THE INSECT: ITS INTERNAL STRUCTURE 19 



spiracles and follows along the tracheae to the living tissues, which take 

 it in place of the oxygen usually received in this way, and are killed. 



It was formerly supposed that certain materials called contact in- 

 secticides which kill insects by contact with their bodies, caused death 

 by entering the spiracles and closing them up, thus producing suffocation. 

 This has now been proved to be incorrect. 



Insects which in their early stages live in water, cannot of course 

 breathe air into their bodies through spiracles during that period of their 

 lives. These are closed in such cases and the animal obtains air usually 

 through special structures called tracheal gills. These will be described 

 in connection with the insects which possess them. In a few small 

 water-inhabiting forms, the chitin covering the surface of the body is so 

 thin that oxygen present in the water can pass directly through it into 

 the body and to the parts there which need it, and carbon dioxid passes 

 in the reverse direction. 



Circulatory Organs. Insects have only an incomplete system of blood vessels. 

 A tube lies in the middle of the body close beneath the back, beginning near the 

 hinder end of the animal and extending forward into the head (Fig. 26). In the 

 abdomen this tube is constricted, forming chambers, and the chambered portion 

 is called the heart. There is a pair of openings on the sides of each chamber 

 through which blood can enter, and valves there which prevent its going out 

 again. The walls of the heart contain muscles and these contract one after the 

 other, forming a sort of wave of contraction which begins at the hinder end and 

 travels forward. Blood in the heart, being unable because of the valves, to pass 

 out at the sides, is pressed forward by this contraction wave, and at the front end 

 of the heart finds itself in a tube without chambers or valves, called the aorta, 

 through which it is led to the head where the aorta may divide into a few short 

 branches or may be unbranched. In either case, at this point the blood pours 

 out of it into the body, the system of blood vessels coming to an end. There is 

 now no definite and particular path for the blood to follow, but it would, in theory 

 at least, remain near where it escaped from the aorta, or gradually pass into any 

 spaces it might find unoccupied between the different structures in the head. 

 With each heart-beat, however, more blood is poured out of the aorta, increasing 

 the pressure upon that already in the head. It therefore is gradually forced 

 backward and to other parts of the body, each particle probably taking the path 

 where there is least resistance to its passage. In this way a general backward 

 direction is given to the flow. 



As it approaches the heart, another influence appears. During each contrac- 

 tion of the heart, it occupies less space, which leads to less than normal pressure 

 near it, and blood close by naturally flows closer to it. Upon its expansion again 

 and the opening of its valves, the direction of least resistance is now through the 

 valves and into the heart. 



As the blood passes back through the body, a given particle may at one circuit 

 go over certain organs and at the next, over entirely different ones. All the 

 internal organs, however, have their surfaces bathed by blood and this as it 

 passes over the stomach or other parts of the alimentary canal will pick up any 



