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; but by the intervention of a spiral 
cartilaginou s thread 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 1 '. The ramifications of 
the tracheal tree may be 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 I have mentioned 
their number and incredible smallness. Nothing but 
* Lyonet 104. Sprengel Commentar. 17- 
b Lyonet 102. Malpigh. De Bombyc. 12. Reaum. i. 13Q. 
c Swamin. Bibl. Kat. t. ii./. 7- 
