How is it that the part of the leaf treated by the Louse is naturally yellow and crimson, when, if simply folded, it retains its normal green hue unimpaired? Again, how is it that in the one case the thickness of the tissues is not increased while in the other it becomes augmented? Why does the spindle keep to the plane of the edge, whereas the ear-shaped gall, or auricle, abruptly bends its leaf and hangs vertically? In all three cases, the implement is the same and the work differs profoundly. Is it the effect of a virus whose properties vary according to the sucker that inoculates it? Is it the result of a change of method in wielding the awl? We are confounded. The problem becomes doubly obscure when we consider the spherical galls. Here the original black Louse settles just at the base of a leaf, on the upper surface, against the median vein. There she takes her stand, motionless and patient. The point abraded by the awl is hollowed into a tiny pit, which soon forms a small protuberance beneath the underside of the leaf. As though its foothold were gradually withdrawn, the insect dives and is swallowed up by a pocket whose opening closes of its own accord by the contact of its lips. Here we have the Plant-louse at home, strictly isolated from the world. Though the edge of the fostering leaflet undergoes no alteration of shape or colour, the pitcher-shaped appendage at its base turns a pale yellow and grows larger day by day, thanks to the centrifugal expansion provoked by the insect’s irritant sucker. The continual punctures of the solitary Louse and presently of her offspring will enlarge it, by the end of the summer, to the dimensions of a fair-sized plum. The horn-shaped galls originate in an entire leaf, selected from among the smallest. On the tops of the boughs there are sickly leaves, the last achievements of an exhausted impulse. Scarcely unfolded and innocent of green, the colour of health, they measure barely a fifth of an inch in length. It is on these vegetable trifles that the enormous horn-shaped structures are based; and even so the leaf is not completely utilized, but only one of its lobes: in short, a speck, a mere nothing. Exploited by the Plant-louse, this mere nothing acquires a peculiar energy. In the first place, it welds itself to the tip of the twig and becomes one with it, so that it lingers on the tree when the leaves fall and, with them, the other galls; next, it excites a flow of sap comparable with that of the pumpkin-stalk nourishing its fruit. The very small begets the huge. The gall is at first a pretty little horn, regular in shape and green all over. Open it. The interior is a magnificent flesh-colour and soft as satin. For the moment, a solitary Louse, a black one, inhabits this attractive residence.