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 trachete, 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 terminal 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. W. Ser. n. 9. 179.) 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 Muscidcs. It does not appear whether he has 

 himself examined this grub, but Reaumur (443) states that it has 

 seven pairs of legs all armed with claws. If this is correct, it is not 

 properly a palmated organ. b Reaum. iv. t. xxx./. 10. 



