64 INTERNAL ANATOMY OF INSECTS. 
small the diameter of these vessels, they are sure to re- 
main constantly open, and pervious to the air. And by 
this circumstance they may be always distinguished iron 
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 powerful mi- 
-eroscope in a recently dissected imsect, present a most 
beautiful and brilliant appearance, resembling a branch- 
ing tree of highly polished silver or pearl :—though 
sometimes they are blue, or of a lead colour, and some- 
times assume a tint of gold. In the dead insect the larger 
tubes soon turn brown, but the finer ones preserve their 
lustre several weeks?. The ramifications of the tracheal 
tree may be seen without dissection through the trans- 
parent skin of the common louse> and most of the thin- 
skinned larvee. 
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 the 
scalpel of a Lyonnet and the most powerful lenses are 
adequate to trace the extremities of these vessels; and 
even with every help, they at last become so inconceiva- 
bly slender as to elude the most piercing sight. That 
illustrious anatomist found that the two trachee of the 
larva of the Cossus gave birth to 236 bronchial tubes, and 
that these ramify into no less than 1336 smaller tubes, 
to which, if 232, the number of the detached bronchie, 
be added, the whole will amount to 1804 branches°. 
Surprising as this number may appear, it is not greater 
* Lyonnet 102. Malpigh. De Bombyc. 12. Reaum. i. 130. 
» Swamm. Bibl. Nat. t. ii. f. 7. ° Lyonnet 411. 
