INTERNAL ANATOMY OF INSECTS. 53 
amongst the mud at the bottom of shallow pools, in 
which they are constantly employed, they require an ap- 
paratus capable of being lengthened or shortened, to suit 
the depth of the water, that they may maintain their 
necessary communication with the atmosphere ; and for 
this purpose a single tube would not have been sufficient : 
therefore Providence has furnished them with two, 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 ex- 
tend their tails to near twelve times their own length. 
The mechanism by which the terminal piece is pushed 
forth or retracted, is very curious, though extremely 
simple. Two large parallel trachcce, the direction of 
which is from the head a of the grub to its tail, occupy a 
considerable portion 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 con- 
tracted space, they form numerous zigzag folds attach- 
ed to the teiTninal tube ; when this issues from the outer 
tube they consequently begin to unfold, and when it is 
intirely disengaged, they are become quite straight and 
parallel to each other. Reaumur has figured them as 
being united at the base of the inner tube b ; most pro- 
bably, however, they do not here stop short, but, as in 
a Mr. W. S. MacLeay (Philos. Mag. N. Ser. n. 9. 170.) asserts that 
what Reaumur (iv. 487. t. xxx./. 6. //) calls the first pair of legs of 
this grub, are the usual palmated stigmata which occur on the hume- 
rus of the larvae of Muscidce. It does not appear whether he has 
himself examined this grub, but Reaumur (443) states that it has 
seven pairs of legs all armed ivith claivs. If this is correct, it is not 
properly a palmated organ. b Reaum. iv. t. xxx./. 10. 
