hose uses are as yet unknown) through the
skin, which finally became specialized into a stigma, or breathing pore;
and from the tracheal system being closed, we now have the open tracheal
system of land insects.

The next inquiry is as to the origin of the wings. Here the question
arises if wingless forms are exceptional among the winged insects, and
the loss of wings is obviously dependent on the habits (as in the lice),
and environment of the species (as in beetles living on islands, which
are apt to lose the hinder pair of wings), why may not their acquisition
in the first place have been due to external agencies; and, as they are
suddenly discarded, why may they not have suddenly appeared in the first
place? In aquatic larvae there are often external gill-like organs, being
simple sacs permeated by tracheae (as in Agrion, Fig. 129, or the May
flies). These organs are virtually aquatic wings, aiding the insect in
progression as well as in aerating the blood, as in the true wings. They
are very variable in position, some being developed at the extremity of
the abdomen, as in Agrion, or along the sides, as in the May flies, or
filiform and arranged in tufts on the under side of the body, as in
Perla; and the naturalist is not surprised to find them absent or
present in accordance with the varying habits of the animal. For
example, in the larvae of the larger Dragon flies (Libellula, etc.) they
are wanting, while in Agrion and its allies they are present.

Now we conceive that wings formed in much the same way, and with no more
disturbance, so to speak, to the insect's organization, appeared during
a certain critical period in the metamorphosis of some early insect. As
soon as this novel mode of locomotion became established we can easily
see how surrounding circumstances would favor their farther development
until the presence of wings became universal. If space permitted us to
pursue this interesting subject farther, we could show how invariably
correlated in form and structure are the wings of insects to the varied
conditions by which they are surrounded, and which we are forced to
believe stand in the relation of cause to effect. Again, why should the
wings always appear on the thorax and on the upper instead of the under
side? As this is the seat of the centre of gravity, it is evident that
cosmical laws as well as the more immediate laws of biology determine
the position and nature of the wings of an insect.

Co