The sub-family Lithosiinae is of great extent; our native "Footmen" give a very good idea of it; the moths are generally of light structure, with long, narrow front wings; a simple system of yellow and black colour is of frequent occurrence. Many of this group feed in the larval state on lichens. Hampson includes in this group the Nyctemeridae—light-bodied diurnal moths, almost exclusively of black and white colours, of Geometrid form, frequently treated as a distinct family.
The sub-family Nolinae is a small group of rather insignificant Insects, in appearance like Pyralids or Geometrids; four or five species are native in Britain. Packard maintains the family Nolidae as distinct.[[301]]
The sub-family Nycteolinae consists of a few small moths the position of which has always been uncertain; Nycteola (better known as Sarrothripus), Halias, and Earias are all British genera that have been placed amongst Tortrices, to which they bear a considerable resemblance. Sarrothripus is at present placed by Hampson in Noctuidae, by others in Lithosiidae, by Meyrick in Arctiidae. The sub-family forms the family Cymbidae of Kirby;[[302]] it includes at present only about 70 species, all belonging to the Eastern hemisphere. Two types of larvae are known in it: one bare, living exposed on leaves; the other, Earias, hairy, living among rolled-up leaves. Halias prasinana is known from the testimony of numerous auditors to produce a sound when on the wing, but the modus operandi has not been satisfactorily ascertained. Sound-production seems to be of more frequent occurrence in Arctiidae than it is in any other family of Lepidoptera; Dionychopus niveus produces a sound by, it is believed, friction of the wings. In the case of the genera Setina and Chelonia the process is said to be peculiar to the male sex: Laboulbène believes it to proceed from drum-like vesicles situate one on each side of the base of the metathorax.[[303]]
Fam. 35. Agaristidae.—An interesting assemblage of moths, many of them diurnal and of vivid colours, others crepuscular. There is considerable variety of appearance in the family, although it is but a small one, and many of its members remind one of other and widely separated families of Lepidoptera. The style and colour of the Japanese Eusemia villicoides are remarkably like our Arctia villica. In some forms the antennae are somewhat thickened towards the tip and hooked, like those of the Skipper butterflies. The family consists at present of about 250 species, but we doubt its being a sufficiently natural one. It is very widely distributed, with the exception that it is quite absent from Europe and the neighbourhood of the Mediterranean Sea. In North America it is well represented. The larvae, so far as known, are not very remarkable; they have some lateral tufts of hair, as well as longer hairs scattered over the body.
The male of the Indian Aegocera tripartita has been noticed to produce a clicking sound when flying, and Sir G. Hampson has shown[[304]] that there is a peculiar structure on the anterior wing; he considers that this is rubbed against some spines on the front feet, and that the sound is produced by the friction. Though this structure is wanting in the acknowledged congeners of A. tripartita, yet it occurs in a very similar form in the genus Hecatesia, already noticed under Castniidae.
Fam. 36. Geometridae (Carpets, Pugs, etc.)—This very extensive family consists of fragile moths, only a small number being moderately stout forms; they have a large wing-area; the antennae are frequently highly developed in the males, but on this point there is much diversity. Either the frenulum or the proboscis is absent in a few cases. The caterpillars are elongate and slender, with only one pair of abdominal feet—placed on the ninth segment—in addition to the anal pair, or claspers. They progress by moving these two pairs of feet up to the thoracic legs, so that the body is thrown into a large loop, and they are hence called Loopers or Geometers. The family is universally distributed, and occurs even in remote islands and high latitudes; in Britain we have about 270 species. The family was formerly considered to be closely connected with Noctuidae, but at present the opinion that it has more intimate relations with the families we have previously considered is prevalent. Packard considers it near to Lithosiidae, while Meyrick merely places the six families, of which he treats it as composed, in his series Notodontina. Hampson adopts Meyrick's six families as subfamilies, but gives them different names, being in this respect more conservative than Meyrick, whose recent revision of the European forms resulted in drastic changes in nomenclature.[[305]] This classification is based almost exclusively on wing-nervuration. The number of larval legs and the consequent mode of walking is one of the most constant characters of the group; the few exceptions that have been detected are therefore of interest. Anisopteryx aescularia has a pair of undeveloped feet on the eighth segment, and, according to Meyrick, its allies "sometimes show rudiments of the other two pairs." The larva of Himera pennaria is said to have in early life a pair of imperfect feet on the eighth segment, which disappear as the larva approaches maturity.
Fig. 202—Larva of Amphidasis betularia, reposing on a rose-twig. × 1. Cambridge.
The position of the abdominal feet and claspers throws the holding power of the larva to the posterior part of the body, instead of to the middle, as in other caterpillars. This, combined with the elongate form, causes these larvae when reposing to assume attitudes more or less different from those of other larvae; holding on by the claspers, some of these Insects allow all the anterior parts of the body to project in a twig-like manner. The front parts are not, however, really free in such cases, but are supported by a thread of silk extending from the mouth to some point near-by. Another plan adopted is to prop the front part of the body against a twig placed at right angles to the supporting leaf, so that the caterpillar is in a diagonal line between the two (Fig. 202). Other Geometers assume peculiar coiled or spiral attitudes during a whole or a portion of their lives; some doing this on a supporting object—leaf or twig—while others hang down (Ephyra pendularia). Certain of the larvae of Geometridae vary in colour, from shades of brown to green; there is much diversity in this variation. In some species it is simple variation; in others it is dimorphism, i.e. the larvae are either brown or green. In other cases the larvae are at first variable, subsequently dimorphic. In Amphidasis betularia it would appear that when the larva is hatched the dimorphism is potential, and that the future colour, whether green or brown, is settled by some determining condition during the first period of larval life and cannot be subsequently modified.[[306]] According to Poulton, the dark tint is due in A. betularia to colouring matter in the skin or immediately below it, and the green tint to a layer of fat between the hypodermis and the superficial muscles; this layer being always green, but more brightly green in the larvae that are of this colour externally. Much discussion has occurred about these larval attitudes and colours, and it seems probable that Professor Poulton has overrated the value of protection from birds, mammals and entomologists; the chief destroying agents being other than these, and not liable to be thus deceived, even if the vertebrates are. In some cases such resemblance as undoubtedly exists is not made the best use of. The larva shown in figure 202 bore a wonderful resemblance, when examined, to the rose-twigs it lived on, but the effect of this as a concealing agent was entirely destroyed by the attitude; for this, being on different lines to those of the plant, attracted the eye at once. This larva, and we may add numerous other larvae, could have been perfectly concealed by adopting a different attitude, but never did so; the position represented being constantly maintained except while feeding.
In some species of this family the adult females are without wings, or have them so small that they can be of no use for flight. This curious condition occurs in various and widely-separated groups of the Geometridae; and it would be naturally supposed to have a great effect on the economy of the species exhibiting it, but this is not the case. Some of the flightless females affect the highest trees and, it is believed, ascend to their very summits to oviposit. It has been suggested that they are carried up by the winged males, but this is probably only an exceptional occurrence; while, as they are known to be capable of ascending with rapidity by means of crawling and running, it may be taken for granted that this is the usual method with them. Some of these wingless females have been found in numbers on gas-lamps, and are believed to have been attracted by the light, as is the case with very many of the winged forms.[[307]] Neither is the geographical distribution limited by this inferior condition of the most important of the organs of locomotion, for Cheimatobia brumata (the Winter-moth) one of the species with flightless female, is a common and widely distributed Insect in Europe and North America.