86 TRUE TALES OF THE INSECTS. 



are present even in apterous forms of AcridiidDe, distinct 

 vesicles are absent in the neighbouring groups of Or- 

 thoptera. Those Orthoptera which do not take long 

 flights have no need of air-vessels. 



No doubt they are connected with the power of flight ; 

 doubtless they assist the insect in its aerial movements. 

 The body of a large grasshopper or locust is naturally 

 of considerable weiofht, and it seems certain true flight 

 can only be effected when the sacs are dilated and filled 

 with air. That by filling and partially emptying them 

 during the process of breathing the insect is enabled 

 to enlarge its bulk and alter its specific gravity at 

 pleasure, so as to render itself capable of rising and 

 supporting itself on the wing with little muscular eftbrt, 

 was first assumed by Sir John Hunter; and inference 

 and observation confirm the view that the use of the 

 vesicles is to lighten the body, to float up the insect in 

 the air. The precise mode in which they are dilated 

 is not understood. With its sacs in full play, plainly a 

 locust becomes an aeronaut, a sort of balloon. 



To this fact are largely due the enormous powers of 

 flight possessed by these insects ; the intimate association 

 of the complex arrangement of air-tubes and air-sacs 

 with the powers of flight is very evident. It will be 

 seen that, once having risen from the ground, the insect 

 can sail for hours in the air, constantly filling and re- 

 filling its internal buoys or balloons, and thus be wafted 



