Читать книгу The Butterflies of the British Isles - Richard South - Страница 10

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Having safely cleared itself free of the chrysalis shell, the butterfly makes its way to some suitable twig, spray, or other object, from which it can hang, sometimes in an inverted position, whilst a very important function takes place. This is the distention and drying of the wings, which at first are very weak and somewhat baggy affairs, although the colour and markings appear upon them in miniature. All other parts of the butterfly seem fully formed, but the helpless condition of the wings alone prevent it as yet from floating off into the air. In a remarkably short time, after the insect has settled to the business, the fluids from the body commence to flow and circulate through the wings, and these are seen gradually expanding and filling out until they attain their proper size. Occasionally there is some obstruction to the equal distribution of the fluids, and when this occurs a greater or lesser amount of distortion, or cockle, in the wing affected is the result. When the inflation is completed the wings are kept straight out for a time; they are then motionless, but all their surfaces are well apart. The wings being now fully developed, the further flow of fluid appears to be arrested. It has been stated by some authorities that this fluid is fibrin held in solution, and that when the work of expansion has been accomplished, the watery medium evaporates, leaving the fibrin to harden, and so fasten together the upper and lower membranes of the wing and to fix the veins, or nerves, in their proper position. Mayer, a specialist on these matters, referring to the expansion of the wings, remarks that the blood [the fluid previously mentioned] forced into the freshly emerged wing would cause it to become a balloon-shaped bag if it were not for fibres that hold the upper and lower walls closely together. The fibres referred to, he states, are derived from those hypodermic cells which do not contribute to the formation of scales, but are stretched out from one wall of the wing to the other.

Fig. 6.

Head of Butterfly.

a, compound eye; b, palp; c, antenna; d, proboscis.

It may be well now to briefly consider some of the structural details of the perfect butterfly, so a beginning will be made with the head (Fig. 6). When looking at the head of a butterfly, the first thing to attract the attention is the very large size of the compound eye (a), which seems to take up the largest share of the whole affair. Although so bulky and so complex in the matter of divisions, or facets, as they are termed (the facets are not shown in figure), the power of sight is not really very keen. A butterfly can see things in a general way readily enough, but it seems unable to clearly distinguish one object from another. When engaged in egg-laying, the female butterfly rarely fails to place her eggs on a leaf or spray of the plant that the future caterpillar will feed upon, and it has been suggested that in making this unerring selection the insect is guided more by the sense of smell than by that of sight.

The horns (c) (antennæ), or feelers, as they are sometimes called, which adorn the head, are now considered to be organs of smell. These are composed of a number of rings or segments, which vary in the different kinds of butterfly, as also does the shape of the terminal rings forming what is known as the club. In Fig. 7, e (Purple Emperor) and f (Marbled White) represent the gradually thickened club; in g (Brimstone) and h (Dark-green Fritillary) the clubs are more or less abruptly formed. Our Skippers have well-developed clubs; these may be hooked at the tip as in i (Large Skipper), or blunt at the tip as in j (Chequered Skipper); at the base of the Skipper's antenna, that is at the point where it is inserted in the head, there is a tuft of rather long hairs.

Of the various mouth parts it will only be necessary to refer to the suction-tube, Fig. 6, d (proboscis), often called the "tongue," which is perhaps the most important, at least to the butterfly itself, as this organ is, in a way, as useful to it in the perfect state as were the very differently constructed strong biting jaws (mandibles) of its caterpillar existence. These latter in the butterfly are only microscopically represented, and the suction-tube of the perfect insect is an extension of the maxillæ, which in the caterpillar are not conspicuous. When not engaged in probing the nectaries of flowers for the sweets they contain, the suction-tube is neatly coiled up between the palpi (Fig. 6, b). Its great flexibility is due to the many rings of which it is composed. Although seemingly entire, it is really made up of two tubes, each being grooved on its inner side, and forming, when the edges are brought together, an additional central canal, through which the sweets from the flowers and other liquids are drawn up into a bulb-like receptacle in the head, whence it passes into the stomach. When it is remembered that the passage of sweet, and no doubt sticky, fluid through the central tube would most probably result in its walls becoming clogged, there is reason to suppose that the method of construction permits of the canal being cleansed from time to time.

Fig. 7.

Antennæ of Butterflies.

Fig. 8.

Leg of Butterfly.

The important divisions of the body are the thorax and the abdomen. The former is made up of three segments (named the pro-, meso-, and meta-thorax), each of which, as in the caterpillar state, is furnished with a pair of legs; the second and third, which are closely united, each bear a pair of wings also. The legs, which in the butterfly are adapted for walking at a leisurely pace, are made up of four main parts; these are (a) the basal joint (coxa, coxæ), (b) the thigh (femur, femora), (c) the shank (tibia, tibiæ), and (d) the foot (tarsus, tarsi). The small joint uniting the coxa with the femur is the trochanter (tr.). The foot usually has five joints, the last of which is provided with claws (e). The abdomen really consists of ten rings or segments according to some specialists. Examined from above, the female butterfly appears to have only seven rings and the male butterfly eight. This discrepancy arises from the fact that in the former sex two rings and in the latter one ring are withdrawn into the body, and so are tucked away out of sight. The organs of reproduction are placed in the terminal ring. The breathing arrangements are pretty much as in the caterpillar, but the external openings are not so apparent owing to the dense clothing of the body.

The beauty of a butterfly's wings is intimately connected with the form and colour of the scales with which they are covered, as with a kind of mosaic; but before the scales and their method of attachment, etc., are referred to, something should be said about the wings themselves. The various shapes of these organs of flight will be seen on turning to the plates, where will be found accurate portraits of every species that will be dealt with in the descriptive section later on.

A butterfly's wing consists of an upper and a lower membrane, with a framework of hollow tubes, acting as ribs, between the two layers. Fig. 9, A, shows a fore and a hind wing of the Swallow-tail butterfly. The point of attachment with the thorax is the base of the wing, and the edge farthest from the base is the outer margin (termen); the upper edge, or front margin, is the costa; and the lower edge is the inner margin (dorsum). The point where the upper margin meets the outer margin on the fore wing is the apex, but on the hind wing it is called the outer angle; the angle formed by the junction of outer and inner margins is the inner angle of the fore wing, but the anal angle of the hind wing. The term tornus is sometimes used for this angle on either wing. Dividing the wings transversely into three portions, we have three areas, termed respectively basal, central or discal, and outer. These are terms used in descriptions of butterflies, and it will be useful to remember them.

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Fig. 9.

Butterflies' Wings.

The ribs of a butterfly's wings are by some authors described as veins, whilst others style the main ones nervures, and the branches nervules. Fig. 9, B, represents the venation, or neuration of the Black-veined White, and the numeral system of indicating the veins has been adopted, as it is the most simple. In another method of referring to the venation, and one that has been much in use, vein 12 of the fore wing would be styled the costal nervure, or vein; veins 11, 10, 9 (absent in figure), 8, and 7 would be the subcostal nervules 1, 2, 3, 4, and 5; 6 would become the upper radial, and 5 the lower radial; 2, 3, and 4 would be the median nervules 1, 2, and 3; vein 1 would be the submedian nervure, or vein. On the hind wing, vein 1a would be the internal vein; 1 the submedian; 2, 3, and 4 the median nervules; 5 the lower and 6 the upper radials; 7 the subcostal, and 8 the costal nervures. Just near the base of the hind wing will be noted a short recurved vein (p.c.); this is the precostal vein, and so named because it comes before the costal. It is always absent in some species. Comparing the venation of A and B, it will be seen that in A the fore wing has 12 veins and the hind wing 8 veins, whilst in B there are only 11 veins on the fore wing, but the hind wing has one vein more than that of A. In the Black-veined White, vein 9 is absent on the fore wing, and on the hind wing there is one internal vein.

Fig. 10.

Arrangement of Scales.

(After Holland.)

Dust-like as they appear to the naked eye, the scales from a butterfly's wing seen under the microscope are found to be exceedingly interesting structures and very varied in shape. Dr. Sharp describes them as "delicate chitinous bags." Chitin, it may be mentioned, is the horny substance of which the chrysalis shell is formed, and this was adverted to when discussing the chrysalis stage as a varnish-like ooze. As seen on the wings, the scales are flattened and the upper and under sides are then almost, or quite, brought together. They are attached in lines on the membrane or covering of the wing by short stalks which fit into sockets in the membrane. The arrangement of the scales, which has often been stated to resemble that of the slates on a roof, is shown in Fig. 10.

Colour is chiefly due to pigment contained in the scale or adhering to the interior of its upper side. Pigments, according to Mayer, are derived, by various chemical processes, from the blood while the butterfly is still in the chrysalis. Some scales have minute parallel lines (striæ) on their upper sides, and rays of light falling on these are turned aside or broken up, and so produce changes in the colouring of a wing, according to the angle from which it is looked at.

The males of many kinds of butterfly have special scales, which are known as androconia, or plumules. It is believed that these are scent organs. Whatever their particular use may be to the possessor, these androconia enable the entomologist to distinguish male specimens from females with great certainty. In the Fritillaries they are placed on one or more of the median nervules (veins 2, 3, and 4) of the fore wing. In the Meadow Brown and its kindred they form brands on the disc of the fore wing. In the Skippers they are placed in a fold of the costa in some species, and in other species they are clustered together, into more or less bar-like marks, about the middle of the fore wings. Some of these various shaped "plumules" are shown in the illustrations.