AND FI.IES
Within the puparium, the larva sheds another skin, and then transforms to the pupa. The pupa (Fig. ?82 F) is thus protected during its transformation to the adult by the puparial skin of the larva, which serres in place of a cocoon. When the adult is fully formed, it pushes off a circular cap from the anterior end of its case, and the fly emerges. The length of the entire cycle from egg to adult vanes according to temperature conditions, but it is usually from twelve to fourteen days. The adult flies are short-lived in sunanaer, thirty days, or not more than two months, being their usual span of life. In cooler weather, however, when their activities are suppressed, they lire ronger, and a few survive the winter in protected places. One of the essential differences between flies of the house tir type and the mosquitoes and horseflies is in the structure of the mouth parts. The house fly lacks mandi- bles and maxillae, but it retains the median members of the normal group of mouth-part pieces, which are the labrum, the hypopharynx, and the labium. These parts are combined to form a sucking proboscis that is ordi- narily folded beneath the head, but which is extended downward when in use (Fig. 183 A, Prb). The labium (Fig. 18.3 ]?, Lb) is the principal component of the proboscis of the house fly, and its terminal lobes, or labella (La), are particularly well developed. From the base of the labium there projects forward a pair of palps (Plp), which are probably the palpi of the maxillae, though those organs are otherwise lacking. The anterior surface of the labium is deeply concave, but its trough- like hollow is closed by the labrum (Lin). Against the labial wall çf the inclosed channel lies the hypopharynx (Hphy). When the lobes of the labium are spread out, the anterior cleft between them is closed except for a small central aperture (a). This opening becomes the func- tional mouth of the fly, though the true mouth is situated, as in other insects, between the bases of the labrum and the hypopharynx, and opens into a large sucking pump
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INSECTS
