INTERNAL ANATOMY OF INSECTS. 191 



they rise ; its head makes way for it ; its abdomen, as a 

 rudder, steers it; and by alternately increasing and di- 

 minishing in volume, and rising and falling, enables it to 

 win an easy way through the fluctuations of the atmo- 

 spheric sea. The trunk by its elasticity admits the in- 

 ternal action of antagonist muscles, which by turns com- 

 press and dilate it ; an action promoting the elevation 

 and depression of the wings, and keeping up the elasti- 

 city of the internal air, which is thus now rarified and 

 now condensed : in the former state flowing like a tide, 

 accompanied by the blood, into the nervures of the 

 wings % and thus increasing their tension and centrifugal 

 force ; in the latter ebbing and receding to the trunk, 

 thus relaxing the one and diminishing the other. The 

 spiracles by which the air enters or is expelled, open 

 and shut at the animal's pleasure 13 ; and besides, many 

 insects are furnished, as we have seen c , with numerous 

 vesicles or reservoirs, which can give out a supply of in- 

 ternal air when wanted : and thus they can vary their 

 aerial motions, diminish or increase the counteracting 

 centrifugal force ; rk d and fall, and move onwards and 

 in different directions, as their occasions demand. 



iii. The Abdomen is perhaps capable of the greatest 

 variety of motions of the three primary sections of the 

 body. Even when the insect is reposing, a constant di- 

 latation and contraction usually takes place in it d ; and 

 from its annular structure, its parts capable of separate 

 motion are numerous: it expands and contracts; it 



a Chabr. Sur le Voldes Ins. c. ii. 336. note 1. VOL. III. p. 292. 

 b Chabr. Ibid. c. i. 447. c See above, p. 66-. 



d See above, p. 73. 



