52 INTERNAL ANATOMY OF INSECTS. 
hairs, which are furnished with some repellent material 
which prevents their becoming wet a : it is this repellent 
quality that probably causes a dimple or depression of 
the surface, which if you look narrowly you will discover 
round the mouth of the tube b . 
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 c . By these tubular horns, which 
Reaumur compares to asses' ears d , they respire, and are 
suspended at the surface. 
Other respiratory tubes or horns are more complex. 
The rat-tailed grub of a fly [Helophilus 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 ac- 
complish the same end, this respiratory organ is of a 
construction totally 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 c . The extremity, which is 
very slender, and through which the air finds admission 
by a pair of spiracles, terminates in five diverging hairs 
or rays, which probably maintain it in equilibrio at its 
station at the surface f . As these larvae seek their food 
a Plate XIX. Fig. 1). b. 
" Compare Swamm, Bibl. Nat. i. 154. t. xxx'i.f. 5. Reaum. iv. 
601—. /. xliii. De Gecr vi. 317—. t. xvii./. 2—8. 
c Swamm. Ibid. t. xxxi./. 7, 8. a Reaum. iv. G07- 
e Plate XIX. Fig. 12. a. f Reaum. iv. /. xxxii./. 2. e. 
