133 HOW INSECTS BREATHE. 



removing it when once there ; it could hardly fail, in the 

 lapse of time, that something would accidentally enter, which 

 would afterwards prove a constant source of annoyance. 

 But insects cast their skin at short intervals, at least as long 

 as they are larvae, and but few live more than a few weeks 

 in the perfect state ; and as the larger tracheae are cast with 

 the skin, it is sufficient to take precautions to prevent any- 

 thing from penetrating through the spiracle. 



In most cases the spiracle is a slit which can be closed, and 

 muscles are provided for the purpose ; sometimes instead of 

 one hole there are a number, separated by a horny net-work. 

 These are found in the larva of Sirex duplex, and may per- 

 haps be required, because these larvae live in galleries which 

 they eat in trees, and the wood dust would be very apt to 

 penetrate an ordinary spiracle. In the large water-beetle 

 (Di/ticus), and in Cicada, bushy hairs project from the 

 margin, and the air being thus strained, all extraneous 

 matters are effectually excluded. The spiracles lead into 

 tubes called tracheae, which in the live insect are constantly 

 full of air. From each spiracle these tubes ramify in all 

 directions, like the roots of a tree, dividing and subdividing 

 until their final branches become so small as to be scarcely 

 discernible. Besides these branching tubes, each spiracle 

 sends a large unbranched one to the tracheae arising from the 

 spiracles on each side. In this manner the whole body is 

 connected into one system; but for this contrivance, if any one 

 spiracle became stopped up, the organs in its neighbourhood 

 would suffer from want of air. 



Besides the tracheae we find also, in a great many cases, 

 air bladders or vessels ; these serve as receptacles of air, and 

 probably assist in giving to the body that lightness which is 

 necessary for flight : in this respect we may compare them to 

 the air vessels which are found in birds ; and we shall ob- 



