The butterfly guide

by W. J. Holland

THE ANATOMY OF BUTTERFLIES

The body of a butterfly consists of the head, the thorax, and the abdomen (see [Plate A], Fig. c).

The head carries two relatively large eyes, one on either side. The eyes of insects are compound, and if examined under a microscope are seen to have a multitude of minute facets, which serve to gather the light from all directions, so that butterflies can look forward and backward, upward and downward, as well as outward, all at one time. Between the eyes on the upper part of the head arise the antennæ, of which we have already spoken. The precise function of these organs in insects has been the subject of much discussion. Supposed by some to be ears, by others to be the seat of the sense of smell, by others to combine within themselves these two senses, and by still others to represent a sense which is not possessed by vertebrate animals, their use in the life of insects is not yet clearly understood. The weight of evidence seems to be in favor of the view that they are organs of smell, and it is now quite firmly established by experiment that the organs of hearing in insects are represented by certain pores and openings on their legs. In front between the eyes and below the antennæ are two little organs, each composed of three joints, which are known as the labial palpi (see [Plate A], Fig. e). Between these, coiled up like a watch-spring, is the proboscis, with which the butterfly sucks up the nectar from flowers or drinks water from moist places (see [Plate A], Figs. c and d). We have not the space in this little manual to go more deeply into the anatomy of these organs, but enough has been said to enable the beginner to recognize the various parts. The student realizes that the head in general supports the principal organs of sense and the proboscis, or mouth.

The thorax carries the organs of locomotion, which consist of four wings and six feet. The thorax is made up of three segments, or rings, the foremost of which is called the prothorax, the next the mesothorax, and the hindmost the metathorax. The subdivisions of the thorax are not easily distinguishable by examining the body of a butterfly even under a microscope, because the bodies of butterflies are generally heavily clothed with hairs and scales. In order to clearly make out the subdivisions, which we are considering, it is necessary to take a specimen and denude it of its scales and hairs, and even dissect it under a glass. The correctness of the foregoing statements then becomes apparent.

The legs of butterflies are arranged in three pairs, the foremost of which are known as prothoracic, being attached to the prothorax; the second pair are called mesothoracic, springing, as they do, from the middle segment of the thorax; and the last are styled metathoracic legs, rising from the hindmost segment of the chest (see [Plate A], Fig. c). It should be noted here that in the great family of the Nymphalidæ, or “Brush-footed Butterflies,” in both sexes the anterior, or prothoracic pair of legs, are not fully developed, being aborted (see [Plate A], Fig. d) and therefore do not serve for walking; and that in the families of the Erycinidæ, or “Metal-marks,” and the Lycænidæ, or “Blues and Coppers,” the females have six legs adapted to walking, while the males possess only four ambulatory legs, the front pair being in the latter sex aborted in these families, as in the Nymphalidæ. The legs of butterflies, like those of all other insects, consist of five parts (see [Plate A], Fig. f) the first of which, nearest the body, is called the coxa, with which articulates a small ring-like piece, known as the trochanter. To the trochanter is attached the femur, and united with the latter, forming an angle with it, is the tibia. The last division of the leg is the tarsus, or foot, composed of a series of joints, to the last of which is attached a pair of claws, which in butterflies are generally rather minute, though in other orders of insects these claws are sometimes long and powerful, this being especially true of some beetles. The prothoracic legs of the Nymphalidæ and of the males of the Erycinidæ and Lycænidæ have lost the use of the tarsus, only retaining it in feeble form, and the tibia has undergone modification. In many of the Nymphalidæ the tibia is densely clothed with long hairs, giving this part of the leg the appearance of a brush, whence the name “Brush-footed Butterflies” (see [Plate A], Fig. d). The tibiæ are often armed with more or less strongly developed spines.

The most striking parts of butterflies are their wings, which in proportion to the size of their bodies are usually very large, and which are remarkable for the beauty of the colors and the markings which they display both on the upper and on the under side.

The wings consist of a framework of horny tubes which are in reality double, the inner tube being filled with air, the outer tube with blood. The blood of insects is not, like that of vertebrates, red in color. It is almost colorless, or at most slightly stained with yellow. The circulation of the blood in the outer wall of the wing-tubes takes place most freely during the brief period in which the insect is expanding its wings after emergence from the chrysalis, concerning which we shall have more to say elsewhere. After the wings of the butterfly have become fully expanded, the circulation of the blood in the wings ceases almost entirely. The horny tubes, which compose the framework of the wings of butterflies, support between them a delicate membrane, to which upon both the upper and lower sides are attached the scales. The two fore wings are more or less triangular in outline; the hind wings are also subtriangular, but are generally more or less rounded on the outer margin, and in numerous forms are provided with tails or tail-like prolongations.

Inasmuch as in describing butterflies authors generally devote a good deal of attention to the markings of the wings, it is important for the student to become acquainted with the terms employed in designating the different parts of the wings (see [Plate B], Fig. 10). That part of the wing which is nearest to the thorax, is called the base; the middle third of the wing is known as the median or discal area; the outer third as the external or limbal area. The anterior margin of the wing is called the costal margin; the outer edge is styled the external margin; the inner edge is known as the inner margin. The tip of the front wing is called the apex, which may be rounded, acute, falcate (sickle-shaped), or square (see [Plate B], Figs. 1-4). 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 of the hind wing, which corresponds with the tip or apex of the fore wing, is designated as the external angle. The margins of wings may have different styles of outline, and are spoken of as entire, crenulate, scalloped, waved, lobed, or tailed (see [Plate B], Figs. 5-8).

A knowledge of the veins which form the framework of the wings is important, because authors have frequently established genera upon the basis of the wing structure. It is desirable on this account to understand the nomenclature which has been applied to the veins. This nomenclature is somewhat variant, different writers having employed different terms to designate the same vein. In what follows the writer has adopted the designations which are most current, and which are generally accepted by authors. The best understanding of this matter is to be derived from the attentive study of the diagrams given on [Plate B], Figs. 9 and 10. 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 radials, the submedian, and the internal; in the hind wing they are the costal, the subcostal, the radials, the submedian, and the internal. 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 the 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. 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, variously arranged. In the hind wing the subcostal is simple. The median vein in the hind wing has three nervules, as in the fore wing. In both wings between the subcostal and the median veins toward the base is enclosed the cell, which may be either closed or wholly or partially open at its outer extremity. The veinlets which close the cell are known as the discocellular veins, of which there are normally three. From the point of union of these discocellular veins go forth the radials, known respectively as upper and lower, though the upper radial in many genera is emitted from the lower margin of the subcostal vein.

EXPLANATION OF PLATE B

Fig. 1.	Angulated or acuminate apex of fore wing ( Grapta).
Fig. 2.	Falcate apex of fore wing ( Pyrrhanæa).
Fig. 3.	Square apex of fore wing ( Smyrna).
Fig. 4.	Rounded apex of fore wing ( Euptoieta).
Fig. 5.	Hind wing rounded at outer angle, twice-tailed ( Thecla).
Fig. 6.	Hind wing crenulate, tailed, lobed at anal angle ( Papilio).
Fig. 7.	Hind wing with outer margin entire ( Parnassius).
Fig. 8.	Hind wing with outer margin waved ( Argynnis).
Fig. 9.	Neuration of wings of Anosia plexippus.
	Veins:	C, costal; SC, subcostal; M, median; SM, submedian; I, internal; PC, precostal; UDC, upper discocellular; MDC, middle discocellular; LDC, lower discocellular; UR, upper radial; LR, lower radial.
	Nervules:	SC 1, 2, 3, 4, 5, first to fifth subcostal nervules; M 1, 2, 3, first to third median nervules.
Fig. 10.	Wing of Papilio turnus, showing the names given to the different parts of the wings of butterflies.

Plate B

Butterflies generally hold their wings erect when they are at rest, with their upper surfaces facing each other, and only the under surfaces displaying their colors to the eye. In the genus Ageronia the insect prefers, like some moths, to settle upon the bark of trees, with the wings spread flat, and the head pointing downward. Many of the Hesperiidæ, or “Skippers,” have the habit when they are at rest of holding the fore wings folded together, while the hind wings are expanded horizontally. Many of the butterflies known as “Hair-streaks,” belonging to the genus Thecla and its allies, have the curious habit, when at rest upon the end of a twig or leaf, of moving their folded wings backward and forward, first on one side and then on the other, thus partially displaying with each movement the splendid blue surfaces of the upper side of the wings.

The abdomen of butterflies consists normally of nine segments (see [Plate A], Fig. c). In most butterflies except the Ithomiids, the end of the abdomen does not extend beyond the anal angle of the hind wings. In the moths, on the other hand, there are multitudes of genera in which the extremity of the abdomen extends far beyond the hind margin of the posterior wings. This is particularly true of the hawk-moths. A minute examination of the abdomen of a butterfly reveals in each segment except the last a little valve-like orifice on either side. These openings are known as spiracles (see [Plate A], Fig. c). Through these the insect breathes. Insects do not breathe through their mouths, like vertebrates, and their lungs, or what correspond to the lungs of the higher animals, are not located in the thorax or chest, but in the abdomen. The last segment of the abdomen carries at its extremity the external organs of generation. The male may be distinguished from the female by the so-called prehensores, or “claspers,” two flattish, scale-like appendages which adhere one on either side to the last segment of the body. These organs are quite peculiar in their structure, and in recent years have received a great deal of study, as it has been found possible by means of them to distinguish closely allied species, especially among the Hesperiidæ. It is, however, not possible within our allotted space to go into a minute discussion of this matter. The abdomen of the female is devoid of these appendages, and in this sex is generally heavier and plumper, especially when the eggs, which fill the ovaries, have not as yet been deposited.

Thus far we have devoted our attention almost exclusively to the consideration of the external organs of the suborder of insects which is under discussion. It may be of interest to devote a few paragraphs to the internal anatomy of butterflies. To do the subject full justice would require a volume; nevertheless some general statements may pave the way for a more studious inquiry on the part of readers.

Butterflies take their nourishment in fluid form, principally from the cups which Flora provides. The organ of ingestion is the proboscis, which communicates with a bulb-like receptacle in the head, known as the pharyngeal sac, controlled by a set of muscles which cause it to alternately expand and contract, very much like the bulb-syringe used by physicians. When the muscles expand a vacuum is created, and the fluid is drawn up from the honeyed chalice of the flower into the receptacle in the head; when they contract, a valve in front closes, a valve behind opens, and the honey in the receptacle is forced backward through the œsophagus into the crop, and thence into the stomach. The stomach lies on the ventral or under side of the body, but above the nervous cord, which lies still more ventrad. The stomach opens posteriorly into the small intestine, which is followed by the colon, the latter in turn being succeeded by the rectum. Connected with the intestines are certain vesicles, which are known as Malpighian vessels, and by some are thought to have the function of the liver in higher animals.

Having thus briefly spoken of the nutritive system we may turn to the circulatory and respiratory systems. The heart of a butterfly, as in all arthropods, lies on the dorsal side of the body. Its location corresponds almost exactly to that occupied in the vertebrate animals by the spinal cord. It is a long tubular organ. It does not possess chambers—ventricles and auricles—such as are discovered in the heart of vertebrates, but it has an enlargement in the mesothoracic region known as the aortal chamber. The movement of the heart is wave-like, analogous to the peristaltic movement in the intestines of the vertebrates. From the heart there go out lateral blood-vessels, which ramify and intermingle with the capillary extremities of the tracheæ, or bronchial tubes, through which air is imported by way of the spiracles, and thus the blood is purified in a manner quite analogous to that in which the blood of the higher vertebrates is purified and freed from waste matter by oxydization. The process is, however, as has already been pointed out, mainly carried on in the abdominal region, and not exclusively in the thorax, as in the case of vertebrates.

The nervous system of butterflies consists of a cord with ganglia, or nerve-knots, one for each segment of the body. As there are thirteen segments in the body of a butterfly, there are normally thirteen such ganglia, or nerve centres. The nervous cord lies in a position exactly opposite to that which is held by the spinal cord in vertebrates. It is situated on the ventral side of the body, and more ventrad than all the other internal organs. The ganglion in the head is the largest of all and forms a rudimentary brain, the greater portion of which consists of two relatively large optic nerves.

The internal organs of reproduction in the female consist of the ovaries, from which the eggs pass by way of the oviduct to the ovipositor, which in butterflies is not so conspicuous or remarkable an organ as is the case in many other insects. Communicating with the oviduct are the spermatothecæ, which are receptacles in which is retained the fertilizing fluid received from the male at the time of coition. As the eggs, one by one, are passed from the ovaries into the oviduct, they are impregnated through absorption of the spermatozoa, which enter their walls, imparting to them vitality. The internal organs of the male are tubular vesicles, or testes, which secrete the seminal fluid, which by means of the intromittent organ is introduced into the spermatothecæ of the female. Union between the sexes among insects generally takes place but once, and is not recurrent. The female, having been impregnated, proceeds at once to lay her eggs upon the tender leaves or the bark of the plants upon which her progeny are to be nourished, and then promptly dies. The life of insects in the winged form is usually very brief. Inasmuch as there are great differences between the sexes in insects alike in size, form, color, and markings, it is well for the collector to carefully preserve specimens which may be captured in copulation. Such specimens should be designated as “Taken in coitu.” The writer in such cases uses the formula “A ♂ in coitu with B ♀,” which is put in minute handwriting upon a label and placed upon the pin bearing the male insect. Upon the pin carrying the female insect there is put the label “B ♀ in coitu with A ♂.” The finding of insects in copula is sometimes the only way in which to definitely settle the question of sexual relationship between forms. So different are some insects that the two sexes have in a number of cases been described by early writers as belonging not merely to different species, but to different genera, and even families.