The Jugum, or Yoke.
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.
Fig. 5.
Wings and Body of a Moth.
h. Head; th. Thorax; col. Collar; lap. Lappet; ab. Abdomen. b.l. basal line; b.s. basal streak; i.l. inner line; c.s. claviform stigma; o.s. orbicular stigma; c. central shade; r.s. reniform stigma; o.l. outer line; a.p. apical patch; s.m. submarginal line.
Fig. 6.