18 LICE AND THEIR MENACE TO MAN 



really ingrowths from the skin, so that the inside 

 of the tubes is part of the outside of the body. 

 This system opens to the exterior by means of 

 small pores called spiracles which are situated along 

 the sides of the body on the thorax and abdomen. 

 The spiracles are round and are protected by 

 a ring of teeth which filter the air and prevent 

 gross particles entering. Nor can water enter 

 owing to its high surface tension, but fluids, such 

 as oils, which have a lower surface tension than 

 water are able to enter and thus clog up the 

 pores and choke the insect. Each of the spiracles 

 is connected by means of a short passage with a 

 horse-shoe-shaped tube, the curve of which is 

 towards the hinder end of the body. From this 

 branching tubes run and ramify right through 

 the body of the insect, forming a network over 

 all the organs, so that no part is left without a 

 supply of air. The system has thus not only 

 the function of our lungs but also one of the main 

 functions of our circulatory organs. Air is forced 

 in and out of the system by contraction and 

 expansion of the body wall with a bellows-like 

 action. 



There are no circulatory organs in insects as 

 we know them in higher animals, and no true 

 blood. The main function of these is taken on 

 by the respiratory system described above. The 

 nutriment from the gut is carried about the body 

 by the fluid of the body space, or haemocoele, 

 which is a cavity extending all through the insect 



