52 INTERNAL ANATOMY OF INSECTS. 
dimple or depression of the surface, which if you look 
narrowly you will discover round the mouth of the tube?. 
When the gnat undergoes its first change and assumes 
the pupa, instead of a single respiratory appendage it is 
furnished with a pair, each in shape resembling a cor- 
nucopia, and, what is remarkable, placed near the oppo- 
site extremity of the body, for they proceed from the up- 
per side of the trunk®. By these tubular horns, which 
Reaumur compares to asses’ ears‘, they respire, and are 
suspended at the surface. 
Other respiratory tubes or horns are more complex. 
The rat-tailed grub of a fly (Elophilus pendulus) like the 
gnat breathes by a tube: but as if the Creator willed 
to show those whose delight it is to investigate his works, 
by how many varying processes he can accomplish the 
same end, this respiratory organ is of a construction to- 
tally different from that we have been considering. It 
is not fixed to the side of the tail, but is a continuation 
of the tail itself, and is composed of two tubes, the inner 
one, like the tube of a telescope, being retractile within 
the other’. ‘The extremity, which is very slender, and 
through which the air finds admission by a pair of spi- 
racles, terminates in five diverging hairs or rays, which 
probably maintain it zn equdlibrio at its station at the 
surface®. As these larvee seek their food amongst the 
mud at the bottom of shallow pools, in which they are 
constantly employed, they require an apparatus capable 
of being lengthened or shortened, to suit the depth of 
* Compare Swamm. Bibl. Nat. i. 154. ¢. xxx1. f. 5. -Reaum. iv. 
601—. ¢. xl. De Geer vi. 317—. ¢. xvii. f. 2—8. 
> Swamm. Ibid. ft. xxxi. f. 7, 8. ¢ Reaum. iv. 607. 
d Pirate XEXS Ric. 12a: * Reaum. iv. ¢. xxxii. fi 2. e. 
