216 INTRODUCTION TO ZOOLOGY CHAP, xv 



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- 

 FIG. 146. Piens brassicae. M1 * ,. 



pillar can cling very firmly. On 



A, Thoracic five- jointed leg; B, the lagfc segment? the pro-legs are 

 pro -leg or "cushion foot of , . 



abdomen. 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 

 fth L^-va severa l P a i rs 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. Head of 

 which feeding is most active, we find that pl^t^icaJ, 

 the mouth-parts are specially adapted for se en from in front. 

 the rapid cutting of the leaves which , Antenna; m, man- 

 form the food. The mandibles, which dible ; P> maxillary 



,..,,. -LI j palp ; s, spinning- 



are absent in the imago, are here large and {. ube< 



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 



