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, Œstrus (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.
Fig. 49.—Cephalemyia ovis.
It is to avoid the attacks of the Cephalemyia that during the hot days of summer sheep lie down with their nostrils buried in dusty ruts, or stand up with their heads lowered between their fore-legs, and their noses nearly in contact with the ground. When these poor beasts are in the open country, they are observed assembled with their nostrils against each other and very near the ground, so that those which occupy the outside are alone exposed ([Plate III.]). The Cephalemyia ovis ([Fig. 49]) has a less hairy head, but larger in proportion to the size of its body than the Gad-fly (Gasterophilus equi). Its face is reddish; its forehead brown with purple bars; its eyes of a dark and changing green; its antennæ black, its thorax sometimes grey, sometimes brown, bristling with small black tubercles; the abdomen white, spotted with brown or black; and the wings hyaline.