The Butterfly Book. W. J. Holland

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СКАЧАТЬ veins in both the fore and hind wings of butterflies may be divided into simple and compound veins. In the fore wing the simple veins are the costal, the radial, and the submedian; in the hind wing, the costal, the subcostal, the upper and lower radial, the submedian, and the internal are simple. The costal vein in the hind wing is, however, generally provided near the base with a short ascending branch which is known as the precostal vein. In addition to these simple veins there are in the fore wing two branching veins, one immediately following the costal, known as the subcostal, and the other preceding the submedian, known as the median vein. The branches of these compound veins are known as nervules. The median vein always has three nervules. The nervules of the subcostal veins branch upwardly and outwardly toward the costal margin and the apex of the fore wing. There are always from four to five subcostal nervules. In the hind wing the subcostal is simple. The median vein in the hind wing has three nervules as in the fore wing. Between the subcostal and the median veins, toward the base in both wings, is inclosed the cell, which may be wholly or partially open at its outer extremity, or closed. The veinlets which close the cell at its outward extremity are known as the discocellular veins, of which there are normally three. From the point of union of these discocellular veins go forth the radial veins known respectively as the upper and lower radials, though the upper radial in many genera is emitted from the lower margin of the subcostal.

      An understanding of these terms is, however, more readily derived from a study of the figure in which the names of these parts are indicated (Fig. 40).

       Fig. 40.—Wing of Anosia plexippus, showing the names of the veins and nervules: C, C, costal veins; SC, subcostal vein; SC_1, etc., subcostal nervules; UR, upper radial; LR, lower radial; M, median veins; M_1, M_2, M_3, median nervules; SM, submedian veins; I, internal veins; PC, precostal nervule; UDC, MDC, LDC, upper, middle, and lower discocellulars.

      Butterflies generally hold their wings erect when they are at rest, with their two upper surfaces in proximity, the under surfaces alone displaying their colors to the eye. Only in a few genera of the larger butterflies, and these tropical species, with which this book does not deal, is there an exception to this rule, save in the case of the Hesperiidæ, or "skippers," in which very frequently, while the anterior wings are folded together, the posterior wings lie in a horizontal position.

Internal Organs.—Thus far we have considered only the external organs of the butterfly. The internal organs have been made the subject of close study and research by many writers, and a volume might be prepared upon this subject. It will, however, suffice for us to call the attention of the student to the principal facts.

       Fig. 41.—Longitudinal section through the larva of Anosia plexippus, ♂, to show the internal anatomy (the Roman numerals indicate the thoracic, the Arabic the abdominal segments): b, brain; sog, subœsophageal ganglion; nc, nervous cord; œ, œsophagus; st, stomach; i, intestine; c, colon; sv, spinning-vessel of one side; s, spinneret; mv, Malpighian vessel, of which only the portions lying on the stomach are shown, and not the multitudinous convolutions on the intestine; t, testis; dv, dorsal vessel; the salivary glands are not shown. (Magnified 3 diameters.) (Burgess.)

The muscular system finds its principal development in the thorax, which bears the organs of locomotion. The digestive system consists of the proboscis, which has already been described, the gullet, or œsophagus, and the stomach, over which is a large, bladder-like vessel called the food-reservoir, a sort of crop preceding the true stomach, which is a cylindrical tube; the intestine is a slender tube, varying in shape in different genera, divided into the small intestine, the colon, and the rectum. Butterflies breathe through spiracles, little oval openings on the sides of the segments of the body, branching from which inwardly are the tracheæ, or bronchial tubes. The heart, which is located in the same relative position as the spine in vertebrate animals, is a tubular structure. The nervous system lies on the lower or ventral side of the body, its position being exactly the reverse of that which is found in the higher animals. It consists of nervous cords and ganglia, or nerve-knots, in the different segments. Those in the head are more largely developed than elsewhere, forming a rudimentary brain, the larger portion of which consists of two enormous optic nerves. The student who is desirous of informing himself more thoroughly and accurately as to the internal anatomy of these insects may consult with profit some of the treatises which are mentioned in the list of works dealing with the subject which is given elsewhere in this book.

       Fig. 42.—Longitudinal section through the imago of Anosia plexippus, ♁, to show the internal anatomy: t, tongue; p, palpus; a, antenna; pr, prothorax; mes, mesothorax; met, metathorax; ps, pharyngeal sac; b, brain; sog, subœsophageal ganglion; 1–2, blended first and second ganglia of the larva; 3–4, blended third and fourth ganglia of the larva; l, l, l, the three legs; ac, aortal chamber; dv, dorsal vessel; œ, œsophagus; res, reservoir for air or food; st, stomach; mv, Malpighian vessels; i, intestine; c, colon; r, rectum; cp, copulatory pouch; o, oviduct; ag, accessory glands; sp, spermatheca; ov, ovaries (not fully developed); nc, nervous cord. (Magnified 3 diameters.) (Burgess.)

      Polymorphism and Dimorphism.—Species of butterflies often show great differences in the different broods which appear. The brood which emerges in the springtime from the chrysalis, which has passed the winter under the snows, may differ very strikingly from the insect which appears in the second or summer brood; and the insects of the third or fall brood may differ again from either the spring or the summer brood. The careful student notes these differences. Such species are called polymorphic, that is, appearing under different forms. Some species reveal a singular difference between the sexes, and there may be two forms of the same sex in the same species. This is most common in the case of the female butterfly, and where there are two forms of the female or the male such a species is said to have dimorphic females or males. This phenomenon is revealed in the case of the well-known Turnus Butterfly; in the colder regions of the continent the females are yellow banded with black, like the males, but in more southern portions of the continent black females are quite common, and these dark females were once thought, before the truth was known, to constitute a separate species.

      Albinism and Melanism.—Albinos, white or light-colored forms, are quite common among butterflies, principally among the females. On the other hand, melanism, or a tendency to the production of dark or even black forms, reveals itself. Melanism is rather more common in the case of the male sex than in the female sex. The collector and student will always endeavor, if possible, to preserve these curious aberrations, as they are called. We do not yet entirely understand what are the causes which are at work to produce these changes in the color, and all such aberrant specimens have interest for the scientific man.

      Monstrosities.—Curious malformations, producing monstrosities, sometimes occur among insects, as in other animals, and such malformed specimens should likewise be preserved when found. One form of malformation which is not altogether uncommon consists in an apparent confusion of sexes in specimens, the wings of a male insect being attached to the body of a female, or half of an insect being male and half female.

      Mimicry.—One of the most singular and interesting facts in the animal kingdom is what has been styled mimicry. Certain colors and forms are possessed by animals which adapt them to their surroundings in such wise that they are in a greater or less degree secured from observation and attack. Or they possess forms and colors which cause them to approximate in appearance other creatures, which for some reason are feared or disliked by animals which might prey upon them, and in consequence of this resemblance СКАЧАТЬ