The inconceivably delicate structure of the maxillæ or tongues (for there are two) of the butterfly, rolled up like the trunk of an elephant, and capable, like it, of every variety of movement, has been carefully examined and described by Mr. Newport. “Each maxilla is convex on its outer surface, but concave on its inner; so that when the two are united they form a tube, haustellium, by their union, through which fluids may be drawn into the mouth. The inner or concave surface, which forms the tube, is lined with a very smooth membrane, and extends throughout the whole length of the organ; while that of each maxilla is hollow in its interior, apparently forming a tube ‘in itself,’ but this is not so; the mistake has arisen from the existence of large tracheæ, or breathing tubes, in the interior of the proboscis. In some species the extremity of the haustellium is studded externally with a number of minute papillæ, or fringes—as in Vanessa atalanta—in which they become small elongated barrel-shaped bodies, terminated by smaller papillæ at their extremities. On alighting on a flower, the insect makes a powerful expiratory effort, by which the air is expelled from the interior air-tubes, and from those with which they are connected in the head and body; and at the moment of applying its proboscis to the food, it makes an inspiratory effort, by which the central canal in the proboscis is dilated, and the food ascends it at the same instant to supply the vacuum produced; and thus it passes into the mouth and stomach, the constant ascent of the fluid being assisted by the action of the muscles of the proboscis, which continues during the whole time that the insect is feeding. By this combined agency of the acts of respiration and the muscles of the proboscis we are also enabled to understand the manner in which the humming-bird sphynx extracts in an instant the honey from a flower while hovering over it, without alighting; and which it certainly would be unable to do were the ascent of the fluid entirely dependent upon the action of the muscles of the organ.”
The trachæal or respiratory system of insects varies, or rather is found to exist in modified forms to suit their varied conditions of life. While in the larval stage the breathing apertures are seen to recur at intervals on each side of the abdomen (as that of the silkworm, [Fig. 401]), thus ensuring a continuous supply of air to the circulating fluids throughout the whole body. These spiracles are usually nine or ten in number, and consist of a membranous ring of an oval form. The air-tubes are exquisitely composed of two thin membranes, between which a delicate elastic thread or spiral fibre is interposed, forming a cylindrical opening and keeping the tube in a distended condition, thus mechanically preserving the sides from collapse or pressure in their passage through the air, which otherwise might occasion suffocation. [Fig. 402] represents the double spiral arrangement of a portion of a trachea of Hydrophilus, which ensures both elasticity and strength.
There are other points of interest confined to the water-beetle tribe, among the more striking of which is the foreleg of the Dytiscus marginalis. Here the first three joints of the tarsus are expanded into a broad surface, and fringed throughout with curved hairs. From the surface of these spring a number of short hairs, with cup-like discs at their extremities, one of which is seen highly magnified in [Plate VI]., No. 142. These are so cup-like in form that they have been hitherto described as “suckers,” but it is believed they are simply a special apparatus for the development of the hairs seen on the leg and foot of the beetle. Another curious example occurs in the Gyrinus, or whirligig-beetle. The front pair of legs are of the ordinary kind, but the under pair are furnished with expanding paddles. The trochanter, femur, and tibia, are flat plates of a triangular shape, pointed at their outer angles, from which the apex springs. But the tarsus is jointed on the inner angle of the furthermost end of the tibia, and each of its four joints expands into a flat paddle blade. In the accompanying [Fig. 403] one paddle is seen expanded, the other closed.
Fig. 403.
1. Leg of Gyrinus, Whirligig, paddle shown expanded. 2. Paddle closed up.
These paddles are adapted with much precision to ensure the most effectual application of the propelling power; as the beetle strikes out in the act of swimming, the membranous expansion described enables it to move about with great rapidity; upon the legs being drawn back towards the body, the membrane closes up, and thus offers no resistance to the water. The eyes are not the least curious part of the merry little beetle: the upper section is fitted for seeing in the air, and is adapted to the upper or superior part of the head; the lower portion, for seeing under the water, being placed at a lower angle, a thin division only separating the two.
Fig. 404.—Scales from Butterflies’ and Moths’ wings, magnified 200 diameters.
1. Scale of Morpho menelaus; 2. Large scale of Polyommatus argiolus, azure blue; 3. Hipparchia janira argiolus; 4. Pontia brassica; 5. Podura plumbea; 6. Small scale of azure blue.