The sheep gad-fly is led by instinct to deposit its eggs within the nostrils of the sheep. Its attempts to do this--most common in July, August, and September--are always indicated by the sheep, which collect in close clumps, with their heads inward, and their noses thrust close to the ground, and into it, if any loose dirt or sand is within reach. If the fly succeeds in depositing its egg, the latter is immediately hatched by the warmth and moisture of the part, and the young grubs, or larvæ, crawl up the nose, finding their devious way to the sinuses, where, by means of their tentaculæ, or feelers, they attach themselves to the mucous membrane lining those cavities. During the ascent of the larvæ, the sheep stamps, tosses its head violently, and often dashes away from its companions wildly over the field. The larvæ remain on the sinuses, feeding on the mucus secreted by the membrane, and apparently creating no further annoyance, until ready to assume their _pupa_ form in the succeeding spring. Having remained in the sinuses during the fall and winter, they abandon them as the warm weather approaches in the latter part of spring. They crawl down the nose, creating even greater irritation and excitement than when they originally ascended, drop on the ground, and rapidly burrow into it. In a few hours, the skin of the larvæ has contracted, become of a dark-brown color, and it has assumed the form of chrysalis. This fly never eats; the male, after impregnating two or three females, dies; and the latter, having deposited their _ova_ in the nostrils of the sheep, also soon perish. The larvæ in the heads of sheep may, and probably do, add to the irritation of those inflammatory diseases, such as catarrh, which attack the membranous lining of the nasal cavities; and they are a powerful source of momentary irritation in the first instance, when ascending to, and descending from, their lodging-place in the head. But in the interval between these events, extending over a period of several months, not a movement of the sheep indicates the least annoyance at their presence. They are, moreover, found in the heads of nearly all sheep, the healthy as well as the diseased, at the proper season.