Among the species of gall to which our oaks are subject, there are some which grow at the end of the twig, as for instance, that called the artichoke-gall, which comes the nearest in appearance to the nut-gall of commerce, since it is, in common with that nut, an irregular development, not of the leaf or flower, but of the bud. The artichoke-gall might indeed be easily mistaken for the fruit of the tree, by any one unacquainted with the habit of the oak. It is a cone-like body, consisting of a number of leafy scales overlapping each other, but on being dissected it is found, like other galls, to contain insects in various stages of their growth, according to the season. In the same way, the nut-gall, being a disease of the bud, or extremity of the young shoots of the oak on which it grows, has the appearance common to some other kinds of fruit, and would seem to be the ordinary produce of the stalk, did we not find that acorns also grow on the same tree. The hard and brittle texture of the gall-nut also, so different from the substance of our own oak-galls, would be still more likely to deceive, and indeed has deceived a highly respectable writer, who declares from his observation of the dried galls of commerce, that the nut is the fruit of a tree, and not a mere excrescence.[4] Footnote 4: Aikin. How it is that the different species of gall-fly should produce such varied results, and why one excrescence should be the oak-apple, another the spangle, a third the artichoke, and a fourth the nut, is indeed a mystery. But that these small insects do really produce the excrescences in question, is an ascertained fact, and a most important one in the commercial world. Nut galls contain a large quantity of the vegetable principle called _tannin_, being the astringent property for which oak-bark is in so much repute. They also abound in an acid called from thence _gallic acid_, which is the important ingredient in black dyes, and in fixing and improving several other colours, as well as in the composition of ink. [Illustration: BLISTERING BEETLE. ] CHAPTER IX. OTHER INSECT PRODUCTIONS USEFUL TO MAN. Almost any article could be better spared from the materia medica, than an insect remedy called _cantharides_, or Spanish flies. This consists of the bodies of small beetles, in which resides an active blistering principle of great importance, not only as an external application, but also sometimes as an internal remedy. The insect chiefly used in Europe is the _cantharis vesicatoria_, for the most part rare in England, but seen on some occasions in great numbers, as in the summer of 1837 in Essex, Suffolk, and the Isle of Wight. Other insects of the same family are employed in foreign countries.