60 THE FLEA [OH. 



than that of the lowest vertebrate. The blood is 

 almost colourless. A large contractile heart drives 

 it into the main blood-vessel. There is, however, no 

 closed system of arteries, capillaries, and veins such 

 as the higher animals possess; and the blood circu- 

 lates in the whole cavity which intervenes between 

 the body-wall and the various internal organs. There 

 is little need for an elaborate system of blood-vessels 

 since the internal tissues are supplied with oxygen 

 by the ramifying air-tubes. Fleas have more of the 

 air-holes called stigmata than any other insects. 

 Each of the three segments of the thorax has a pair, 

 as well as the second to the eighth segments of the 

 abdomen. The spiracles or apertures lie free on the 

 outside of the body. In beetles, and other insects 

 which run through dusty places, they are lodged in 

 the thin membrane between the segments. 



The heart of a flea is a very delicate pulsating 

 tube which lies along the back, above the digestive 

 canal and immediately beneath the integument. One 

 may attribute some of the extraordinary strength 

 and vital energy of a flea to the fact that, by the 

 blood-system and the air-system, the tissues of the 

 body are kept richly supplied with oxygen. The 

 blood of a flea is a thin fluid and, of course, without 

 red corpuscles. The blood that is shed when a flea 

 is crushed comes from the stomach and not from the 

 blood-vessels of the insect. 



