INTERNAL ANATOMY OF INSECTS. 63 
vicinity of each spiracle, may be considered as consisting 
in general of three packets; — dorsal ones, which are 
distributed to the back and sides of the animal; visceral 
ones, which enter the cavity of the body, and are lost 
amongst the viscera and the caul; and ventral ones, 
which dipping from the trachea overrun the lower part 
of the sides and belly a . 
The trachea; and bronchia consist of three tunics b : the 
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 d . 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 c . 
The Bronchia? are cylindrical or slightly conical, insen- 
sibly diminishing in size as they leave the trunk, in which 
they originate. In larvae, after losing their spiral fibre, 
they appear to terminate in membrane, but in perfect 
insects they pass into vesicles f . 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- 
3 Lyonet Anat. 101. 
h Sprengel {ubi. sup?: 1G.) says that he never found more than two ; 
but as Lyonet affirms that he has very often separated them (102), 
his accuracy cannot be questioned. c Lyonet Anat. 103. 
J Ibid. Cuv. Anat. Comp. iv. 438. This author says that the in- 
termediate tunic is the spiral thread (437). c Lyonet 102. 
f Ibid. 104. Sprengel Commentar. 17. 
