64? INTERNAL ANATOMY OF INSECTS. 



traction of which Lyonet supposes the trachea may be 

 shut when it is necessary to stop the passage of the air, 

 and direct it to any particular point*. The structure 

 here described is admirably adapted for the purpose it is 

 intended to serve ; for had these vessels been composed 

 of membrane, they could not possibly have been prevent- 

 ed from collapsing; byt by the intervention of a spiral 

 cartilaginou sthread this accident is effectually guarded 

 against, and the necessary tension of the tubes provided 

 for. However violent the contortions of the insect, how- 

 ever small the diameter of these vessels, they are sure to 

 remain constantly open, and pervious to the air. And 

 by this circumstance they may be always distinguished 

 from the other organs of the animal, and likewise by 

 their pearly or silvery hue, for from being constantly 

 filled with air, these tubes, when viewed under a power- 

 ful microscope in a recently dissected insect, present a 

 most beautiful and brilliant appearance, resembling a 

 branching tree of highly polished silver or pearl : 

 though sometimes they are blue, or of a lead colour, and 

 sometimes assume a tint of gold. In the dead insect 

 the larger tubes soon turn brown, but the finer ones pre- 

 serve their lustre several weeks b . The ramifications of 

 the tracheal tree maybe seen without dissection through 

 the transparent skin of the common louse c and most of 

 the thin skinned larvae. 



You will not expect to view in this way the minuter 

 ramifications of the bronchia, when 15* have mentioned 

 their number and incredible smallness. Nothing but 



a Lyonet 104. Sprengel Comment ar. 17- 



b Lyonet 102. Malpigh. Be Bombyc. 12. Reaum. i. 130. 



c Swamm. BibL Nat. t. ii./. 7- 



