INTERNAL ANATOMY OF INSECTS. 53 
the water, that they may maintain their necessary com- 
munication with the atmosphere; and for this purpose a 
single tube would not have been sufficient: therefore 
ProviDENCcE has furnished them with fwo, and both are 
extremely elastic, consisting of annular fibres, so as to 
admit their being stretched to an extraordinary length. 
Reaumur found that these animals could extend their 
tails to near twelve times their own length. The me- 
chanism by which the terminal piece is pushed forth or 
retracted, is very curious, though extremely simple. Two 
large parallel ¢rachee, the direction of which is from the 
head of the grub to its tail, occupy a considerable por- 
tion of its interior: near the origin of the tail, where 
they are very ample, they suddenly grow very small, so 
as to form a pair of very slender tubes, but so long that, 
in order to find room in a very contracted space, they 
form numerous zigzag folds attached to the terminal 
tube; when this issues from the outer tube they conse- 
quently begin to unfold, and when it is entirely disen- 
gaged, they are become quite straight and parallel to each 
other. Reaumur has figured them as being united at 
the base of the inner tube?; most probably, however, 
they do not here stop short, but, as in other instances, 
proceed to the end, and terminate in the two spiracles 
mentioned above : he conjectures that when the animal 
has occasion to push forth its respiratory apparatus, it 
injects into these vessels part of the air contained in the 
body of the trachea, which of course would cause them 
to unfold and push forth the tube®. When this insect 
assumes the pupa, instead of its anal respiratory or- 
* Reaum. iv. ¢. xxx. f 10. ® Toid. 447—. 
