framework usually of twelve hollow tubes or veins that are so
connected with the respiratory organs as to be pneumatic. These
tubes support double membranes covered above and below with down.
At the bases of the wings lie their nerves. The fore-wings each
have a heavy rib running from the base and gradually decreasing
to the tip. This is called the costa. Its purpose is to bear
the brunt of air-pressure in flight. On account of being compelled
to fly so much more than the females, the back wings of the males
of many species have developed a secondary rib that fits under
and supports the front, also causing both to work together with
the same impulse to flight. A stiff bunch of bristles serves the
same purpose in most females, while some have a lobe extending
from the fore-wing. As long as the costa remains unbroken to
preserve balance, a moth that has become entangled in bushes
or suffered rough treatment from birds can fly with badly
damaged wing surfaces.

In some species, notably the Attacine group and all non-feeding,
night-flying moths, the legs are short, closely covered with long
down of the most delicate colours of the moth, and sometimes
decorated with different shades. Luna has beautiful lavender legs,
Imperialis yellow, and Regalis red-brown. The day-flying, feeding
group have longer, slenderer legs, covered with shorter down, and
carry more elaborate markings. This provision is to enable them
to cling firmly to flower or twig while feeding, to help them to
lift the body higher, and walk dextrously in searching for food.
It is also noticeable that these moths have, for their size,
comparatively much longer, slenderer wings than the non-feeders,
and they can turn them back and fold them together in the fly
position, thus enabling them to force their way into nectar-bearing