MOSQUITOES AND FLIES
the common large black horsefly (Fig. 169 B) will show the nature of the feeding organs with which these flies are equipped. Projecting downward from the lower part of the head are a numbeof appendages; these are the mouth parts. They correspond in number and in relative position with the mouth appendages of the grasshopper (Fig. 66), but they differ from the latter very much in form because they are adapted to quite a different manner of feeding. The horsefly does hot truly bite; it pierces the skin of its victim and sucks up the exuding blood.
By spreading apart the various pieces that compose the group of mouth parts of the horsefly, it will be seen that there are nine of them in all. Three are median in position, and therefore single, but the remaining six occur in duplicate on the two sides, forming thus three sets of paired structures. The large club-shaped pieces, however, that lie at the sides of the others, are attached at their bases to the second paired organs and constitute a part of the latter, so that there are really only two sets of paired organs. The anteriormost single piece is the labrum (Fig. 169 B, Lm); the first paired organs are the mandibles (Md); the second are the maxillae (Mx); the second median piece is the hypopharynx (not seen in Fig. 169 B); and the large, unpaired, hindmost organ is the labium (Lb). The lateral club-shaped pieces are the palpi of the maxillae (MxPlp).
The labrum is a strong, broad appendage projecting downward from the lower edge of the face (Figs. 169 B, 170 A, Lm). Its extremity is tapering, but the tip is blunt; its under surface is traversed by a median groove extending from the tip to the base but closed normally by the hypopharynx (Fig. 170 D, Hphy), which fits against the under side of the labrum and converts the groove into a tube. The upper end of this tube leads directly into the mouth, a small aperture situated between the base of the labrum and the base of the hypopharynx and opening into a large, stiff-walled, bulblike structure (Fig. 170 A, Pmp)
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