INSECTS
(AL), the first four pairs being on the third, fourth, fifth and sixth segments, and the last on the tenth segment. The region of the three segments in the caterpillar bearing the jointed legs corresponds with the thorax of an adult insect (Fig. 63, Th), and that following corresponds with the abdomen (Ab). The thorax of the adult insect constitutes the locomotor center of the body, but the wormlike caterpillar has no special locomotor region, and hence its body is not separated into thorax and abdomen. The thoracic legs of the caterpillar terminate each in a single claw, but the foot of each of the abdominal legs has a broad sole provided with a series or circlet of claws and with a central vacuum cup. The abdominal legs of the caterpillar, therefore, are important organs of progression, and are the chief organs of grasping or of clinging to hard or flat surfaces.
The jaws of the caterpillar consist of a pair of large, strong mandibles (Fig. 152, Md) concealed, when closed, behind the labrum. Each jaw is hinged to the lower edge of the cranium at the side of the mouth by two ball-and-socket hinges in such a manner that, when in action, it swings outward and inward on a lengthwise axis. The cutting edges are provided with a number of strong teeth (Fig. 153), the points of which come together or slide past each other when the jaws are closed.
The large complex organ that projects behind or below the mouth like a thick under lip (Fig. 152 C) is a combination of three parts that are separate in other insects. These are the second pair of soft jaw appendages, called maxillae (B, C, Mx), and the true under lip, or labium (Lb). The most important part of this composite structure in the caterpillar, however, is a hollow spine (A, B, C, Spt) pointed downward and backward from the end of the labium. This is the spinneret. From it issues the silk thread with which the caterpillar weaves its tent and its cocoon.
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