"Nature," says Réaumur, "has taught this worm the surest, the gentlest, and the most simple of methods, the one to which surgeons often have recourse to hold wounds open, or to enlarge them. They press _tents_ into a wound they wish to enlarge. Two or three days before the worm wishes to come out, it commences to make use of its posterior part as a tent, to increase the size of its exit from its habitation. It thrusts it into the hole and draws it out again many times in the course of two or three days, and the oftener this is repeated, the longer it is able to retain its posterior end in the opening, as the hole becomes larger. On the day preceding that on which the worm is to come out, the posterior part is to be found almost continually in the hole. At last, it comes out backwards, and falls to the ground, when it gets under a stone, or buries itself in the turf; remaining quiet and preparing for its last transformation. Its skin hardens, the rings disappear, and it becomes black. Thenceforth the insect is detached from the outer skin, which forms a cocoon, or box. At the front and upper part of the cocoon is a triangular piece, which the fly gets rid of when it is in a fit state to come into the open air." Fig. 47, taken from drawings in Réaumur's Memoirs, represents the imago of the _OEstrus_ leaving the cocoon. [Illustration: Fig. 47.--Imago of Bot-fly emerging.] [Illustration: Fig. 48.--Ovipositor of the Bot-fly (_OEstrus bovis_).] The reader is, most likely, desirous to know with the aid of what instrument the _OEstrus_ is able to pierce the thick skin of the ox. The female only is possessed of this instrument, which is situated in the posterior extremity of the body. It is of a shiny blackish brown colour, and as it were covered with scales. By pressing the abdomen of the fly between one's two fingers it is thrust out. Réaumur observed that it was formed of four tubes, which could be drawn the one into the other, like the tubes of a telescope (Fig. 48). The last of these appears to terminate in five small scaly knobs, which are not placed on the same line, but are the ends of five different parts. Three of these knobs are furnished with points, which form an instrument well fitted to operate upon a hard thick skin. United together, they form a cavity similar to that of an auger, and terminating in the form of a spoon. The Gad-fly, or Breeze-fly of the sheep, _OEstrus_ (_Cephalemyia ovis_), has obtained notoriety on account of its attacking those animals. Even at the sight of this insect the sheep feels the greatest terror. As soon as one of them appears, the flock becomes disturbed, the sheep that is attacked shakes its head when it feels the fly on its nostril, and at the same time strikes the ground violently with its fore-feet; it then commences to run here and there, holding its nose near the ground, smelling the grass, and looking about anxiously to see if it is still pursued. [Illustration: Fig. 49.--Cephalemyia ovis.]