TRACHEAL SYSTEM OF INSECTS 101 



the skin. The respiratory organs of land animals, there- 

 fore, are built on a different plan from those of aquatic 

 species, and generally speaking are ingrowths of the body 

 wall, communicating with the outside by narrow pas- 

 sages, and, buried deep among the tissues of the body, 

 are kept sufficiently moist to meet the requirements. 

 Breathing organs of this type comprise two classes: the 

 tracheal system of insects and related forms, and lungs, 

 chiefly a characteristic of higher animals. 



The Tracheal System of Insects. ^ — In all but a few 

 small insects respiration is carried on by a unique mecha- 

 nism known as the tracheal system (Fig. 20). If the body 



Fig. 20. — The Tracheal System of the Cockroach. 



of a grasshopper, for example, is carefully examined, sev- 

 eral small openings can be seen along the sides of the 

 body, and further investigation shows that each of these 

 leads into a delicate horny tube that penetrates the tis- 

 sues and breaks up into a large number of exceedingly fine 

 branches. A swelling movement of the body-wall like 

 that of the human chest in breathing, draws air into 

 these canals and distributes it to the cells. The oxygen 

 is then absorbed by the cells and carbon dioxid is liber- 

 ated by them and is then discharged by a reverse 

 contraction. 



In some of the water bugs and beetles that hunt their 

 food in ponds and gently running streams, periodical 

 visits are made to the surface where a small quantity of 

 air is imprisoned under the wing-covers or among hairs 



