hindwing subserves the same function. In the most primitive
moths a small lobate outgrowth—the jugum (fig. 6, j.)—from the
dorsum of the forewing is present, but it can be of little service in
keeping the two wings together. A jugum may be also present on
the hindwing. The legs, which are generally used for clinging rather
than for walking, have five-segmented feet and are covered with
scales. In some families the front pair are reduced and without
tarsal segments.
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| After Packard, Mem. Nat. Acad. Sci. vol. vii. |
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Fig. 6.—Wing neuration of a Swift Moth (Hepialid). j, Jugum. Nervures numbered as in fig. 5. Note that there are five branches to the radial nervure (No. 3) in both fore- and hindwing, and that the median trunk nervures (No. 4) traverse the discoidal areolet. |
Ten abdominal segments are recognizable in many Lepidoptera, but the terminal segments are reduced or modified to form external organs of reproduction. In the male, according to the interpretation of C. Peytoureau, the lateral plates belonging to the ninth segment form paired claspers beset with harpes, or series of ridges or teeth, while the tergum of the tenth segment forms a dorsal hook—the uncus—and its sternum a ventral process or scaphium. In the female the terminal segments form, in some cases, a protrusible ovipositor, but the typical hexapodan ovipositor with its three pairs of processes is undeveloped in the Lepidoptera.
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Fig. 7.—A, Arrangement of scales in rows on wing of Butterfly. n, Nervure; c, collar-like outgrowths of cuticle. Magnified. B, single scale, and C, an androconium more highly magnified. |
As already mentioned, the characteristic scales on the wings, legs and body of the Lepidoptera are cuticular structures. A complete series of transitional forms can be traced between the most elaborate flattened scales (fig. 7, B) with numerous longitudinal striae and a simple arthropod “hair.” Either a “hair” or a scale owes its origin to a special cell of the ectoderm (hypodermis), a process from which grows through the general cuticle and forms around itself the substance of the cuticular appendage. The scales on the wings are arranged in regular rows (fig. 7, A), and the general cuticle is drawn out into a narrow neck or collar around the base of each scale. The scales can be easily rubbed from the surface of the wing, and the series of collars in which the scales rest are then evident (fig. 7, A, c) on the wing-membrane. On the wings of many male butterflies there are specially modified scales—the androconia (fig. 7, C)—which are formed by glandular cells and diffuse a scented secretion. In some cases, the androconia are mixed among the ordinary scales; in others they are associated into conspicuous “brands” (see fig. 66). The admirable colours of the wings of the Lepidoptera are due partly to pigment in the scales—as in the case of yellows, browns, reds and blacks—partly to “interference” effects from the fine striae on the scales—as with the blues, purples and greens.
A few points of interest in the internal structure of the Lepidoptera deserve mention. The mouth opens into a sub-globular, muscular pharynx which is believed to suck the liquid food through the proboscis, and force it along the slender gullet into a crop-like enlargement or diverticulum of the fore-gut known as a “food-reservoir” or “sucking-stomach.” The true stomach is tubular, and beyond it lies the intestine into which open the three pairs of excretory (Malpighian) tubes. The terminal part of the intestine is of wide diameter, and in some cases gives off a short caecum. The brain and the sub-oesophageal ganglia are closely approximated; there are two or three thoracic and four (rarely five) abdominal ganglia. In the female each ovary has four ovarian tubes, in which the large egg-cells are enclosed in follicles and associated with nutritive cells. There is a special bursa which in the Hepialidae opens with the vagina on the eighth abdominal sternum. In the Micropterygidae, Enocraniidae and the lower Tineides, the duct of the bursa leads into the vagina, which still opens on the eighth sternum. But in most Lepidoptera, the bursa opens by a vestibule on the eighth sternum, distinct from the vagina, whose opening shifts back to the ninth, the duct of the bursa being connected with the vagina by a canal which opens opposite to the spermatheca. In the male, the two testes are usually fused into a single mass, and a pair of tubular accessory glands open into the vasa deferentia or into the ejaculatory duct. In a few families—the Hepialidae and Saturniidae for example—the testes retain the primitive paired arrangement. These details have been worked out by various students, among whom W. H. Jackson and W. Petersen deserve special mention. Summing up the developmental history of the genital ducts, Jackson remarks that there is “an Ephemeridal stage, which ends towards the close of larval life, an Orthopteran stage, indicated during the quiescent period preceding pupation, and a Lepidopteran stage which begins with the commencement of pupal life.”
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Fig. 8 A.—Cossus macmurtrei. (MacMurtrie’s Goat Moth.) N. America. |
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| Fig. 8 B.—Larva of Cossus cossus. (Goat Moth.) Europe. |
Development.—Many observations have been made on the embryology of the Lepidoptera; for some of the more important results of these see Hexapoda. The post-embryonic development of Lepidoptera is more familiar, perhaps, than that of any other group of animals. The egg shows great variation in its outward form, the outer envelope or chorion being in some families globular, in others flattened, in others again erect and sub-conical or cylindrical; while its surface often exhibits a beautifully regular series of ribs and furrows. Throughout the order the larva is of the form known as the caterpillar (fig. 1, a, b, fig. 8 B) characterized by the presence of three pairs of jointed and clawed legs on the thorax and a variable number of pairs of abdominal “prolegs”—sub-cylindrical outgrowths of the abdominal segments, provided with a complete or incomplete circle of hooklets at the extremity.
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Fig. 9.—Head of Goat Moth Caterpillar (Cossus) from behind. Magnified. (From Miall and Denny after Lyonnet.) | ||||||||
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There are ten abdominal segments—the ninth often small and concealed; prolegs are usually present on the third, fourth, fifth, sixth and tenth of these segments. The head of the caterpillar (fig. 9) is large with firmly chitinized cuticle; it carries usually twelve simple eyes or ocelli, a pair of short feelers (fig. 9 At) and a pair of strong mandibles (fig. 9, Mn), for the caterpillar feeds by biting leaves or other plant-tissues. The first maxillae, so highly developed in the imago, are in the larva small and inconspicuous appendages, each bearing two short jointed processes,—the galea and the palp (fig. 9, Mx). The second maxillae form a plate-like labium on whose surface projects the spinneret which is usually regarded as a modified hypopharynx (fig. 9, Lm). The silk-glands whose ducts open on this spinneret are paired convoluted tubes lying alongside the elongate cylindrical stomach. In the common “silkworm” these glands are five times as long as the body of the caterpillar. They are regarded as modified salivary glands, though the correspondence has been doubted by some students. The body of the caterpillar is usually cylindrical and wormlike, with the
