ABOUT CATERPILLARS 197 



thoracic from the abdominal segments without 

 looking at those parts which characterize them 

 distinctively, such as the legs or spiracles. Thus, 

 even in the mere disposition of the spines along 

 a caterpillar's back, the future separation of the 

 thorax and abdomen is foreshadowed. This is 

 wholly independent of the larger amount of space 

 upon the thoracic tract due to the absence of spir- 

 acles ; for, when the spines are well developed on 

 the first thoracic segment, wliich bears an unusu- 

 ally large spiracle, they are aligned with those of 

 the other thoracic segments and not with those of 

 the abdominal segments. 



This, like the absence of spiracles from the sec- 

 ond and third thoracic segments, might be ex- 

 plained on the theory that the transformations of 

 the insects are an acquired characteristic, a devel- 

 opment backward from the imago. But this will 

 not explain another peculiarity which one observes 

 in the general arrangement of the spines and other 

 dermal appendages on the back of caterpillars, a 

 feature which is extremely common, though per- 

 haps not universal, with all vermiform creatures. 

 I refer to what might be called the polar or an- 

 tithetic arrangement of these appendages, which 

 shows itself in a multitude of ways. As a general 



