THE CATERPILLAR. 19 



of the creature, when the thoracic segments become 

 the thorax and the abdominal segments the abdo- 

 men a distinction which is merely foreshadowed 

 now in the difference between the anterior and 

 posterior feet. We have, therefore, chosen not to 

 treat of these two tracts separately, but to discuss 

 the diiferent characteristics of the body as a 

 whole, pointing out, however, some further dis- 

 tinctions between those which occur in the tho- 

 racic and those in the abdominal tracts. To return 

 to the consideration of the legs, it should be 

 noticed that while every one of the thoracic seg- 

 ments bears a true leg, it is not every one of the 

 abdominal segments which bears a proleg. In- 

 deed in the Lepidoptera generally there is consid- 

 erable variation in this respect, but among the but- 

 terflies these appendages are always borne by the 

 third to the sixth abdominal segments, and by the 

 last segment, leaving thus a similar space without 

 support between the true and false legs, and be- 

 tween the terminal and preceding false legs. In 

 a single group of butterflies (Plebeii), that to 

 which our tiny blues (Adolescentes) belong, the 

 prolegs are excessively small, and can indeed only 

 be readily detected when the skin of the caterpil- 

 lar is prepared by inflation. These caterpillars 

 seem to glide rather than to creep [Fig. 25] ; and 

 indeed their movements are undoubtedly aided 

 by muscular contractions and expansions of the 



