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.