Wing-nervures.—The nervures or ribs of the wings are of great importance in Lepidoptera, as at present they furnish the chief characters for classification and for the discussions of phylogeny that are so numerous in entomological literature. On looking at wings that have been deprived of their scales it will be noticed (Fig. 161) that the ribs are much more numerous at the outer margins than they are near the points of attachment of the wings, and that there is usually but one cell (or area completely enclosed by ribs). This latter point is one of the chief peculiarities of the Lepidopterous wing; in Insect-wings generally the number of cells in proportion to the area of the wings and to the number of nervures is greater than it is in Lepidoptera, for in the latter there are few or no cross-nervures. Hence there is sometimes no closed cell at all on the wing (Fig. 161, II. B). The maximum number of closed cells is six; this is found in some species of Micropteryx, while in Hepialus there may be three or four; but the rule is that there is only one cell in the Lepidopterous wing. When the number of cells is increased this is not necessarily due to an increase in the cross-nervures; and in fact it is generally due to irregular forking or to the sinuous form of the longitudinal nervures themselves (see wing of Castnia, Fig. 162, A.). Some authorities consider that all transverse or cross-veins in Lepidoptera are merely portions of longitudinal veins having diverted courses. When a portion of a nervure beyond the basal or primary portion serves as a common piece to two forked parts external to it, it is called a stalk (Fig. 162, A, e). There are cases in which the furcation takes place in the opposite direction, so that a nervure is double at the base of the wing (Fig. 161, I, A, 1a, and B, 1b). This important condition has not yet been adequately discussed.
Fig. 161.—Wing-nervuration of Lepidoptera. I, Diagram of moths' wings (after Hampson); II, of a butterfly's wings (Morpho menelaus ♂, after Staudinger and Schatz). A, front, B, hind wing. I.—c, costal; sc, subcostal; m, median; 1a, 1b, 1c, internal nervures; f, frenulum; 2, 3, 4, branches of median nervure; 5, lower radial; 6, upper radial; 7-11, divisions of the subcostal; 12, termination of costal; c, cell; d, discocellular nervure. II.—C, costal; SC, subcostal; M, median; SM and SN, submedian nervures; 1A, inner-margin nervure; UR, lower radial; OR, upper radial; SC1 to SC5, divisions of subcostal; M1 to M3, divisions of median nervure; C, cell; DC, discocellulars.
Turning to the mode of designation of the nervures,[[177]] we may commence by remarking that no system satisfactory from a practical as well as from a theoretical point of view has yet been devised. The diagrams given in figure 161 will enable us to explain the methods actually in vogue; I. representing the system, dating from the time of Herrich-Schaeffer, chiefly used by British naturalists, and II. that adopted by Staudinger and Schatz in their recent great work on the Butterflies of the world. The three anterior nervures in both front and hind wings correspond fairly well, and are called, looking at them where they commence at the base of the wing, "costal," "subcostal," and "median" nervures. The nervures near the inner margin of the wing (that is the lower part in our figures) differ much in the front and hind wings, consisting either of two or of three separate portions not joined even at the base. British entomologists call these "branches or divisions of the internal nervure": the Germans call the more anterior of them the "submedian," and the more internal the "inner-margin nervure"; they are also frequently called anal nervures. The cross-nervure that closes the cell is called discocellular; when apparently composed of two or three parts joined so as to form angles, the parts are called, according to position, upper, lower, and middle discocellulars. One or more short spurs may exist on the front part of the basal portion of the hind wing; these are called praecostal. The branches or terminal divisions of the nervures should be called nervules; they are usually mentioned by the numbers shewn in the diagram (Fig. 161, I.). In addition to this, it is only necessary to remember that number 2 is always assigned to the posterior division of the median nervure, the nervules below this being all called 1, and distinguished by the addition of a, b, c when requisite. This course is necessary, because if it were not adopted the corresponding nervules on the front and hind wings would bear different numbers.
The use of this system of numbers for the nervules is becoming general, and it answers fairly well for practical purposes. On the other hand, extreme discrepancy exists as to the nomenclature of the nervures and nervules, and there are almost as many systems as there are authorities.
The normal number of nervules is, on the front wing, 11 + 1 or 2 inner marginal, and on the hind wing 7 + 2 or 3 inner marginal. In the aberrant moths of the genus Castnia the nervuration is unusually complex and irregular (Fig. 162), and an analogous condition occurs in our common Goat-moth (Cossus ligniperda). In Hepialus and Micropteryx (the jugate moths of Comstock) the hind wings are less dissimilar in nervuration from the front wings than they are in other Lepidoptera.[[178]]
Fig. 162—Wing-nervuration of Castnia. Undersides of, A, front, B, hind wings. 1a, 1b, 1c, 1d, Inner marginal nervures; 2, lower branch of median; 8, subcostal of hind wing; 12, subcostal of front wing; e, "stalk" of 8 and 9; f, frenulum; r, retinaculum; s, shoulder; g, articulation of wing.
Internal Anatomy.[[179]]—The alimentary canal extends as a long, slender oesophagus through the length of the thorax, dilating when it reaches the abdomen to form a tubular stomach; before this it is somewhat enlarged to form an indistinct crop, and gives off a large diverticulum usually called a sucking stomach. According to Burgess, this structure does not possess the function ascribed to it by this name, and he terms it a food-reservoir. The Malpighian tubes are six in number, three on each side, and each set of three unite to form a common tube opening into the posterior extremity of the stomach; behind them the alimentary canal continues in the form of a slender, tortuous intestine, expanding at the extremity of the body to form a rectum. The dorsal or circulatory vessel commences near the posterior extremity of the body, but in the front part of the abdomen is deflexed to pass under the great phragma into the thorax, where it rises abruptly to the dorsal wall, but is again abruptly deflexed, forming a loop, and is then prolonged above the oesophagus into the head: at the summit of the thoracic loop there may be a dilatation called the aortal chamber. The supra- and infra-oesophageal ganglia are consolidated into a mass pierced by the oesophagus: there is a minute frontal ganglion; the ventral chain consists of three much approximated thoracic ganglia and four abdominal ganglia separated from the thoracic by a long interval.
