a flattened bladder,
extended from the general skin of the body. The sides of this bladder
are pressed closely together, and would be in absolute contact but for
a series of branching rigid tubes that are spread out in the
intervening cavity. These tubes are air-vessels; their interiors are
lined with elastic, spirally-rolled threads, that serve to keep the
channels constantly open; and through these open channels the vital
atmosphere rushes with every movement of the membraneous organ. The
wing of the May-fly flapping in the air is a respiratory organ, of as
much importance to the wellbeing of the creature in its way, as the
gill-plate of its grub prototype is when vibrating under the water.
But the wing of the insect is not the only respiratory organ: its
entire body is one vast respiratory system, of which the wings are
offsets. The spirally-lined air-vessels run everywhere, and branch out
everywhere. The insect, in fact, circulates air instead of blood. As
the prick of the finest needle draws blood from the flesh of the
backboned creature, it draws air from the flesh of the insect. Who
will longer wonder, then, that the insect is so light? It is aerial in
its inner nature. Its arterial system is filled with the ethereal
atmosphere, as the more stolid creature's is with heavy blood.

If the reader has ever closely watched a large fly or bee, he will
have noticed that it has none of the respiratory movements that are so
familiar to him in the bodies of quadrupeds and birds. There is none
of that heaving of the chest, and out-and-in movement of the sides,
which constitute the visible phenomena of breathing. In the insect's
economy, no air enters by the usual inlet of the mouth. It all goes in
by means of small air-mouths placed along the sides of the body, and
exclusively appropriated to its reception. Squeezing the throat will
not choke an insect. In order to do this effectually, the sides of the
body, where the air-mouths are, must be smeared with oil.

In the vertebrated animals, the blood is driven through branching
tubes to receptacles of air placed within the chest; the air-channels
terminate in blood extremities, and the blood-vessels cover these as a
net-work. The mechanical act of respiration merely serves to change
the air contained within the air-receptacles. In the insects, this
entire process is reversed; the air is carried by branching tubes to
receptacles of blood scattered throughout the body; the bloo