As mentioned in "Butterflies of the British Isles," there is, speaking generally, no clear line of division between moths and butterflies, and, as Dr. Sharp, in "Insects," puts the case, "the only definition that can be given of Heterocera [moths] is the practical one that all Lepidoptera that are not butterflies are Heterocera." Now, it happens that all the butterflies occurring in these islands have the tips of the horns (antennæ) clubbed; and, although there is much variety in the structure of the horns of our moths, none of them have the tips knobbed.
Like the butterflies, moths pass through the stages of egg, caterpillar, and chrysalis before they attain the perfect state (imago), and the duration of the several stages is just as variable. The majority assume the moth condition but once in the year, but some species have two, or even three, generations in the twelve months, whilst others occupy twenty-four months in completing the life cycle. In one or two species the chrysalis stage may last four, five, or even six years.
Diversity of form and structure is considerable in the early stages as well as in the perfect insects, and this is shown in the selection of life-history details figured on the black and white plates in this volume.
Except that it is generally less prominent, the head, with the various parts thereof, is pretty much the same as in the butterflies. In a few families, however, the "tongue" (proboscis) is only rudimentary or even entirely absent; while in others it is very long. The Convolvulus Hawk-moth has the proboscis of such length that it is able to reach the deep-seated nectary of such tubular flowers as those of Nicotiana affinis. In the illustration the "tongue" of the moth and the sweet-scented tobacco blossom are shown on exactly the same scale.
Fig. 2 represents some forms of antennæ found in moths. A, thickened and spreading out towards the tip (dilate); B, simple, thread-like (filiform) structure, without teeth, hairs, or bristles; C, fringed with fine hairs (ciliate); D, fringed with fine hairs, and with longer bristles at the joints (setose ciliate); E, the fringe in tufts (fasciculate); F, toothed, with fine hairs on the teeth (dentate ciliate); G, toothed with hairs in tufts from the teeth (dentate fasciculate); H, I, with double rows of hair scales (bipectinate); in I the projections are continued to the tip, and are themselves fringed with fine hairs, giving the antennæ a very feather-like appearance—the term "plumose" is sometimes used to describe this form of antennæ; in H the pectinations do not reach the apical fourth, which is simple. J, the lamellate, that is, the undersides of the rings or joints are made up of minute plates.
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 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.
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.
