90 



Insects and Disease 



THE BLOOD 



After a mosquito has been feeding on a man or 

 some other animal it is often so distended that the 

 blood shows rich and red through the thin sides of 

 the walls of the abdomen. This, however, is the 

 blood of the victim and not of the mosquito. The 

 blood of insects is not red but pale yellowish or 

 greenish. It is not confined in definite vessels, but 

 fills all the space inside the body cavity that is 

 not occupied by some of the tissues or organs. 

 It bathes the walls of the alimentary canal and 

 gathers there the nourishment which it carries to 

 all parts of the body. It does not carry oxygen or 

 collect the carbon dioxid as does the blood of 

 higher animals. That work, as we have just seen, 

 is done by the air-tubes. Above the alimentary 

 canal, extending almost the whole length of the ab- 

 domen and thorax, is a thin-walled pulsating ves- 

 sel, the heart. This consists of a series of chambers 

 each communicating with the one in front of it by 

 an opening which is guarded by a valve. When 

 one of these chambers contracts it forces the blood 

 that is in it forward into the next chamber which, in 

 its turn, sends it on. As the walls relax the valves 

 at the sides are opened and the blood that is in the 

 body-cavity rushes in to fill the empty chamber. 



