INSECTS
threadlike and provided with comparatively few short hairs (A). The sexes differ also in the mouth parts, for, as in the horseflies, the males lack mandibles.
The mouth parts of the mosquito, in the natural position, do not appear as separate pieces, as do those of the horsefly. The various elements, except the palpi, are compressed into a beak that projects forward and downward from the lower part of the head (Fig. 176 A, Prb). The length of the beak varies in different kinds of mosquitoes; it is particularly long in the large South American species shown in Figure 176.
When the beak of the female mosquito is dissected (Fig. 176 B), the same equipment of parts is revealed as is possessed by the female horsefly (Fig. 169 B), namely, a labrum (Lm), two mandibles (Md), two maxillae (Mx), a hypopharynx (Hphy), and a labium (Lb). It is the labium that forms most of the visible part of the beak, the other pieces being concealed within a deep groove in its upper surface.
The labrum (Fig. 176 B, Lm) is a long median blade, concave below, terminating in a hard, sharp point; it is probably the principal piercing tool of the mosquito's outfit. The mandibles of the mosquito (Md) are very slender, delicate bristles; those of the species figured are so weak that it would seem they can be of little use to the insect. The maxillae (Mx) are thin, flat organs with thickened bases, each terminating in a sharp point armed on its outer edge with a row of backward-pointing, saw-like teeth which probably serve to keep the mouth parts fixed in the puncture as the piercing labrum is thrust deeper into the flesh. The palpi (MxPlp) arise from the bases of the maxillae. The hypopharynx (Hphy) is a slender blade with a median rib which is traversed by the channel of the salivary duct. Its upper surface is concave and, in the natural position, is closed against the concave lower side of the labrum, the two apposed pieces thus forming between them a tube which leads up to the
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