80 THE FLEA [OH. 



two main fixing and clinging organs are the mouth 

 and the claws. Weak mouth-parts accompany strong 

 legs. We observe, accordingly, two lines of develop- 

 ment. The chigoes and their allies present an extreme 

 case : for the legs are practically useless for holding 

 on. The bristles, and the claws as well, are exceed- 

 ingly thin. In this family the mandibles serve the 

 purpose of claws. The other line of development is 

 best seen in a genus of fleas from South America 

 (Malacopsylla\ where the piercing organs are short 

 and weak, whilst the claws and bristles of the legs 

 are very strongly developed. 



The modification found in the rostrum of the 

 chigoes has already been referred to, and the ex- 

 planation of this will now be understood. The 

 rostrum is a sheath, on either side of the piercing 

 organs, consisting of an under lip and two labial 

 palpi. When the flea sucks, the labial palpi are 

 pushed asunder, as the piercing organs are driven 

 in, and lie flat on the skin of the host. In this family 

 the rostrum is almost white in colour and soft instead 

 of being horny or chitinised. Where the rostrum is 

 strongly chitinised or very horny the flea has to use 

 a certain amount of force to counteract the spring- 

 like action of the labial palpi. It is conceivable that 

 rigid labial palpi would prove inconvenient to fleas 

 which remain permanently attached to their host by 

 their mouth organs. 



