The Ammophilae 



upon the victim. The method adopted is 

 the following: when the caterpillar is quite 

 still, I try each segment with the point of a 

 fine needle and thus measure the amount of 

 sensibility by the more or less manifest signs 

 of pain in the insect. When the needle 

 pricks the fifth segment or the sixth, even 

 piercing it right through, the caterpillar does 

 not stir. But if you prick even slightly a 

 second segment, behind or in front of that 

 insensible segment, the caterpillar wriggles 

 and struggles with a violence which increases 

 in proportion to the distance of the point 

 attacked from the original segment. At the 

 hinder end in particular, the least touch pro- 

 vokes wild contortions. There was only one 

 sting, therefore, and it was administered to 

 the fifth or sixth ring. 



What peculiarity then do these two seg- 

 ments possess that one or other of them 

 should be the target of the assassin's weapon? 

 None whatever in their organization; but 

 their position is another matter. Leaving 

 the Silky Ammophila's Measuring-worms on 

 one side, I find that the prey of the others 

 is organized as follows, the head being 

 counted as the first segment: three pairs of 

 real legs on the second, third and fourth 

 265 



