ABOUT CATERPILLARS 195 



difference between the structures found on the 

 thoracic and the abdominal segments. Thus, on 

 account of the development of the future wings, the 

 spiracles of the second and third thoracic segments 

 are omitted, and the legs found upon the thoracic 

 and abdominal segments are very different in 

 character, those of the abdominal segments being 

 a temporary expedient for the long trailing abdo- 

 men, while those of the thoracic segments are more 

 highly developed. But these exceptions, which 

 have a direct relation to the future needs of the 

 animal, cannot explain certain other features which 

 show similar differences. In general terms, the 

 entire surface structure of the upper part of the 

 animal is practically identical on the thoracic and 

 abdominal segments, but there are certain differ- 

 ences which appear, to which no explanation is 

 readily given. 



For instance : By the transverse creases which 

 simulate the incisures between the segments, each 

 segment is ordinarily divided into two or more 

 transverse sections. Now these sections, at least 

 when there are more than one or two, invariably 

 differ upon the thoracic and abdominal segments, 

 the abdominal segments having a system of their 

 own, distinct from that of the thoracic. The lii-st 



