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 " claspers." 

 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 the caterpillar. The labium, or lower lip, bears a little 



tubular projecting structure known as the spinneret, for into 



- Head of 

 the Caterpillar of 

 Pieris brassicae, 

 seen from in front. 

 Antenna ; m, man- 

 dible ; pj maxillary 

 palp ; s, spinning- 

 tube. 



