Fig. 158—External structure of a female butterfly, Anosia plexippus. (After Scudder.) a, Base of antenna; b, pronotum; b2, 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.
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, 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.
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.
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 is not the case.[[165]] The mandibles are usually indistinguishable, though they, or some prominence possibly connected with them,[[166]] 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.[[167]] 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 articulation—on each side of the labrum; and even this projection is usually absent. Meinert recognised these projections as mandibles in Smerinthus populi, and Kellogg in Protoparce carolina, another large Sphinx moth. They appear to be unusually well developed in that group. In Castnia they are even more definite than they are in Sphingidae.
The Maxillae are chiefly devoted to the formation of the proboscis. Their basal portions are anatomically very indefinite, though they exist very intimately connected with the labium. Each usually bears a small tubercle or a segmented process, the representative of the maxillary palpus. The proboscis itself consists of the terminal, or outer, parts of the two maxillae, which parts are closely and beautifully coadapted to form the spirally coiled organ, that is sometimes, though incorrectly, called the tongue. The exact morphology of the Lepidopterous proboscis has not been established. The condition existing in the curious family Prodoxidae (see p. [432]), where a proboscis coexists with another structure called a maxillary tentacle, suggests a correspondence between the latter and the galea of a typical maxilla; and between the proboscis and the lacinia or inner lobe of a maxilla: but J. B. Smith is of opinion that the tentacle in question is a prolongation of the stipes. The condition of the parts in this anomalous family (Prodoxidae) has not, however, been thoroughly investigated, and Packard takes a different view of the proboscis; he considers that "it is the two galeae which become elongated, united and highly specialised to form the so-called tongue or glossa of all Lepidoptera above the Eriocephalidae."[[168]] The proboscis in some cases becomes very remarkable, and in certain Sphingidae is said to attain, when unrolled, a length of ten inches. In some cases the maxillary lobes do not form a proboscis, but exist as delicate structures, pendulous from the mouth, without coadaptation (Zeuzera aesculi, the Wood-leopard moth). In other forms they are absent altogether (Cossus, e.g.), and in Hepialus we have failed to detect any evidence of the existence of the maxillae. On the other hand, in Micropteryx the maxillae are much more like those of a mandibulate Insect; and various other Microlepidoptera approach more or less a similar condition. In the genus last mentioned the maxillary palpi are largely developed, flexible and slender. According to Walter various forms of palpus intermediate between that of Micropteryx and the condition of rudimentary tubercle may be found amongst the Microlepidoptera.[[169]]
Labium.—The labial palpi are usually largely developed, though but little flexible; they form conspicuous processes densely covered with scales or hairs, and curve forwards or upwards, rarely downwards, from the under side of the head, somewhat in the fashion of tusks. The other parts of the labium are frequently represented merely by a membranous structure, united with the maxillae and obstructing the cavity of the pharynx. Where the proboscis is absent it is difficult to find any orifice leading to the alimentary canal, such opening as may exist being concealed by the overhanging clypeus and labium. In some forms, Saturnia, e.g., there appears to be no buccal orifice whatever. In Hepialus the labium is in a very unusual condition; it projects externally in the position usually occupied by the labial palpi, these organs being themselves extremely short. It is very difficult to form an opinion as to the structure of the labium and other mouth-parts when the maxillae are not developed, as in these cases the parts are of a delicate membranous nature, and shrivel after death. This is the explanation of the fact that in descriptive works we find vague terms in use such as "mouth aborted" or "tongue absent."
The mouth of the Lepidopterous imago is a paradoxical structure; it differs very greatly from that of the larva, the changes during metamorphosis being extreme. We should thus be led to infer that it is of great importance to the creatures; but, on the other hand, the various structures that make up the mouth, as we have remarked, are frequently absent or reduced to insignificant proportions; and even in forms where the apparatus is highly developed the individuals seem to be able to accomplish oviposition without taking food, or after taking only very minute quantities. It is therefore difficult to understand why so great a change should occur during the metamorphosis of the Insects of this Order. It has been ascertained that in some forms where the mouth is atrophied the stomach is in a correlative condition; but we are not aware that any investigations have been made as to whether this correspondence is general or exceptional.