52 THE FLEA [OH. 



sexual organs of insects, of both sexes, are of special 

 importance to the systematist or classifying naturalist. 

 They often enable him to recognise the species when 

 other organs do not show sufficiently striking cha- 

 racters. A minute study of the genitalia of fleas is 

 an absolute necessity to the systematic entomologist, 

 the more so as fleas do not present nearly as many, 

 or nearly as varied, external differences as do the 

 species of most winged insects where colour and 

 pattern of wings are both important. 



CHAPTER IV 



THE INTERNAL ORGANS OF A FLEA 



A FLEA like every other animal must feed and 

 breathe, which leads to a consideration of the internal 

 organs of digestion and respiration. The digestive 

 canal is a slender tube which connects the mouth 

 and the anus, and which is less convoluted and much 

 straighter than in the higher vertebrates. Fig. 6 

 will show the relative positions of the various parts, 

 namely, the mouth, pharynx, gullet, gizzard, stomach, 

 and rectum. Connected with the digestive canal are 

 certain glands and organs of excretion. The ali- 

 mentary tube itself passes through the middle of the 

 flea's body, and is kept in that position partly by 



