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 c ; 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 ojf considerable 

 tension, after which they return to their usual length 6 . 

 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- 



a Lyonet Anat. 101. 



b Sprengel (ubi. supr. 16.) says that be 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. 



d Ibid. Cuv. Anat. Comp. iv. 438. This author says that the in- 

 termediate tunic is the spiral thread (437). e Lyonet 102. 



f Ibid. 104. Sprengel Commentar. 17. 



