It will be remarked that the body of each worm does not exceed five lines in length. The tail is a peculiar organ, by the aid of which the worm breathes, although its body may be covered by water to the depth of several inches. It is composed of two tubes, one of which shuts into the other, like a telescope. Réaumur calls it the breathing tube. It terminates in a little brown knob, in which, according to Réaumur, are two holes for the purpose of receiving the air, and which have five little tufts of hair, which float on the surface of the water. When the time comes for the metamorphosis of these worms, they come out of the water and bury themselves in the earth; the skin then hardens and becomes a sort of cocoon. In this cocoon the insect loses the form of a worm, and takes by degrees that of the pupa, which it keeps until circumstances cause it to throw off its last coverings, and to appear in the winged state. What an eventful life! what a life full of changes and turns of fortune is that of these insects, which pass the first and longest period of their existence under water, another part of their life under the ground, and, finally, after having existed in these two elements, enjoy, high in the air, the pleasures of flight! The third group of Brachycera is that of the _Dichæta_; that is, those flies having two-fibred suckers. Among these are classed the _OEstri_, the _Conopes_, and the flies properly so called. The genus _OEstrus_, the Gad, Bot-fly, or Breeze, comprises those formidable insects which attack the horse, the sheep, and the ox.[15] The labours of Réaumur, in his admirable Memoirs, and those of M. Joly, Professor of Zoology to the Faculté des Sciences de Toulouse, who published some most valuable researches on this subject, in 1846, will guide us in the following brief explanation. [15] Mr. Bates, in his interesting "Naturalist on the Amazons," mentions an _OEstrus_ as occurring in those regions, which deposits its eggs in the human flesh, the larva causing a swelling which resembles a boil.--ED. The following is the description given by M. Joly of the Gad-fly (_OEstrus equi_) represented in Figs. 41, 42, which are taken from a drawing which accompanies that naturalist's Memoirs. The head of this insect is large and obtuse; the face light yellow, with whitish silky fur; the eyes blackish; the antennæ ferruginous; the thorax grey; and the abdomen of a reddish yellow, with black spots. The wings are whitish, not diaphanous, with a golden tint, and divided by a winding band of blackish colour. The feet are palish yellow. [Illustration: I. A Herd of Horses attacked by Gad-flies (_OEstrus equi_).] [Illustration: Fig. 41.--Horse-fly, male (_OEstrus_ [_gasterophilus_] _equi_).] [Illustration: Fig. 42.--Horse-fly, female (_OEstrus_ [_gasterophilus_] _equi_).]