INTERNAL ANATOMY OF INSECTS. 6% 
Jirst or external one is a thickish membrane, strength- 
ened by a vast number of fibres or vessels, which form 
round it a number of irregular circles; the second is a 
membrane more thin and transparent, without a vascu- 
lar covering?; the third is formed of a cartilaginous 
thread running in a spiral direction, which may be easily 
unwound’, ‘This structure gives a great elasticity to 
these organs, so that they are capable of considerable 
tension, after which they return to their usual length‘. 
The Bronchi@ are cylindrical or slightly conical, insen- 
sibly diminishing in size as they leave the trunk, in which 
they originate. In larve, after losing their spiral fibre, 
they appear to terminate in membrane, but in perfect 
insects they pass into vesicles’. In the Cossus the trachea 
is flattened, and in every segment, except the first and 
two last, is bound by a fleshy cord four or five times as 
thick as its threads. Where this occurs, there is a slight 
constriction,—probably here is a sphincter, by the con- 
traction of which Lyonnet 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 prevented 
from collapsing; but by the intervention of a spiral cartila- 
ginous thread this accident is effectually guarded against, 
and the necessary tension of the tubes provided for. 
However violent the contortions of the insect, however 
4 Lyonnet Anat. 103. 
b Ibid. Cuv. Anat. Comp. iv. 438. This author says that the in- 
termediate tunic is the spiral thread (437). © Lyonnet 102. 
4 Ibid. 194. Sprengel Commentar. 17. * Lyonnet Ibid. 
