4 A BOOK OF INSECTS 



closed by muscular contraction. The air which enters 

 the spiracles is conveyed direct to every part of the body 

 through a system of minute pipes, known as trachea?, the 

 final ramifications of which are so delicate that they pene- 

 trate the most distant extremities, such as the eyes and 

 the tips of the antenna?. The trachea? are soft tubes 

 within which run spirally coiled threads of chitin. They 

 are, in fact, exact counterparts in miniature of our wire- 

 lined rubber gas-tubing. Their structure is a safeguard 

 against short circuit. The elastic chitinous coil keeps the 

 trachea open even when it is subjected to pressure, as by 

 the bending of a joint through which it passes, while the 

 flexibility of the tube is not impaired. Owing to this 

 singular method of breathing, the blood of the typical 

 insect has little or nothing to do with the aeration of its 

 tissues. Its function is almost wholly nutritive. More- 

 over, the blood (which is usually colourless, though some- 

 times green or yellow) does not flow through a system of 

 arteries and veins. True, the insect has a chambered 

 heart from which the blood passes into a pulsating tube 

 or aorta ; but from this it is pumped direct into the body 

 cavity, where it circulates freely, bathing all the organs, 

 receiving nutriment from the food canal, giving up its 

 waste matter to the kidneys and trachea?, and visiting the 

 limbs. Finally, it re-enters the heart through valvular 

 slits in the walls of its chambers. The insect's heart is a 

 comparatively large, long organ, situated above the diges- 

 tive canal, and immediately beneath the chitinous armour 

 of the back. Its aorta, or pumping tube, passes into 

 the head and discharges its stream of blood above the 

 brain. 



Of the insect's digestive system little need be said. It 

 comprises, among other parts and appendages, a crop, 

 a gizzard, a stomach, and an intestine. Moreover (and 



