Imágenes de páginas
PDF
EPUB

The wings of insects present different degrees of opacity— those of the moths and butterflies being non-transparent; those of the dragon-flies, bees, and common flies presenting a delicate, filmy, gossamer-like appearance. The wings in every case are composed of a duplicature of the integument or in

b

f

b

a

a

FIG. 91.-The Dragon-fly (Petalura gigantea). In this insect the wings are finely curved and delicately transparent, the nervures being most strongly developed at the roots of the wings and along the anterior margins (e e, ff), and least so at the tips (bb), and along the posterior margins (a a). The anterior pair (e e) are analogous in every respect to the posterior (ff). Both make a certain angle with the horizon, the anterior pair (e e), which are principally used as elevators, making a smaller angle than the posterior pair (ff), which are used as drivers. The wings of the dragon-fly make the proper angles for flight even in repose, so that the insect can take to wing instantly. The insect flies with astonishing velocity.-Original.

vesting membrane, and are strengthened in various directions by a system of hollow, horny tubes, known to entomologists as the neuræ or nervures. The nervures taper towards the extremity of the wing, and are strongest towards its root and anterior margin, where they supply the place of the arm in bats and birds. They are variously arranged. In the beetles they pursue a somewhat longitudinal course, and are jointed to admit of the wing being folded up transversely beneath the elytra.1 In the locusts the nervures diverge from a common centre, after the manner of a fan, so that by their aid the wing is crushed up or expanded as required; whilst in the dragon-fly, 1 The wings of the May-fly are folded longitudinally and transversely, so that they are crumpled up into little squares.

where no folding is requisite, they form an exquisitely reticulated structure. The nervures, it may be remarked, are strongest in the beetles, where the body is heavy and the wing small. They decrease in thickness as those conditions are reversed, and entirely disappear in the minute chalcis and psilus. The function of the nervures is not ascertained; but as they contain spiral vessels which apparently communicate with the trachea of the trunk, some have regarded them as being connected with the respiratory system; whilst others have looked upon them as the receptacles of a subtle fluid, which the insect can introduce and withdraw at pleasure to obtain the requisite degree of expansion and tension in the wing. Neither hypothesis is satisfactory, as respiration and flight can be performed in their absence. They appear to me, when present, rather to act as mechanical stays or stretchers, in virtue of their rigidity and elasticity alone,— their arrangement being such that they admit of the wing being folded in various directions, if necessary, during flexion, and give it the requisite degree of firmness during extension. They are, therefore, in every respect analogous to the skeleton of the wing in the bat and bird. In those wings which, during the period of repose, are folded up beneath the elytra, the mere extension of the wing in the dead insect, where no injection of fluid can occur, causes the nervures to fall into position, and the membranous portions of the wing to unfurl or roll out precisely as in the living insect, and as happens in the bat and bird. This result is obtained by the spiral arrangement of the nervures at the root of the wing; the anterior nervure occupying a higher position than that further back, as in the leaves of a fan. The spiral arrangement occurring at the root extends also to the margins, so that wings which fold up or close, as well as those which do not, are twisted upon themselves, and present a certain degree of convexity on their superior or upper surface, and a corresponding concavity on their inferior or under surface; their free edges supplying those fine curves which act with such efficacy upon the air, in obtaining the maximum of resistance and the minimum of displacement; or what is the same thing, the maximum of support with the minimum of slip (figs. 92 and 93).

1 Kirby and Spence, vol. ii. 5th ed., p. 352.

The wings of insects can be made to oscillate within given areas anteriorly, posteriorly, or centrally with regard to the plane of the body; or in intermediate positions with regard to it and a perpendicular line. The wing or wings of the one

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small]

FIG. 92.-Right wing of Beetle (Goliathus micans), dorsal surface. This wing somewhat resembles the kestrel's (fig. 61, p. 136) in shape. It has an anterior thick margin, d e f, and a posterior thin one, b a c. Strong nervures run along the anterior margin (d) until they reach the joint (e), where the wing folds upon itself during repose. Here the nervures split up and divaricate and gradually become smaller and smaller until they reach the extremity of the wing (f) and the posterior or thin margin (b); other nervures radiate in graceful curves from the root of the wing. These also become finer as they reach the posterior or thin margin (c a). r, Root of the wing with its complex compound joint. The wing of the beetle bears a certain analogy to that of the bat, the nervures running along the anterior margin (d) of the wing, resembling the humerus and forearm of the bat (fig. 94, d, p. 175), the joint of the beetle's wing (e) corresponding to the carpal or wrist-joint of the bat's wing (fig. 94, e), the terminal or distal nervures of the beetle (b) to the phalanges of the bat (fig. 94, fb). The parts marked fb may in both instances be likened to the primary feathers of the bird, that marked a to the secondary feathers, and c to the tertiary feathers. In the wings of the beetle and bat no air can possibly escape through them during the return or up stroke -Original.

FIG. 93.-Right wing of the Beetle (Goliathus micans), as seen from behind and from beneath. When so viewed, the anterior or thick margin (df) and the posterior or thin margin (b x c) are arranged in different planes, and form a true helix or screw. Compare with figs. 95 and 97.-Original.

side can likewise be made to move independently of those of the opposite side, so that the centre of gravity, which, in insects, bats, and birds, is suspended, is not disturbed in the endless evolutions involved in ascending, descending, and wheeling. The centre of gravity varies in insects according to the shape of the body, the length and shape of the limbs and antennæ, and the position, shape, and size of the

pinions. It is corrected in some by curving the body, in others by bending or straightening the limbs and antennæ, but principally in all by the judicious play of the wings themselves.

The wing of the bat and bird, like that of the insect, is concavo-convex, and more or less twisted upon itself (figs. 94, 95, 96, and 97).

[graphic][subsumed][subsumed][subsumed][merged small][ocr errors][ocr errors][merged small][merged small]

FIG. 94.-Right wing of the Bat (Phyllorhina gracilis), dorsal surface. def, Anterior or thick margin of the wing, supported by the bones of the arm, forearm, and hand (first and second phalanges); c a b, posterior or thin margin, supported by the remaining phalanges, by the side of the body, and by the foot.-Original.

FIG. 95.-Right wing of the Bat (Phyllorhina gracilis), as seen from behind and from beneath. When so regarded, the anterior or thick margin (df) of the wing displays different curves from those seen on the posterior or thin margin (b c); the anterior and posterior margins being arranged in different planes, as in the blade of a screw propeller.-Original.

The twisting is in a great measure owing to the manner in which the bones of the wing are twisted upon themselves, and the spiral nature of their articular surfaces; the long axes of the joints always intersecting each other at nearly right angles. As a result of this disposition of the articular surfaces, the wing is shot out or extended, and retracted or flexed in a variable plane, the bones of the wing rotating in the direction of their length during either movement. This secondary action, or the revolving of the component bones upon their own axes, is of the greatest importance in the movements of the wing, as it communicates to the hand and forearm, and

consequently to the membrane or feathers which they bear, the precise angles necessary for flight. It, in fact, insures that the wing, and the curtain, sail, or fringe of the wing shall be screwed into and down upon the air in extension, and unscrewed or withdrawn from it during flexion. The wing of the bat and bird may therefore be compared to a huge gimlet or auger, the axis of the gimlet representing the bones of the wing; the flanges or spiral thread of the gimlet the frenum or sail (figs. 95 and 97).

[graphic][subsumed][merged small][merged small][merged small][merged small]

FIG. 96.-Right wing of the Red-legged Partridge (Perdix rubra), dorsal aspect. Shows extreme example of short rounded wing; contrast with the wing of the albatross (fig. 62, p. 137), which furnishes an extreme example of the long ribbon-shaped wing; def, anterior margin; b a c, posterior ditto, consisting of primary (b), secondary (a), and tertiary (c) feathers, with their respective coverts and subcoverts; the whole overlapping and mutually supporting each other. This wing, like the kestrel's (fig. 61, p. 136), was drawn from a specimen held against the light, the object being to display the mutual relation of the feathers to each other, and how the feathers overlap.-Original.

FIG. 97.-Right wing of Red-legged Partridge (Perdix rubra), seen from behind and from beneath, as in the beetle (fig. 93) and bat (fig. 95). The same lettering and explanation does for all three.-Original.

THE WINGS OF BATS.

The Bones of the Wing of the Bat-the spiral configuration of their articular surfaces.-The bones of the arm and hand are especially deserving of attention. The humerus (fig. 17, 7, p. 36) is short and powerful, and twisted upon itself to the extent of something less than a quarter of a turn.

« AnteriorContinuar »