d-channels
terminate in blood-extremities, and a capillary net-work of
air-vessels is spread over these. Now, in the vertebrated creature,
the chest is merely the grand air-receptacle into which the blood is
sent to be aerated; while in the insect, the chest contains but its
own proportional share of the great air-system. In the latter case,
therefore, there is a great deal of available space, which would have
been, under other circumstances, filled with the respiratory
apparatus, but is now left free to be otherwise employed. The thoracic
cavity of the insect serves as a stowage for the bulky and powerful
muscles that are required to give energy to the legs and wings. The
portion of the body that is almost exclusively respiratory in other
animals, becomes almost as exclusively motor in insects. It holds in
its interior the chief portions of the cords by which the moving
levers and membranes are worked, and its outer surface is adorned by
those levers and membranes themselves. Both the legs and wings of the
insect are attached to the thoracic segment of its body.

The extraordinary powers of flight which insects possess are due to
the conjoined influences of the two conditions that have been
named--the lightness of their air-filled bodies, and the strength of
their chest-packed muscles. Where light air is circulated instead of
heavy blood, great vascularity serves only to make existence more
ethereal. Plethora probably takes the insect nearer to the skies,
instead of dragging it towards the dust. The hawk-moth, with its burly
body, may often be seen hovering gracefully, on quivering wings, over
some favourite flower, as if it were hung there on cords, while it
rifles it of its store of accumulated sweets by means of its long
unfolded tongue. The common house-fly makes 600 strokes every second
in its ordinary flight, and gets through five feet of space by means
of them; but when alarmed, it can increase the velocity of its
wing-strokes some five or six fold, and move through thirty-five feet
in the second. Kirby believed, that if the house-fly were made equal
to the horse in size, and had its muscular power increased in the same
proportion, it would be able to traverse the globe with the rapidity
of lightning. The dragon-fly often remains on the wing in pursuit of
its prey for hours at a stretch, and yet will sometimes baffle the
swallow by its speed, although that bird is calculated to be able to
move at the rate of a