108 THE FLEA 



fleas which are parasites of strictly nocturnal animals, 

 lends colour to the suggestion that fleas which are 

 blind have lost their eyes because they had no need 

 of them. Disuse is speedily followed by degeneration. 



APPENDIX A 



SYSTEMATIC VIEW OF THE ORDER 

 SIPHONAPTERA 



Order SIPHONAPTERA. Latreille (1825). 



Insects with body laterally compressed. Head 

 rounded and fixed by the whole posterior part to the 

 thorax. Mouth-parts for piercing and sucking, con- 

 sisting of paired mandibles with serrate margins and 

 unpaired labrum. These are sheathed by the labium 

 and labial palpi. Maxillae usually triangular with 

 four-jointed palpi. Eyes simple, placed in front of 

 the antenna), occasionally rudimentary or absent. 

 Antennae of three main segments which lie when at 

 rest in a groove. Three thoracic segments, always 

 free, each consisting of a no turn and a sternum. The 

 sterna of the second and third segments are further 

 divided into a sternum, an episternum and an 

 epimeron, the two latter constituting the pleura. 

 Wings and rudiments of wings entirely absent. 



