42 THE FLEA [OH. 



The small bristles at the extreme tip of the 

 rostrum seem to be sensory organs. They are like 

 those at the apex of the maxillary palpus. When 

 a hungry flea is put on one's arm, it appears to test 

 the skin with these bristles before it ventures to 

 make a puncture. 



The mandibles. These are a pair of sharp lancets 

 with serrated edges. They make the puncture and 

 are interlocked with the labrum to form a sucking 

 tube. 



The labrum. This is the central portion of the 

 mouth-parts and is in fact a prolongation of the 

 upper lip of the flea. It is a hard, sharp, awl-like 

 instrument : in shape like a horny trough. Its edges 

 are more or less toothed. Its apex is pointed and it 

 is as long as the mandibles. 



The general appearance and the relative positions 

 of the mouth-parts are shown in Fig. 4. 



Bearing in mind, then, that the piercing organs 

 are the labrum and the two mandibles, and that the 

 rostrum (composed of labium and labial palpi) is 

 merely a sheath, it is easy to form a clear picture of 

 a flea feeding. Anyone who is bold enough to place 

 a hungry flea on the bare skin of the arm can readily 

 observe through a powerful lens what happens. 

 When the flea has chosen a spot to pierce the skin, 

 the rostrum, with the mandibles and long upper lip 

 or labrum inside it, is moved a little forward. The 



