CHAPTER VI

LEPIDOPTERA- -OR BUTTERFLIES AND MOTHS

Order VI. Lepidoptera.

Wings four; body and wings covered with scales usually variegate in colour, and on the body frequently more or less like hair: nervures moderate in number, at the periphery of one wing not exceeding fifteen, but little irregular; crossnervules not more than four, there being usually only one or two closed cells on each wing, occasionally none. Imago with mouth incapable of biting, usually forming a long coiled proboscis capable of protrusion. Metamorphosis great and abrupt; the wings developed inside the body; the larva with large or moderate head and strong mandibles. Pupa with the appendages usually adpressed and cemented to the body so that it presents a more or less even, horny exterior, occasionally varied by projections that are not the appendages and that may make the form very irregular: in many of the smaller forms the appendages are only imperfectly cemented to the body.

LEPIDOPTERA, or butterflies and moths, are so far as ornament is concerned the highest of the Insect world. In respect of intelligence the Order is inferior to the Hymenoptera, in the mechanical adaptation of the parts of the body it is inferior to Coleoptera, and in perfection of metamorphosis it is second to Diptera. The mouth of Lepidoptera is quite peculiar; the proboscis the part of the apparatus for the prehension of foodis anatomically very different from the proboscis of the other Insects that suck, and finds its nearest analogue in the extreme elongation of the maxillae of certain Coleoptera, e.g. Nemognatha.

The female has no gonapophyses, though in certain exceptional forms of Tineidae, there are modifications of structure connected with the terminal segments, that have as yet been only imperfectly investigated. As a rule, the egg is simply deposited on some living vegetable and fastened thereto. Lepidoptera are the most exclusively vegetarian of all the Orders of Insects; a certain number of their larvae prey on Insects that are themselves filled with vegetable juices (Coccidae,

FIG. 157. Metamorphosis of a Lepidopteron (Rhegmatophila alpina, Notodontidae). (After Poujade, Ann. Soc. ent. France, 1891.) Europe. A, Egg; B, young larva, about to moult; C, adult larva; D, head and first body-segment of adult larva, magnified; E, pupa, x; F, male moth in repose; G, female moth in repose.

Aphidae) and a very small number (Tinea, etc.) eat animal matter. In general the nutriment appears to be drawn exclusively from the fluids of the vegetables, the solid matter passing from the alimentary canal in large quantity in the form of little pellets usually dry, and called frass. Hence the quantity of food ingested is large, and when the individuals unduly increase in number, forest trees over large areas are sometimes completely defoliated by the caterpillars.

Lepidoptera pass a larger portion of their lives in the pupal stage than most other Insects do; frequently during nine months of the year the Lepidopteron may be a pupa. In other Orders of

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Insects it would appear that the tendency of the higher forms is to shorten the pupal period, and when much time has to be passed between the end of the feeding up of the larva and the appearance of the imago, to pass this time as much as possible in the form of a resting-larva, and as little as may be in the form of a pupa; in Lepidoptera the reverse is the case; the resting-larva period being usually reduced to a day or two. Hence we can understand the importance of a hard skin to the pupa. There are, however, numerous Lepidopterous pupae where the skin does not attain the condition of hardness that is secured for the higher forms by the chitinous exudation we have mentioned; and there are also cases where there is a prolonged resting-larva period: for instance Galleria mellonella spins a cocoon in the autumn and remains in it as a resting larva all the winter, becoming a pupa only in the spring. In many of these cases the resting-larva is protected by a cocoon. It is probable that the chief advantage of the perfect chitinous exudation of the Lepidopterous pupa is to prevent the tiny, complex organisation from the effects of undue transpiration. Bataillon has suggested that the relation of the fluid contents of the pupa to air and moisture are of great importance in the physiology of metamorphosis.

The duration of life is very different in various forms of Lepidoptera. It is known that certain species (Ephestia kuehniella, e.g.) may go through at least five generations a year. On the other hand, certain species that feed on wood or roots may take three years to complete their life-history; and it is probable that some of the forms of Hepialidae are even longer lived than this.

Lepidoptera have always been a favourite Order with entomologists, but no good list of the species has ever been made, and it would be a difficult matter to say how many species are at present known, but it can scarcely be less than 50,000. In Britain we have about 2000 species.

The close affinity of the Order with Trichoptera has long been recognised: Réaumur considered the latter to be practically Lepidoptera with aquatic habits, and Speyer pointed out the existence of very numerous points of similarity between the two. Brauer emphasised the existence of mandibles in the nymph of Trichoptera as an important distinction: the pupa

of Micropteryx (Fig. 211) has however been recently shown to be similar to that of Trichoptera, so that unless it should be decided to transfer Micropteryx to Trichoptera, and then define Lepidoptera and Trichoptera as distinguished by the condition of the pupa, it would appear to be very difficult to retain the two groups as distinct.

Structure of Imago. The head of a Lepidopteron is in large part made up of the compound eyes; in addition to these it frequently bears at the top a pair of small, simple eyes so much concealed by the scales as to cause us to wonder if seeing be carried on by them. The larger part of the front of the head is formed by the clypeus, which is separated by a well

FIG. 158.-External structure of a female butterfly, Anosia plexippus. (After Scudder.) a, Base of antenna; b, pronotum; 62, scutum of mesothorax ; c, clypeus; cx, coxa; d, scutellum; d1, scutellum of metathorax; e, post-scutellum (= base of phragma); em, epimeron; ep, episternum; f, scutum of metathorax; m, basal part of proboscis (= maxilla); o, eye; p, labial palp; r, mesosternum; s, prothoracic spiracle; t, tegula; tr, trochanter; 1-9, dorsal plates of abdomen.

marked line from the epicranium, the antennae being inserted on the latter near its point of junction with the former. There is sometimes (Saturnia, Castnia) on each side of the clypeus a deep pocket projecting into the head-cavity. The other parts of the head are but small. The occipital foramen is very large.1

The antennae are always conspicuous, and are very various in form; they are composed of numerous segments, and in the males of many species attain a very complex structure, especially in Bombyces and Psychidae; they doubtless function in such cases as sense-organs for the discovery of the female.

The largest and most important of the mouth-parts are the maxillae and the labial palpi, the other parts being so small as to render their detection difficult. The labrum is a very short,

1 Kellogg, Kansas Quarterly, ii. 1893, p. 51, plate II.

comparatively broad piece, visible on the front edge of the clypeus; its lateral part usually forms a prominence which has often been mistaken for a mandible; Kellogg has applied the term "pilifer" to this part. In the middle of the labrum a small angular or tongue-like projection is seen just over the middle of the base of the proboscis; this little piece is considered by several authorities to be an epipharynx.

MANDIBLES.—Savigny, Westwood, and others considered the parts of the labrum recently designated pilifers by Kellogg to be the rudimentary mandibles, but Walter has shown that this

FIG. 159.-Mouth of Lepidoptera. Tiger-moth, Arctia caja. A, Seen from front; B, from front and below. a, Clypeus; b, labrum; c, epipharynx; d, mandibular area; d', prominence beneath mandibular area; e, one side of haustellum or proboscis ; f, maxillary palp; g, labial palp.

is not the case. The mandibles are usually indistinguishable, though they, or some prominence possibly connected with them, may frequently be detected in the neighbourhood of the pilifers; they are, according to Walter, largest and most perfectly developed in Eriocephala, a genus that was not distinguished by him from Micropteryx and was therefore termed "niedere Micropteryginen," i.e. lower Micropteryges. The opinion entertained by Walter that Micropteryx proper (his "höhere Micropteryginen ") also possesses rudimentary mandibles is considered by Dr. Chapman, no doubt with reason, to be erroneous.3 The mandibles, however, in the vast majority of Lepidoptera can scarcely be said to exist at all in the imago; there being only an obtuse projection without trace of

1 Jena. Zeitschr. Naturw. xviii. 1885, p. 751.

2 The writer is not quite convinced that the supposed mandibles of these Macrolepidoptera are really entitled to be considered as such.

3 Tr. ent. Soc. London, 1893, p. 263.