The exact mode in which the proboscis acts is in several respects still obscure, the views of Burmeister and Newport being in some points erroneous. Towards the tip of the proboscis there are some minute but complex structures considered by Fritz Müller to be sense-organs, and by Breitenbach to be mechanical instruments for irritating or lacerating the delicate tissues of blossoms. It is probable that Müller's view will prove to be correct. Nevertheless the proboscis has considerable power of penetration; there being a moth, "Ophideres fullonica" that causes considerable damage to crops of oranges by inserting its trunk through the peel so as to suck the juices.[[170]] The canal formed by each maxilla opens into a cavity inside the front part of the head. This cavity, according to Burgess,[[171]] is a sort of sac connected with five muscles, and by the aid of this apparatus the act of suction is performed: the diverticulum of the alimentary canal, usually called a sucking-stomach, not really possessing the function formerly attributed to it.
The Prothorax is very small, being reduced to a collar, between the head and the alitrunk, of just sufficient size to bear the front pair of legs. Its most remarkable feature is a pair of processes, frequently existing on the upper surface, called "patagia." These in many cases (especially in Noctuidae) are lobes capable of considerable movement, being attached only by a narrow base. In Hepialus, on the contrary, they are not free, but are merely indicated by curved marks on the dorsum. The patagia are styled by many writers "tegulae." They are of some interest in connection with the question of wing-like appendages on the prothorax of Palaeozoic insects, and they have been considered by some writers[[172]] to be the equivalents of true wings. The Mesothorax is very large, especially its upper face, the notum, which is more or less convex, and in the higher forms attains a great extension from before backwards. The notum consists in greater part of a large anterior piece, the meso-scutum, and a smaller part, the meso-scutellum behind. In front of the scutum there is a piece termed prae-scutum by Burgess. It is usually small and concealed by the front part of the scutum; but in Hepialus it is large and horizontal in position. It is of importance as being the chief point of articulation with the prothorax. The scutellum is more or less irregularly rhomboidal in form; its hinder margin usually looks as if it were a lobe or fold placed in front of the base of the abdomen or metathorax, according to whether the latter is concealed or visible. In some of the higher forms this meso-scutellar lobe is prominent, and there may be seen under its projection a piece that has been called the post-scutellum, and is really the base of the great mesophragma, a chitinous piece that descends far down into the interior of the body. In addition to the front pair of wings the mesothorax bears on its upper surface another pair of appendages, the tegulae: in the higher forms they are of large size; they are fastened on the front of the mesothorax, and extend backwards over the joint of the wing with the body, being densely covered with scales so that they are but little conspicuous. These appendages are frequently erroneously called patagia, but have also been called scapulae, pterygodes, paraptera, and shoulder-tufts, or shoulder-lappets. The lower surface of the mesothorax is much concealed by the large and prominent coxae, but the sternum and the two pleural pieces on each side, episternum and epimeron, are easily detected. The area for attachment of the anterior wing on each side is considerable, and appears to be of rather complex structure; its anatomy has been, however, but little studied.
The Metathorax is small in comparison with the preceding segment, to which it is intimately co-adapted, though the two are really connected only by delicate membrane, and can consequently be separated with ease by dissection. The metanotum consists of (1) the scutum, which usually appears externally as an anterior piece on each side; (2) the scutellum, forming a median piece placed behind the scutum, which it tends to separate into two parts by its own extension forwards. In order to understand the structure of the metathorax it is desirable to dissect it off from the larger anterior segment, and it will then be found that its appearance when undissected is deceptive, owing to its being greatly arched, or folded in the antero-posterior direction. A broad, but short phragma descends from the hind margin of the metascutellum into the interior of the body. It should be noted that though the metanotum is forced, as it were, backwards by the great extension of the mesonotum in the middle line of the body, yet at the sides the metanotum creeps forward so as to keep the points of attachment of the hind wings near to those of the front wings. In many forms of Hesperiidae, Sphingidae, Noctuidae, etc. the true structure of the metanotum is further concealed by the back of the mesoscutellum reposing on, and covering it.
Difference of opinion exists as to the thoracic Spiracles; there is one conspicuous enough in the membrane behind the pronotum, and it is thought by some writers that no other exists. Westwood and Scudder, however, speak of a mesothoracic spiracle, and Dr. Chapman considers that one exists. Minot describes[[173]] a structure behind the anterior wing, and thinks it may be an imperfect spiracle, and we have found a similar stigma in Saturnia pavonia. At the back of the thorax there is on each side in some Lepidoptera (Noctuidae, Arctia, etc.), a curious large cavity formed by a projection backwards from the sides of the metasternum, and a corresponding development of the pleura of the first abdominal segment. Minot and others have suggested that this may be an organ of hearing.
The Abdomen differs according to the sex. In the female seven segments are conspicuous dorsally, but only six ventrally, because the first segment is entirely membranous beneath, and is concealed between the second abdominal ventral plate and the posterior coxae. Besides these segments there are at the hind end two others smaller, more or less completely withdrawn into the body, and in certain cases forming an ovipositor. These nine segments are usually considered to constitute the abdomen; but according to Peytoureau,[[174]] a tenth dorsal plate is represented on either side of the anal orifice, though there is no trace of a corresponding ventral plate. In the male the segments, externally conspicuous, are one more than in the female. According to the authority quoted,[[175]] this sex has also truly ten abdominal segments, the ninth segment being withdrawn to a greater or less extent to the inside of the body, and modified to form part of a copulatory apparatus; its dorsal portion bears a process called the "uncus"; the anal orifice opens on the inner face of this process, and below it there is another process—developed to a greater or less extent—called the "scaphium." The ventral portion of the ninth segment bears a lobe, the "saccus" (Peytoureau, l.c.). On each side of the ninth abdominal segment there is a process called the "valve," the internal wall of which bears some hook-like or other processes called "harpes"; it is continued as a membrane surrounding the "oedeagus," or penis, and—bearing more or less distinct prominences—connects with the scaphium. In many forms the parts alluded to, other than the valves, are concealed by the latter, which come together when closed, and may be covered externally with scales like the rest of the abdomen. Peytoureau considers that the uncus is really the dorsal plate of a tenth segment, and that the scaphium is the tenth ventral plate. Thus, according to this view, the ninth segment is extensive and complex, being very highly modified in all its parts: while the tenth segment is greatly reduced. The structure of the male organs is simpler in Lepidoptera, and less varied than it is in the other great Orders of Insects. There are seven pairs of abdominal spiracles on the upper parts of the membranous pleurae.
Fig. 160—Acherontia atropos. The termination of ♂ body, one side removed. IX, Ninth dorsal plate; IX’, ninth ventral; s, lobe, saccus, of ninth ventral plate; X, tenth dorsal plate, or uncus; sc, scaphium, or tenth ventral plate; a, position of anus; b, chitinised band of scaphium; V, valve or clasper; c, hooks, or harpes, of clasper; p, penis (or oedeagus). (After Peytoureau.)
Legs.—The legs are long, slender, covered with scales, and chiefly remarkable from the fact that the tibiae sometimes bear articulated spurs on their middle as well as at the tip. The front tibia usually possesses on its inner aspect a peculiar mobile pad; this seems to be in some cases a combing organ; it also often acts as a cover to peculiar scales. The tarsi are five-jointed, with two small claws and a small apparatus, the functional importance of which is unknown, between the claws.
Wings.—The wings are the most remarkable feature of this Order; it is to them that butterflies owe their beauty, the surfaces of the wings being frequently adorned with colours and patterns of the most charming and effective nature. These effects are due to minute scales that are implanted in the wing-membrane in an overlapping manner, somewhat similar to the arrangement of slates on the roof of a house. The scales are very readily displaced, and have the appearance of a silky dust. We shall describe their structure and allude to their development subsequently. The wings are usually of large size in comparison with the Insect's body: in the genus Morpho, the most gorgeous of the butterflies, they are enormous, though the body is small; so that when deprived of these floats the Insect is insignificant. The great expanse of wing is not correlative with great powers of flight, though it is perhaps indicative of flying with little exertion; for the small-winged Lepidoptera, Sphingidae, etc., have much greater powers of aërial evolution than the large-winged forms. The area of the wing is increased somewhat by the fact that the scales on the outer margin, and on a part or on the whole of the inner margin, project beyond the edges of the membrane that bears them: these projecting marginal scales are called fringes. In many of the very small moths the actual size of the wing-membranes is much reduced, but in such cases the fringes may be very long, so as to form the larger part of the surface, especially of that of the hind wings. Frequently the hind wings are of remarkable shape, being prolonged into processes or tails, some of which are almost as remarkable as those of Nemoptera in the Order Neuroptera.
The wings are very rarely absent in Lepidoptera; this occurs only in the female sex, no male Lepidopterous imago destitute of wings having been discovered. Although but little is known of the physiology of flight of Lepidoptera, yet it is clearly important that the two wings of the same side should be perfectly coadapted or correlated. This is effected largely by the front wing overlapping the hind one to a considerable extent, and by the two contiguous surfaces being pressed, as it were, together. This is the system found in butterflies and in some of the large moths, such as Lasiocampidae and Saturniidae; in these cases the hind wing always has a large shoulder, or area, anterior to its point of insertion. In most moths this shoulder is absent, but in its place there are one or more stiff bristles projecting forwards and outwards, and passing under a little membranous flap, or a tuft of thick scales on the under face of the front wing; the bristle is called the "frenulum," the structure that retains it a "retinaculum." In Castnia (Fig. 162) and in some Sphingidae there is the unusual condition of a highly-developed shoulder (s) coexisting with a perfect frenulum (f) and retinaculum (r). The frenulum and retinaculum usually differ in structure, and the retinaculum in position, in the two sexes of the same moth; the male, which in moths has superior powers of flight, having the better retaining organs. Hampson says "the form of the frenulum is of great use in determining sex, as in the males of all the forms that possess it, it consists of hairs firmly soldered together so as to form a single bristle, whilst in nearly all females it consists of three or more bristles which are shorter than that of the male; in one female Cossid I have found as many as nine. Also in the large majority of moths the retinaculum descends from the costal nervure in the male, while in the female it ascends from the median nervure."[[176]] This sexual difference in a structure for the discharge of a function common to the two sexes is a very remarkable fact. There are a few—very few—moths in which the bases of the hind wings are not well coadapted with the front wings, and do not possess a frenulum, and these species possess a small more or less free lobe at the base of the front wing that droops towards the hind wing, and may thus help to keep up an imperfect connexion between the pair; this lobe has been named a jugum by Professor Comstock. Occasionally there is a jugum on the hind as well as on the front wing. There is usually a very great difference between the front and the hind wings; for whereas in the front wing the anterior portion is doubtless of great importance in the act of flight and is provided with numerous veins, in the hind wing, on the other hand, the corresponding part has not a similar function, being covered by the front wing; hence the hind wing is provided with fewer nervures in the anterior region, the divisions of the subcostal being less numerous than they are in the front wing. In the moths possessing a jugum the two wings differ but little from one another, and it is probable that they function almost as four separate wings instead of as two pairs.