70 INTERNAL ANATOMY OF INSECTS. 
insects, amongst other functions, give more fixity and 
force to the muscles for flight*. 
Many physiologists have seen an analogy between the 
spiral vessels of plants and the trachee of insects ; and 
some of great name, as Comparetti, Decandolle, and 
Kieser, have thought that in some instances they termi- 
nated in the oscula or cortical pores: but Sprengel con- 
tends that they are not accurate in this opinion’. In 
fact, the principal analogy seems to be in the spzral 
structure of both these vessels. 
Having considered the different organs of respiration 
both external and internal, I shall make a few further 
observations upon this function. We know little more 
respecting the mode in which insects respzre, except that 
they breathe out the air by the same kind of organs by 
which they receive it,—namely, the spiracles, or their re- 
presentatives. This has been satisfactorily proved by 
Bonnet, who showed that the experiments by which 
Reaumur thought it established that insects inspire by 
their spiracles, but exspire through the mouth, anus, or 
pores of the skin, are founded on an erroneous assump- 
tion. This physiologist, having observed on the surface 
of submerged insects numerous bubbles of air, concluded 
that they had passed through the above orifices®: but 
Bonnet found by various experiments carefully conduct- 
ed, that this appearance was caused by air which ad- 
hered to the skin and its hairs, and that when the access 
* Sur le Vol des Ins. c. ii. 336. note |. 
> Sprengel Comment. 13—. These oscula or pores in the straw 
of Triticum hybernum, as figured by Mr. Bauer’s admirable pencil 
(Sir J. Banks On the Blight, §c. t, ii. J. 3.) exactly resemble the a 
racles of insects, © Reaum. i. 136. 
