Border cut through at each side, and short Footstalk. B, 1, Three Galls of Spathegaster vesicatrix (Blister Galls of Oak Leaves) in a Leaf, natural size; 2, Transverse Section of Leaf, with Gall, showing very small Larval Chamber. In acorns, at times, Galls are to be found in the seed, though the outside of the acorn shows no sign of insect work. The interior is broken up by numerous small oval spaces, in each of which lives a small larva of the Gall-maker (Andricus glandium). These brief notes must suffice for the more common forms of Oak Galls; and attention must now be directed for a little to certain points in the life of the Gall-makers--points which, for a considerable time, seemed to defy explanation, but which are gradually emerging into the light, thanks to the careful experiments and observations of entomologists, pre-eminent among whom stands Dr. Adler. The following are two very noteworthy peculiarities connected with many Oak Galls: (1) From certain kinds, e.g., Devonshire Gall, Oak Spangles, &c., females alone have been reared (sometimes in thousands), while from others males as well as females are readily obtained in confinement. Where females alone emerge from Galls, they lay eggs that produce larvae, though unimpregnated. But we can hardly imagine that this mode of reproduction can go on perpetually without requiring to be re-invigorated by the influence of males. (2) Certain kinds of Galls appear only in early summer; e.g., Currant Gall, Blister Gall, and Oak Apple. The insects emerge from the Galls in June or July, and may be observed to lay their eggs at once on the twigs, buds, or leaves; yet no trace of the Galls can be seen before the succeeding spring. In like manner, many Galls are met with only in autumn, e.g., Oak Spangles, Silky Button Galls, &c., and the insects emerge from them in spring, lay eggs on the Oaks, and disappear; but the Galls re-appear only in autumn. The insects reared from each form of Oak Gall are distinguishable, by an expert, from those reared from every other form, the differences affecting form and size of the ovipositor, wings, and other important organs, as well as such minor points as colour. Formerly, it was very difficult to understand alike the constancy with which only females emerged from some Galls, and the (supposed) length of time between the puncture of the plant by the parent Gall Fly, and its result in the formation of a Gall, some months later. The explanation of both difficulties now seems afforded to us in the discovery, first announced from the United States by Mr. Walsh, in 1870, but chiefly wrought out and enforced in its application to the life-history of Gall Flies (Cynipidae) by Dr. Adler. This explanation is, that most of these insects appear in two annual broods; that these broods differ from one another in the Galls formed by them, as well as in the structural characters of the mature insects; in short, that the insects and their Galls alike are dimorphic--a unisexual brood appearing in winter or spring, laying eggs, though unimpregnated, and producing Galls in early summer, from which emerge, in a month or two, insects differing so much from their parents (makers of the Galls) that they have been referred to genera different from the latter. The later brood possess males as well as females: these pair, and the females produce Galls similar to those with which the cycle was begun. Such a cycle has now been traced in many of our native Gall Flies. Subjoined are noted only five such cycles, all figured above, since even to enumerate all the cases would exceed the space available here. The two forms of Galls are, in each case, placed in the same line, the spring or early summer form to the left, the autumn form to the right. They are as follows: Andricus curvator. Aphilothrix collaris. Dryoteras terminalis. Biorhiza aptera. Spathegaster baccarum. Neuroterus lenticularis. Spathegaster Taschenbergi. Dryophanta folii. Spathegaster vesicatrix. Neuroterus numismatis. It is at once evident that, if this theory of the existence of dimorphism among Oak Gall Flies be true, it sufficiently explains the problems, stated above, in regard to unisexual insects alone being reared frequently, and to the supposed interval between the puncture and the formation of the Gall. Remedies. It is seldom necessary to take active means to reduce the numbers of Oak Galls, although, at times, the lower surface of the leaves--almost every square line upon it--bears one or more Galls, such as Oak Spangles and others of the smaller kinds. Their presence does not seem to weaken the trees very materially--a fortunate circumstance, since remedies could scarcely be applied with success to huge trees, or even to Oak-bushes. Almost the only Gall deserving to be mentioned as distorting twigs and leaves, is the work of Andricus curvator. The supposed second stage of this Gall is a tiny egg-shaped excrescence, half hidden among the leaf scales in buds; the insect reared from it has been named Aphilothrix collaris. Where so many occur as to injure the plants, the best remedies are to hand-pick and to destroy the Galls. Another Gall that occasionally distorts the branches of Oaks, though much less often than the last-mentioned kind, is that of Andricus inflator: it is a swelling at the tips of the young twigs. In the centre is a cup-shaped hollow, closed above by a thin membrane; and at the bottom of the hollow is a small, ovate, brown, very thin inner Gall. The insects emerge in July, and there are both males and females amongst them. The Gall believed to alternate with this is globular, scarcely over 1/8in. across, green, and smooth; and is formed in the buds in autumn, remaining concealed in the bud scales. The insects emerge in early spring, are all females, and have been named Aphilothrix globuli.