THE HOUSE FLY AND ITS ALLIES. [Illustration: 84. Mouth-parts of the House fly.] The common House fly, Musca domestica, scarcely needs an introduction to any one of our readers, and its countenance is so well known that we need not present a portrait here. But a study of the proboscis of the fly reveals a wonderful adaptability of the mouth-parts of this insect to their uses. We have already noticed the most perfect condition of these parts as seen in the horse fly. In the proboscis of the house fly the hard parts are obsolete, and instead we have a fleshy tongue like organ (Fig. 84), bent up beneath the head when at rest. The maxillae are minute, their palpi (_mp_) being single-jointed, and the mandibles (_m_) are comparatively useless, being very short and small, compared with the lancet-like jaws of the mosquito or horse fly. But the structure of the tongue itself (labium, l) is most curious. When the fly settles upon a lump of sugar or other sweet object, it unbends its tongue, extends it, and the broad knob-like end divides into two broad, flat, muscular leaves (_l_), which thus present a sucker-like surface, with which the fly laps up liquid sweets. These two leaves are supported upon a framework of tracheal tubes. In the cut given above, Mr. Emerton has faithfully represented these modified trachae, which end in hairs projecting externally. Thus the inside of this broad fleshy expansion is rough like a rasp, and as Newport states, "is easily employed by the insect in scraping or tearing delicate surfaces. It is by means of this curious structure that the busy house fly occasions much mischief to the covers of our books, by scraping off the albuminous polish, and leaving tracings of its depredations in the soiled and spotted appearance which it occasions on them. It is by means of these also that it teases us in the heat of summer, when it alights on the hand or face to sip the perspiration as it exudes from, and is condensed upon, the skin." [Illustration: 85. Larva; _a_, Pupa-case of House fly.] [Illustration: 86. Larva of Flesh fly.]