. An introduction to zoology, with directions for practical work (invertebrates). 216 INTRODUCTION TO ZOOLOGY CHAP. XV. FiQ; 146.—Pieris brassicae. Af Thoracic five-jointed leg; B, pro-leg or "cushion foot" of, abdomen. Bespiratlon. On some of the abdominal segments are structures known as cushion feet or pro-legs. THese are fleshy protuberances from the body, which are not jointed, though the very elastic skin covering them may be thrown into transverse wrinkles when the foot is retracted. Each such foot has, at its tip, a half-circle of little hooks by means of which the cater- pil
. An introduction to zoology, with directions for practical work (invertebrates). 216 INTRODUCTION TO ZOOLOGY CHAP. XV. FiQ; 146.—Pieris brassicae. Af Thoracic five-jointed leg; B, pro-leg or "cushion foot" of, abdomen. Bespiratlon. On some of the abdominal segments are structures known as cushion feet or pro-legs. THese are fleshy protuberances from the body, which are not jointed, though the very elastic skin covering them may be thrown into transverse wrinkles when the foot is retracted. Each such foot has, at its tip, a half-circle of little hooks by means of which the cater- pillar can cling very firmly. On the last segment, the pro-legs are turned backwards, and are gener- ally distinguished as the " ; Respiration takes place through spiracles, little openings on each- side of the first thoracic and first eight abdominal segments. They appear as light-brown oval dots with a narrow dark rim round them ; the actual aperture is not visible to the naked eye (see Fig. 145 where the spiracles appear as dark dots just above the legs in the segments mentioned above). The head bears only rudimentary antennae and of the La^a ®^'^^^*' pairs of small simple eyes, or ocelli. Cater- pillars apparently have very dim sight, and even this is limited to the recognition of objects quite close to them. However, it suffices, no doubt, for their limited larval life, during which they rarely leave their food-plant, escaping the attentions of their enemies, not by their own activity, but by their secluded habits and protective coloration, and, in some cases, by their objectionable taste or hairiness. Since the caterpillar stage is that in Fig. 147 which feeding is most active, we find that the mouth-parts are specially adapted for the rapid cutting of the leaves which form the food. The mandibles, which are absent in the imago, are here large and powerful, whilst the maxillae and labial palps, so greatly developed in the butterfly, are rudimentary in t
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