in] MOUTH-PARTS AND SENSE-ORGANS 41 



comparatively long branches. These are the labial 

 palpi. The actual piercing organs, which will be 

 described below, are the mandibles and labruin. They 

 are not so conspicuous as the rostrum which protects 

 them. 



When the piercing organs are at rest they are 

 partly retracted. The external portion is encased in 

 the tubular rostrum. The tube is formed by the two 

 labial palpi which are situated at the apex of the 

 short non-divided labium. The number of segments 

 composing each labial palpus in fleas varies, so far as 

 we know, from two to seventeen. In most fleas, 

 however, the labial palpus consists of five segments. 

 This appears to have been the original state of things 

 in the ancestral flea ; the palpus with more and the 

 palpus with less segments being derived from the 

 normal five-jointed one. The rostrum of a flea is 

 not a piercing organ like that of a fly and a bug. 

 The two labial palpi separate and lie flat, right and 

 left, on the skin when the true piercing organ is 

 driven into the host. The labial palpi therefore 

 require to be flexible, and this is attained by in- 

 creasing the number of segments or by reducing the 

 amount of chitinisation or horniness. We shall find 

 in the chigoes and their allies a rostrum which is pale, 

 weak, soft and scarcely horny. Among other fleas 

 where the rostrum is prolonged and strongly chitinised 

 we shall find greater segmentation. 



