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 b ; 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 ; rise 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 
' Chabr. Sur le Voldes Ins. c. ii. 336. note 1. Vol. III. p. 292—. 
b Chabr. Ibid. e. i. 447. c See above, p. 66-. 
'' See above, p. 73 — . 
