n] EXTERNAL STRUCTURE OF A FLEA 33 



Having now given a rough outline of the external 

 skeleton of a flea, it only remains to say something 

 about the muscular system. Attached to the inside 

 of the chitinous armature are an enormous number 

 of muscles, whitish and almost transparent. They 

 act as extensors, retractors, flexors, elevators and 

 depressors. The joints and hinges of the skeleton 

 allow of considerable, but not perfect, freedom. The 

 muscles of locomotion are partly in the thorax 

 and partly in the several joints of the legs. Our 

 knowledge of the muscular system of fleas is 

 very imperfect. But, as in other insects, the general 

 arrangement of the muscles is based on the 

 segmented structure of the body. 



For the reader who can accurately picture to 

 himself the external structure of a flea and of the 

 typical insects belonging to other orders, a few words 

 may be said on the probable ancestry of fleas and 

 their relationship to other living insects. This vexed 

 and much debated question is still, as the older 

 naturalists would have said, treniendum mysterium. 

 Very little light has yet been thrown upon it, and 

 the most divergent views have been expressed by 

 learned and competent entomologists. A historic 

 survey of the various opinions that have been held 

 since the days of Linnaeus would fill many pages ; but 

 a short summary of the different orders to which fleas 

 have been referred by different zoologists will suffice. 



R. F. 3 



