MOTIONS OF INSECTS. 347 
nervures* are a kind of hollow tube,—above elastic, hor- 
ny, and convex; and flat and nearly membranaceous 
below,—which take their origin in the trunk, and keep 
diminishing gradually, the marginal ones excepted, to 
their termination. The vessels contained in the nervures 
consist of a spiral thread, whence they appear to be 
air-vessels communicating with the tracheze in the trunk. 
—The expansion of the wing at the will of the insect, is a 
problem that can only be solved by supposing that a 
subtile fluid is introduced into these vessels, which seem 
perfectly analogous to those in the wings of birds; and 
that thus an impulse is communicated to every part of 
the organ, sufficient to keep it in proper tension. We 
see by this, that a wing is supported in its flight like a 
sail by its cordage”’. It is remarkable that those insects 
which keep the longest on the wing, the dragon-flies 
(Libellulide) for instance, have their wings most covered 
with nervures. The wings of insects in flying, like 
those of other flying animals, you are to observe, move 
vertically, or up and down. 
In considering the flight of insects, I shall treat of that 
of each order separately, beginning with the Coleoptera or 
beetles. Their subsidiary instruments of flight are their 
wing-cases (Zytra), and in one instance, winglets (Alu). 
The former ‘—which in some are of a hard horny sub- 
stance, and in others are softer and more like leather, 
though they are kept immoveable in flight, are probably, 
by their resistance to the air, not without their use on this 
4 French naturalists use this term (nervure) for the veins of wings, 
leaves, &c., restricting nerve (nerf ) to the ramifications from the brain 
and spinal marrow. We have adopted the term, which we express 
in Latin by xeura;from the Greek vevex. 
> Jurine Hymenopl. 19, ePrate X. Wie. 1, 
