Gall-flies of various kinds attack our oak trees in this country, and also our willows, hawthorns, roses, &c. A few words respecting the excrescences they produce, may therefore assist our ideas of the proceedings of the valuable insect described above. Very few persons are unacquainted with oak-apples, or small roundish, flattened bodies growing on the leaves of the oak. These are sometimes tinged with brown, or pink, or pale yellow, so as to appear not much unlike very small apples, attached to the leaf. Each of these curious bodies is formed by a small fly of the same nature as that which forms the gall-nut. This insect alights on the leaf, pierces it with a very sharp instrument with which she is provided for this purpose, and deposits an egg so minute as to be almost invisible to the human eye. The puncture of this insect produces a diseased action in the leaf, so that the parts immediately surrounding the egg swell and harden until the apple-shaped body is produced. This is now an admirable place of shelter, and a reservoir of food for the insect within, which remains snugly encased until it arrives at its perfect state, when it cuts its way out through the solid substance of the gall. [Illustration: LEAF GALLS. ] [Illustration: OAK SPANGLE. ] This is only one out of many forms of gall that may be found on the oak, where sometimes the leaf is attached, sometimes the catkin, and sometimes the young bud. Another form of that on the leaf consists of very small circular discs fixed to the under part by their central points. The outer side of the disc is red and hairy, the inner side smooth. Each disc contains a single insect, which remains in it long after the leaves have fallen to the ground. The common name of this curious excrescence is “oak spangle.” [Illustration: CURRANT GALLS. ] Still more singular is the gall found in the catkins of the oak. The flies deposit their eggs in the stalk of the stamen-bearing flowers, which in consequence become adorned with what appear to be straggling bunches of currants or bird-cherries. Placed at short distances from each other on the thread-like stem, these excrescences so much resemble currants in size, shape, and mode of growth, that they have been named “currant galls.” [Illustration: ARTICHOKE-GALL. ]