TWO-WINGED FLIES AND FLEAS 105 



popular notice by causing horses to stampede during 

 army manoeuvres. 



The seventeenth order, or Siphonaptera, comprises 

 the fleas, of which upwards of forty British species 

 are known, the largest being HystrichopsyUa talpa, 

 which infests moles. These insects are probably 

 allied to the Diptera, but they differ from them in 

 many important respects. The piercing mouth-parts 

 are the lancet-like mandibles, with serrated edges, and 

 an unpaired organ which some authorities regard as a 

 prolongation of the labrum or upper lip, others as the 

 modified " tongue " (hypopharynx). These three 

 mouth-parts interlock, so as to form a sucking-tube. 

 When not in use, they are encased in a tubular 

 rostrum, formed by the two labial palpi. Fleas are 

 further characterized by the entire absence of wings, 

 and by the lateral flattening of their bodies. They 

 may be described as " intermittent parasites," since 

 they are not permanently attached to their victims. 

 Moreover, only adult fleas suck blood. The white, 

 worm-like larvae have biting mouth-parts. They live 

 among dust and refuse, and feed on the organic matter 

 which it contains. When full-grown, each larva spins 

 a small silken cocoon, wherein the pupal stage is 

 passed. 



