THE CATERPILLAR. 



in proportion and absolutely retractile within this 

 segment, so that when the creature is not feeding 

 its head is invisible [see Fig. 25]. In another 

 group, the swallow-tails, an approach to this state 

 is seen in the concealment of the 

 hinder half of the head by a fold 

 in the cuticle of the segment behind 

 [Fig. 31]. Any striking differences 

 of size seen in the middle segments 

 of the body are generally due not 

 so much to the enlargement or con- 

 traction of single segments as to 

 the presence, at such points, of 

 fleshy tubercles [see Fig. 29] ; but the body al- 

 ways tapers somewhat posteriorly, and the last 

 two segments (especially in the blues and their 

 allies) are generally so closely coalesced as readily 

 to be mistaken for a single one. 



FIG. 31. Front 

 portion of caterpil- 

 lar of Princeps Po- 

 lyxenes, to enow 

 the partial covering 

 of the head by the 

 first thoracic seg- 

 ment, X 1|. 



FIG. 32. Caterpillar of Princeps Polyxenes with extended osmateria, 

 nat. size (the head is drawn too large). 



It only remains to speak of a certain isolated 

 extensile organ borne by the first segment behind 

 the head. I refer to the prong, termed, in the swal- 

 low-tails (Equites), the osmateria or scent-organ 

 [Fig. 32] ; it is here developed to an extraordinary 



