Fig. 3 shows the arrangement by which the upper and lower wings of a moth
are united in flight. The bristle (_frenulum_) arising from the base of the
hind wing is held in place by the catch (_retinaculum_) on the costal
nervure of the fore wing. It will be noticed that the bristle of the male
is longer and firmer than that of the female. The latter, moreover, is
usually made up of two or more strands; the catch, too, in the female is on
the median instead of the costal nervure. These structures are found on the
under side of the wings of most moths, but they are absent in all
butterflies. The Emperor, Kentish Glory, and Eggars may be mentioned as
examples of moths lacking the connecting bristle, but all these have
feather-like antennæ, which terminate in a point. The Burnets have the
antennæ gradually thickened towards the tip (A. Fig. 2), and in this
respect are somewhat butterfly-like, but these moths have a frenulum.
The hind wings of the Swifts, and a few others grouped with them, have
twelve veins, and these moths have a _jugum_, or yoke (see Fig. 4). This is
a flap-like projection from the inner margin, near the base, of the fore
wing; it may serve to connect the wings when the insect flies, but it does
not seem to be capable of giving much help in that way. {5}
[Illustration: FIG. 5.