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Fig. 32.—Interior view of head of milkweed butterfly: cl, clypeus; cor, cornea of the eye; œ, œsophagus, or gullet; fm, frontal muscle; dm, dorsal muscles; lm, lateral muscles; pm, muscles moving the palpus (Burgess).

Fig. 33.—Labial palpus of Colias, magnified 10 diameters.

It will be observed from what has been said that the head in these creatures is to a large extent the seat of the organs of sense and alimentation. What the function of the antennæ may be is somewhat doubtful, the opinion of scientific men being divided. The latest researches would indicate that these organs, which have been regarded as the organs of smell and sometimes as the organs of hearing, have probably a compound function, possibly enabling the creature to hear, certainly to smell, but also, perhaps, being the seat of impressions which are not strictly like any which we receive through our senses.

Fig. 34.—Colias philodice: a, antenna; p, extremity of palpus; pl, prothoracic leg; ml, mesothoracic leg; hl, metathoracic or hind leg; t, proboscis.

Thorax.—The thorax is more or less oval in form, being somewhat flattened upon its upper surface. It is composed of three parts, or segments, closely united, which can only be distinguished from one another by a careful dissection. The anterior segment is known as the prothorax, the middle segment as the mesothorax, and the after segment as the metathorax. The legs are attached in pairs to these three subdivisions of the thorax, the anterior pair being therefore sometimes spoken of as the prothoracic legs, the second pair as the mesothoracic legs, and the latter pair as the metathoracic legs (Fig. 34). On either side of the mesothorax are attached the anterior pair of wings, over which, at their insertion into the body, are the tegulæ, or lappets; on either side of the metathorax are the posterior pair of wings. It will be seen from what has been said that the thorax bears the organs of locomotion. The under side of the thorax is frequently spoken of by writers, in describing butterflies, as the pectus, or breast.

The Abdomen.—The abdomen is formed normally of nine segments, and in most butterflies is shorter than the hind wings. On the last segment there are various appendages, which are mainly sexual in their nature.

Fig. 35.—Leg of butterfly: c, coxa; tr, trochanter; f, femur; t, tibia; tar, tarsi.

The Legs.—Butterflies have six legs, arranged in three pairs, as we have already seen. Each leg consists of five parts, the first of which, nearest the body, is called the coxa, with which articulates a ring-like piece known as the trochanter. To this is attached the femur, and united with the femur, forming an angle with it, is the tibia. To the tibia is attached the tarsus, or foot, the last segment of which bears the claws, which are often very minute and blunt in the butterflies, though in moths they are sometimes strongly hooked. The tibiæ are often armed with spines. In some groups of butterflies the anterior pair of legs is aborted, or dwarfed, either in one or both sexes, a fact which is useful in determining the location of species in their systematic order.

Fig. 36.—Magnified representation of arrangement of the scales on the wing of a butterfly.

Fig. 37.—Androconia from wings of male butterflies: a, Neonympha eurytus; b, Argynnis aphrodite; c, Pieris oleracea.

The Wings.—The wings of butterflies consist of a framework of horny tubes which are in reality double, the inner tube being filled with air, the outer tube with blood, which circulates most freely during the time that the insect is undergoing the process of development after emergence from the chrysalis, as has been already described. After emergence the circulation of the blood in the outer portion of the tubes is largely, if not altogether, suspended. These horny tubes support a broad membrane, which is clothed in most species upon both sides with flattened scales which are attached to the membrane in such a way that they overlap one another like the shingles on a roof. These scales are very beautiful objects when examined under a microscope, and there is considerable diversity in their form as well as in their colors. The males of many species have peculiarly shaped scales arranged in tufts and folds, which are called androconia, and are useful in microscopically determining species (Fig. 37). The portion of the wings which is nearest to the thorax at the point where they are attached to the body is called the base; the middle third of the wing is known as the median or discal area, the outer third as the limbal area. The anterior margin of the wings is called the costal margin; the outer edge is known as the external margin, the inner edge as the inner margin. The shape of the wings varies very much. The tip of the front wing is called the apex, and this may be rounded, acute, falcate (somewhat sickle-shaped), or square. The angle formed by the outer margin of the front wing with the inner margin is commonly known as the outer angle. The corresponding angle on the hind wing is known as the anal angle, and the point which corresponds to the tip or apex of the front wing is known as the external angle (Fig. 38). A knowledge of these terms is necessary in order to understand the technical descriptions which are given by authors.

Plate III

If a wing is examined with the naked eye, or even with a lens, a clear conception of the structure of the veins can rarely be formed. Therefore it is generally necessary to remove from the wings the scales which cover them, or else bleach them. The scales may be removed mechanically by rubbing them off. They may be made transparent by the use of chemical agents. In the case of specimens which are so valuable as to forbid a resort to these methods, a clear knowledge of the structure of the veins may be formed by simply moistening them with pure benzine or chloroform, which enables the structure of the veins to be seen for a few moments. The evaporation of these fluids is rapid, and they produce no ill effect upon the color and texture of the wings. In the case of common species, or in the case of such as are abundantly represented in the possession of the collector, and the practical destruction of one or two of which is a matter of no moment, it is easy to use the first method. The wing should be placed between two sheets of fine writing-paper which have been moistened by the breath at the points where the wing is laid, and then by lightly rubbing the finger-nail or a piece of ivory, bone, or other hard substance over the upper piece of paper, a good many of the scales may be removed. This process may be repeated until almost all of them have been taken off. This method is efficient in the case of many of the small species when they are still fresh; in the case of the larger species the scales may be removed by means of a camel's-hair pencil such as is used by painters. The chemical method of bleaching wings is simple and inexpensive. For this purpose the wing should be dipped in alcohol and then placed in a vessel containing a bleaching solution of some sort. The best agent is a solution of chloride of lime. After the color has been removed from the wing by the action of the chloride it should be washed in a weak solution of hydrochloric acid. It may then be cleansed in pure water and mounted upon a piece of glass, as microscopic slides are mounted, and thus preserved. When thus bleached the wing is capable of being minutely studied, and all points of its anatomy are brought clearly into view.

Fig. 38.—Outline of wing, giving names of parts.

Fig. 39.—Arrangement of scales on wing of butterfly.

The 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 enjoy partial or entire immunity. Some butterflies, for instance, resemble dried leaves, and as they are seated upon the twigs of trees they wholly elude the eye. This illustrates the first form of mimicry. Other butterflies so closely approximate in form and color species which birds and other insects will not attack, because of the disagreeable juices which their bodies contain, that they are shunned by their natural enemies, in spite of the fact that they belong to groups of insects which are ordinarily greedily devoured by birds and other animals. A good illustration of this fact is found in the case of the Disippus Butterfly, which belongs to a group which is not specially protected, but is often the prey of insect-eating creatures. This butterfly has assumed almost the exact color and markings of the milkweed butterfly, Anosia plexippus, which is distasteful to birds, and hence enjoys peculiar freedom from the attacks of enemies. Because this adaptation of one form to another evidently serves the purpose of defense this phenomenon has been called "protective mimicry." The reader who is curious to know more about the subject will do well to consult the writings of Mr. Alfred Russel Wallace and Mr. Darwin, who have written at length upon mimicry among butterflies. There is here a field of most interesting inquiry for the student.

The Distribution of Butterflies.—Butterflies are found everywhere that plant life suited to the nourishment of the caterpillars is found. There are some species which are arctic and are found in the brief summer of the cold North and upon the lofty summits of high mountains which have an arctic climate. Most of them are, however, children of the sun, and chiefly abound in the temperate and tropical regions of the earth. While the number of species which are found in the tropics vastly exceeds the number of species found in the temperate zone, it is apparently true that the number of specimens of certain species is far more numerous in temperate regions than in the tropics. Very rarely in tropical countries are great assemblages of butterflies to be seen, such as may be found in the summer months in the United States, swarming around damp places, or hovering over the fields of blooming clover or weeds. In the whole vast region extending from the Rio Grande of Texas to the arctic circle it is doubtful whether more than seven hundred species of butterflies are found. On the continent of Europe there are only about four hundred and fifty species. The number of species of butterflies and the number of species of birds in the United States are very nearly the same.