iv] INTERNAL ORGANS OF A FLEA 59 



The urinary tubules are excretory organs which 

 carry off, in solution, the waste products of the flea's 

 body. They are sometimes also called Malpighian 

 tubes (Fig. 6). This name they received after 

 Malpighi (1628-94), a famous Italian anatomist, who, 

 four years after Harvey's death, saw with his own 

 eyes the capillary circulation of which Harvey had 

 only inferred the existence. He also was the 

 first to detect the urinary tubes of insects. These 

 tubules answer to the kidneys of the higher verte- 

 brate. They vary in number in different insects from 

 two to over a hundred. In fleas there are four. They 

 are longish, slender, tubular glands which are closed 

 at one end, but, at the other, open into the rectum. 

 The urinary excretions come from the blood, pass 

 down the tubes into the rectum, and so leave the 

 flea's body by the anus. In insects the urinary 

 excretion is, generally, only partially liquid. 



The organs of respiration in a flea consist of a 

 series of tracheae, or air-tubes, which open by aper- 

 tures, called stigmata, at the sides of the body. These 

 air-tubes branch and form an elaborate system of 

 ramifications. They have a horny lining and are 

 supported by a spirally-wound thread-like thickening. 

 In this way air is conveyed from the external world, 

 and the oxygen, which vital processes require, is 

 conducted to all parts of the insect's body. 



The blood-system of a flea is far less complete 



