SOME HUMAN PARASITES 283 



saliva, or arises because the parasite has previously fed 

 upon some decaying animal matter, has not been proved. 



LICE 



Even more disgusting than the bed bugs and fleas are 

 the Pediculce or lice, of which three species are associated 

 with the human body. These little animals are constantly 

 confined to their hosts, they spend their entire life on the 

 mammalian body, and from it derive all their nourishment. 

 Their courtship takes place there, their eggs are attached 

 to hair, and, except by accident, they are never dissociated 

 from their host ; in short, they are true parasites, and, as 

 such, have specially modified organs. All true lice are 

 confined to mammals, they are wingless, their feet are 

 adapted for holding and clasping hairs, their mouth parts, 

 which are always completely withdrawn into the head 

 when not in use, consist of tubular sectorial organs, pro- 

 vided with an armature of lancets for piercing the skin 

 (fig. 80). Writing of these organs, Uhler, in his Standard 

 Natural History, says they have "a fleshy unjointed 

 rostrum, capable of great extension by being rolled inside 

 out (like a glove finger), this action serving to bring for- 

 ward a chaplet of barbs which embed themselves in the 

 skin to give a firm hold for the penetrating bristles, 

 arranged as chitinous strips in a long, slender, flexible 

 tube, terminated by four very minute lobes, which probe 

 to the capillary vessels of a sweat pore. The blood being 

 once reached, a current is maintained by the pulsations of 

 the pumping ventricle and the peristaltic movements of the 

 stomach." 



One interesting feature, common to all the lice, is that 

 they are very rarely found on any other species of mammal 

 than the one they normally infest, and then only on closely 

 related species; thus human lice would not be found on 

 the elephant, nor the elephant louse on the mouse. This 



