Fig. 163—Internal anatomy of Lepidoptera. Section of the body of a female butterfly, Anosia plexippus. (After Scudder.) The portion to the left of the vertical line more magnified. I. II. III. thoracic segments; 1-9, abdominal segments; a, antenna; a, anus; ac, aortal chamber; ag1, etc., abdominal ganglia; agl1, agl2, accessory glands; ao, aorta; br, brain; c, colon; cp, copulatory pouch; cx1, cx2, cx3, coxae; fr, food-reservoir; g1, suboesophageal ganglion; h, dorsal vessel; i, intestine; lm, area filled by wing muscles; l.or, ovary, or egg-tubes of left side; mv, Malpighian tube (the two others of the right side cut away, except small portions); mx, maxilla; o, oviduct; oo, its orifice; oe, oesophagus; ov.c, end of left ovary; p, labial palp; ph, pharynx; r.ov, terminal parts of right ovarian tubes, turned to one side, after the tubes have been cut away; sd, salivary duct; sgl, salivary gland; sp, spermatheca; st, stomach; tg, thoracic ganglia; v, copulatory orifice.
The male sexual organs consist of the two testes placed in a common capsule, from which proceed a pair of contiguous vasa deferentia (dilated soon after their origin to form the vesiculae seminales); into each vas there opens a long, tubular gland; the two vasa subsequently unite to form a long, coiled, ejaculatory duct. It is in the structure of the female sexual organs that the most remarkable of the anatomical characters of Lepidoptera is found, there being two external sexual orifices. The imago has, in the great majority of cases, four egg-tubes in each ovary; the pair of oviducts proceeding from them unite to form a single unpaired (azygos) oviduct which terminates by an orifice quite at the posterior extremity of the body. There is a sac, the bursa copulatrix or copulatory pouch, which is prolonged in a tubular manner, to open externally on the eighth ventral plate: a tube, the seminal duct, connects the bursa with the oviduct, and on this tube there may be a dilatation—the spermatheca. Besides these structures two sets of accessory glands open into the oviduct, an unpaired gland, and a pair of glands. The development of these structures has been described by Hatchett Jackson,[[180]] and exhibits some very interesting features. The exact functions of the bursa copulatrix and of the other structures are by no means clear. According to Riley,[[181]] the spermatheca in Pronuba contains some curious radiate bodies, and Godman and Salvin describe something of the same sort as existing in butterflies. Several variations in the details of the structure of these remarkably complex passages have been described, and the various ducts are sometimes rendered more complex by diverticula attached to them. Some noteworthy diversities in the main anatomical features exist. According to Cholodkovsky, there is but one sexual aperture—the posterior one—in Nematois metallicus; while, according to Brandt, the number of egg-tubes in a few cases exceeds the normal—four—being in Sesia scoliaeformis fourteen. In Nematois metallicus there is individual variation, the number of tubes varying from twelve to twenty.
The egg has been more extensively studied in Lepidoptera than in any other Order of Insects. It displays great variety: we meet with elongate forms (Fig. 164) and flat forms like buttons, while in Limacodes (Fig. 83, Vol. V.) the egg is a transparent scale of somewhat inconstant outline. Some are coloured and mottled somewhat after the fashion of birds'-eggs; this is the case with some eggs of Lasiocampidae and Liparidae; in some the sculpture of the egg-shell is of the most elaborate character (Figs. 77, 78, Vol. V.). The egg-shell or chorion is, according to Korschelt[[182]] and others, a cuticular product of the epithelium of the egg-chambers of the ovaries. The number of eggs deposited by an individual differs greatly in different species, and has been ascertained to be variable within certain limits in the same species. Speyer thought about 250 to be the average number of eggs deposited by an individual. The number in the case of Aporia crataegi is believed to be from 60 to 100, and in some Hepialus to be several thousands. The mode of deposition also differs greatly; where the eggs are very numerous they seem to be discharged almost at random in suitable spots; but moths such as Clisiocampa neustria fasten their eggs round the stems of the food-plant in a very perfect and artistic manner. Butterflies seem as a rule to prefer to oviposit by placing an egg here and there rather than risk many in one situation; but to this there are many conspicuous exceptions especially in the cases where the larvae live gregariously, as in the Vanessae. Some moths cover the eggs with fur from their own body, which, in the case of certain of the Eggers (Lasiocampidae), seems to have a special supply for the purpose. The period that intervenes between deposition and hatching of the eggs varies from a few days to many months. There seems to be, as a rule, comparatively little power of extending the period of latency beyond a single season; though certain facts have been recorded that would lead us to believe that in Australia eggs may last over the proper time during a drought, and be hatched as soon as rain falls.
Fig. 164—Egg of Orange-tip butterfly, Euchloe cardamines, magnified. a, The egg of natural size on a stalk.
Larva.—The young condition or larva of the Lepidopterous Insect is commonly called a caterpillar. It is a somewhat worm-like creature—in old English it was sometimes called palmer-worm—and is composed of a head and thirteen divisions or segments of the body; the first three of the latter are called thoracic, the other ten, abdominal segments; in most caterpillars the terminal two or three abdominal segments are more or less run together, and the ninth may be very small, so that the true number is indistinct. The first three segments bear each, on either side, a short limb, ending in a curved spine; the next two (or three or more) segments are destitute of legs, but on some of the following divisions another kind of leg of a more fleshy character appears, while the body is terminated by a pair of these thick legs of somewhat different form. The front legs are usually called the true legs, the others prolegs, but this latter designation is a most unfortunate one, the term "pro" being in entomology used to signify anterior; it is therefore better to call the three anterior pairs thoracic legs, and the others abdominal feet, distinguishing the hind pair of these latter as claspers. There is, too, an unfortunate discrepancy amongst entomologists in their manner of counting the body-segments, some count the head as the first segment, while others apply this term to the first thoracic segment. The latter is the more correct course, for, as the head is not a single segment it should not be called such in a terminology that affects to be morphologically exact, not simply descriptive. The thoracic legs are transversely jointed (Fig. 165, B), but this is not the case with the abdominal feet, which are usually armed beneath with a circle, or with rows, of little hooks. The thoracic legs are, independent of their form, of a different nature from the abdominal, for these latter disappear subsequently, while the former give rise to the legs of the imago. The number of thoracic legs is always six, except in a few cases where there are none at all; the abdominal feet are much more variable, and exhibit so many distinctions that we cannot here attempt to deal with them. M. Goossens has given a concise and interesting account of this subject,[[183]] and Speyer[[184]] a summary of the variety in number and position.
The anatomy of the larva is simple in comparison with that of the perfect Insect; its main features will be appreciated from Fig. 165, from which it will be seen that the stomach is enormous, and the silk-vessels are also very extensive.
Fig. 165.—A, Section of male caterpillar of Anosia plexippus—muscular and tracheal systems and fat-body not shown: I, pro-, II, meso-, III, meta-thorax; 1-10, abdominal segments; b, supra-oesophageal ganglion; c, rectum; d.v, dorsal vessel; g3-g9, ganglia of ventral chain; h, head; i, intestine; m.v, Malpighian tube; n.c, nerve-cord of ventral chain; oe, oesophagus; s, spinneret; s.o.g, infra-oesophageal ganglion; st, stomach; s.v, silk-vessel; t, testis. B, One of the jointed prothoracic legs. C, An abdominal foot with its hooks. (After Scudder and Burgess, magnification about 2⁄1.)
There are three sets of glands opening by canals on the head, viz. the salivary glands proper, which open into the cavity of the mouth, one close to the base of each mandible; the silk-glands, which terminate by a common canal, continued externally as the spinneret; and the glands of Filippi situate in the head itself, and opening into the ducts of the silk-glands, near their union into a common duct. It should be recollected that Fig. 165 does not indicate all the details of the anatomy; the muscular system, for instance, being entirely omitted, though there are an enormous number of muscles; these however are not very complex, they being mostly repetitions in the successive segments.[[185]] The mouth-parts are very different from those of the perfect Insect, inasmuch as the maxillae and labial palpi, which are the most remarkable structures of the imago, are small, and are differently constructed in the caterpillar, while the mandibles, which are the largest organs of the caterpillar, disappear in the adult. The little organ by which the caterpillar exudes its silk is called a spinneret; according to Packard it is a "homologue of the hypopharynx." It is a more or less prominent point on the middle of the labium (Fig. 166, g) and sometimes forms a conspicuous spine projecting downwards. The eyes are extremely imperfect organs, consisting merely of six, in some cases fewer, transparent, somewhat prominent, little spaces placed on each side of the lower part of the head; they are called "ocelli," by Landois "ocelli compositi." Under each of these external facets there are placed percipient structures, apparently very imperfect functionally, the caterpillar's sight being of the poorest character.[[186]] The spiracles of the caterpillar are nine on each side, placed one on the first thoracic segment and one on each of the first eight abdominal segments; there are no true stigmata on the second and third thoracic segments, though traces of their rudiments or vestiges are sometimes visible.