INSECTS
forms of the disease. One is Glossina palpalis (Fig. 185), distributor of the tropical variety of the disease; the other is Glossina morsitans, carrier both of the southern variety of sleeping sickness and of nagana.
The stable fly, the horn fly, and the tsetse fly, we have said, belong to the same family as the house fly, namely, the Muscidae; and yet they appear to have mouth parts of a very different type. The differences, however, are of a superficial nature. All the muscid flies, biting and non-biting, have the same mouth-part pieces, which are the labrum (Figs. 183 B, 186 C, Lm), the hypopharynx (Hphy), and the labium (Lb). They lack mandibles and maxillae, though the maxillary palps (Plp) are retained. In the biting species, the labium is drawn out into a long, slender rod (Fig. 186 C, Lb), and its terminal lobes, the labella (La), are reduced to a pair of small, sharp-edged plates armed on their inner surfaces with teeth and ridges. In the natural position, the deflected edges of the labrum (Fig. 186 B, Lm) are held securely within the hollow of the upper surface of the labium (Lb), the two parts thus in-closing between them a large food canal (FC) at the bottom of which lies the slender hypopharynx (Hphy), containing the exit tube of the salivary duct.
The biting muscids, therefore, have a strong, rigid, beaklike proboscis formed of the same pieces that compose the sucking proboscis of the house fly (compare Fig. 183 A with Figs. 184 and 186 A), but the labium is so modified that it becomes an effective piercing organ. When
one of these files bites, it sinks the entire beak into the flesh of its victims. The tsetse fly is said to spread its front legs apart when it alights for the purpose of feeding, and to insert its beak by several quick downward thrusts of the head and thorax. The insect then quickly fills itself with blood, with which it may become so distended that it can scarcely fly. The bulb at the base of the tsetse fly's labium (Fig. 186 C, b) is no part of the sucking apparatus; it is merely an enlargement for the accommodation of
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