The wings of a moth are practically identical with those of a butterfly
(see diagram "Butterflies of the British Isles," p. 12). Normally the fore
wings have twelve ribs or veins, and the hind {4} wings eight, but in some
genera a rib, or perhaps two, may be absent from the fore or the hind
wings; or an extra rib, sometimes two, may be found on the hind wings.
These modifications, and others, of the general plan of neuration have been
employed as a basis upon which to found genera, or to group them together
in classification.
[Illustration: FIG. 4.