ANATOMY AND PHYSIOLOGY. 143 
regularly asin the tubes. When the vesicles are torn, 
as often happens by an unlucky scratch in dissecting, 
the fibres sprmg back and carry the tear, transversely 
to their direction, across the whole sac, leaving, as 
the result of our labours, nothing more than a mem- 
brane spread out, like a drop of wax, on the surface of 
the water. Such a membrane, however, displays well 
the structure of the air vesicles. As the trachez ap- 
proach the vesicles their spiral fibre becomes broader 
and less defined, and is arranged with a little less scru- 
pulous accuracy. In the walls of the air vesicle the 
threads are still more distant and less regular, and 
_ they have a woolly, even a wavy outline. On the 
coats of the air vesicles, and of the large air tubes in 
their immediate neighbourhood, many little specks 
are occasionally to be seen, the nature of which is a 
matter of uncertainty. I must ask pardon for ven- 
turing to express an opimion on a point which has 
been left unsettled by competent authorities, but 
these little specks appear to me to be oil-globules. 
_ The irregular mode of their arrangement, their occur- 
rence in a part distorted from its original form, and 
above all, the fact that they may be removed and 
examined separately from the membrane, strongly 
confirms the view which their appearance suggests, 
that they are oil globules. 
Insects breathe, as we have seen, and many of them 
audibly express their feelings, by means of the air; 
much as the higher animals do. But their respira- 
tory system is not concentrated in one place, it is 
made up of many smaller similar systems, repeated 
over and over again, with a longitudinal as well as 
bi-lateral symmetry which is only one stage removed 
