Chapter VIII: Part 8

_The Wing acts upon yielding Fulcra._--The chief peculiarity of the wing, as has been stated, consists in its being a twisted flexible lever specially constructed to act upon yielding fulcra (the air). The points of contact of the wing with the air are represented at _a b c d e f g h i j k l_ respectively of figs. 84 and 85, p. 160; and the imaginary points of rotation of the wing upon its long and short axes at 1, 2, 3, and 4 of the same figures. The assumed points of rotation advance from 1 to 3 and from 2 to 4 (_vide_ arrows marked _r_ and _s_, fig. 85); these constituting the steps or pulsations of the wing. The actual points of rotation correspond to the little loops _a b c d f g h i j l_ of fig. 85. The wing descends at _A_ and _C_, and ascends at _B_ and _D_.

_The Wing acts as a true Kite both during the Down and Up Strokes._--If, as I have endeavoured to explain, the wing, even when elevated and depressed in a strictly vertical direction, inevitably and invariably darts forward, it follows as a consequence that the wing, as already partly explained, flies forward as a true kite, both during the down and up strokes, as shown at _c d e f g h i j k l m_ of fig. 88; and that its under concave or biting surface, in virtue of the forward travel communicated to it by the body in motion, is closely applied to the air, both during its ascent and descent--a fact hitherto overlooked, but one of considerable importance, as showing how the wing furnishes a persistent buoyancy, alike when it rises and falls.

In fig. 88 the greater impulse communicated during the down stroke is indicated by the double dotted lines. The angle made by the wing with the horizon (_a b_) is constantly varying, as a comparison of _c_ with _d_, _d_ with _e_, _e_ with _f_, _f_ with _g_, _g_ with _h_, and _h_ with _i_ will show; these letters having reference to supposed transverse sections of the wing. This figure also shows that the _convex_ or non-biting surface of the wing is always directed upwards, so as to avoid unnecessary resistance on the part of the air to the wing during its ascent; whereas the _concave_ or biting surface is always directed downwards, so as to enable the wing to contend successfully with gravity.

_Where the Kite formed by the Wing differs from the Boy’s Kite._--The natural kite formed by the wing differs from the artificial kite only in this, that the former is capable of being moved in all its parts, and is more or less flexible and elastic, the latter being comparatively rigid. The flexibility and elasticity of the kite formed by the natural wing is rendered necessary by the fact that the wing is articulated or hinged at its root; its different parts travelling at various degrees of speed in proportion as they are removed from the axis of rotation. Thus the tip of the wing travels through a much greater space in a given time than a portion nearer the root. If the wing was not flexible and elastic, it would be impossible to reverse it at the end of the up and down strokes, so as to produce a continuous vibration. The wing is also practically hinged along its anterior margin, so that the posterior margin of the wing travels through a greater space in a given time than a portion nearer the anterior margin (fig. 80, p. 149). The compound rotation of the wing is greatly facilitated by the wing being flexible and elastic. This causes the pinion to twist upon its long axis during its vibration, as already stated. The twisting is partly a vital, and partly a mechanical act; that is, it is occasioned in part by the action of the muscles, in part by the reaction of the air, and in part by the greater momentum acquired by the tip and posterior margin of the wing, as compared with the root and anterior margin; the speed acquired by the tip and posterior margin causing them to reverse always subsequently to the root and anterior margin, which has the effect of throwing the anterior and posterior margins of the wing into figure-of-8 curves. It is in this way that the posterior margin of the outer portion of the wing is made to incline forwards at the end of the down stroke, when the anterior margin is inclined backwards; the posterior margin of the outer portion of the wing being made to incline backwards at the end of the up stroke, when a corresponding portion of the anterior margin is inclined forwards (figs. 69 and 70, _g_, _a_, p. 141; fig. 86, _j_, _f_, p. 161).

_The Angles formed by the Wing during its Vibrations._--Not the least interesting feature of the compound rotation of the wing--of the varying degrees of speed attained by its different parts--and of the twisting or plaiting of the posterior margin around the anterior,--is the great variety of kite-like surfaces developed upon its dorsal and ventral aspects. Thus the tip of the wing forms a kite which is inclined upwards, forwards, and outwards, while the root forms a kite which is inclined upwards, forwards, and inwards. The angles made by the tip and outer portions of the wing with the horizon are less than those made by the body or central part of the wing, and those made by the body or central part less than those made by the root and inner portions. The angle of inclination peculiar to any portion of the wing increases as the speed peculiar to said portion decreases, and _vice versâ_. The wing is consequently mechanically perfect; the angles made by its several parts with the horizon being accurately adjusted to the speed attained by its different portions during its travel to and fro. From this it follows that the air set in motion by one part of the wing is seized upon and utilized by another; the inner and anterior portions of the wing supplying, as it were, currents for the outer and posterior portions. This results from the wing always forcing the air outwards and backwards. These statements admit of direct proof, and I have frequently satisfied myself of their exactitude by experiments made with natural and artificial wings.

In the bat and bird, the twisting of the wing upon its long axis is more of a vital and less of a mechanical act than in the insect; the muscles which regulate the vibration of the pinion in the former (bat and bird), extending quite to the tip of the wing (fig. 95, p. 175; figs. 82 and 83, p. 158).

_The Body and Wings move in opposite Curves._--I have stated that the wing advances in a waved line, as shown at _a c e g i_ of fig. 81, p. 157; and similar remarks are to be made of the body as indicated at 1, 2, 3, 4, 5 of that figure. Thus, when the wing descends in the curved line _a c_, it elevates the body in a corresponding but minor curved line, as at 1, 2; when, on the other hand, the wing ascends in the curved line _c e_, the body descends in a corresponding but smaller curved line (2, 3), and so on _ad infinitum_. The undulations made by the body are so trifling when compared with those made by the wing, that they are apt to be overlooked. They are, however, deserving of attention, as they exercise an important influence on the undulations made by the wing; the body and wing swinging forward alternately, the one rising when the other is falling, and _vice versâ_. Flight may be regarded as the resultant of three forces:--the _muscular and elastic force_, residing in the wing, which causes the pinion to act as a true kite, both during the down and up strokes; the _weight of the body_, which becomes a force the instant the trunk is lifted from the ground, from its tendency to fall downwards and forwards; and _the recoil obtained from the air_ by the rapid action of the wing. These three forces may be said to be active and passive by turns.

When a bird rises from the ground it runs for a short distance, or throws its body into the air by a sudden leap, the wings being simultaneously elevated. When the body is fairly off the ground, the wings are made to descend with great vigour, and by their action to continue the upward impulse secured by the preliminary run or leap. The body then falls in a curve downwards and forwards; the wings, partly by the fall of the body, partly by the reaction of the air on their under surface, and partly by the shortening of the elevator muscles and elastic ligaments, being placed above and to some extent behind the bird--in other words, elevated. The second down stroke is now given, and the wings again elevated as explained, and so on in endless succession; the body falling when the wings are being elevated, and _vice versâ_, (fig. 81, p. 157). When a long-winged oceanic bird rises from the sea, it uses the tips of its wings as levers for forcing the body up; the points of the pinions suffering no injury from being brought violently in contact with the water. A bird cannot be said to be flying until the trunk is swinging forward in space and taking part in the movement. The hawk, when fixed in the air over its quarry, is simply supporting itself. To fly, in the proper acceptation of the term, implies to support and propel. This constitutes the difference between a bird and a balloon. The bird can elevate _and carry itself forward_, the balloon can simply elevate itself, and must rise and fall in a straight line in the absence of currents. When the gannet throws itself from a cliff, the inertia of the trunk at once comes into play, and relieves the bird from those herculean exertions required to raise it from the water when it is once fairly settled thereon. A swallow dropping from the eaves of a house, or a bat from a tower, afford illustrations of the same principle. Many insects launch themselves into space prior to flight. Some, however, do not. Thus the blow-fly can rise from a level surface when its legs are removed. This is accounted for by the greater amplitude and more horizontal play of the insect’s wing as compared with that of the bat and bird, and likewise by the remarkable reciprocating power which the insect wing possesses when the body of the insect is not moving forwards (figs. 67, 68, 69, and 70 p. 141). When a beetle attempts to fly from the hand, it extends its front legs and flexes the back ones, and tilts its head and thorax upwards, so as exactly to resemble a horse in the act of rising from the ground. This preliminary over, whirr go its wings with immense velocity, and in an almost horizontal direction, the body being inclined more or less vertically. The insect rises very slowly, and often requires to make several attempts before it succeeds in launching itself into the air. I could never detect any pressure communicated to the hand when the insect was leaving it, from which I infer that it does not leap into the air. The bees, I am disposed to believe, also rise without anything in the form of a leap or spring. I have often watched them leaving the petals of flowers, and they always appeared to me to elevate themselves by the steady play of their wings, which was the more necessary, as the surface from which they rose was in many cases a yielding surface.

THE WINGS OF INSECTS, BATS, AND BIRDS.

_Elytra or Wing-cases, Membranous Wings--their shape and uses._--The wings of insects consist either of one or two pairs. When two pairs are present, they are divided into an anterior or upper pair, and a posterior or under pair. In some instances the anterior pair are greatly modified, and present a corneous condition. When so modified, they cover the under wings when the insect is reposing, and have from this circumstance been named elytra, from the Greek ἔλυτρον, a sheath. The anterior wings are dense, rigid, and opaque in the beetles (fig. 89, _r_); solid in one part and membranaceous in another in the water-bugs (fig. 90, _r_); more or less membranous throughout in the grasshoppers; and completely membranous in the dragon-flies (fig. 91, _e e_, p. 172). The superior or upper wings are inclined at a certain angle when extended, and are indirectly connected with flight in the beetles, water-bugs, and grasshoppers. They are actively engaged in this function in the dragon-flies and butterflies. The elytra or anterior wings are frequently employed as _sustainers_ or _gliders_ in flight,[81] the posterior wings acting more particularly as _elevators_ and _propellers_. In such cases the elytra are twisted upon themselves after the manner of wings.

[81] That the elytra take part in flight is proved by this, that when
they are removed, flight is in many cases destroyed.

FIG. 89.--The Centaur Beetle (_Augusoma centaurus_), seen
from above. Shows elytra (_r_) and membranous wings (_e_) in the
extended state. The nervures are arranged and jointed in such a
manner that the membranous wings can be folded (_e_) transversely
across the back beneath the elytra during repose. When so folded,
the anterior or thick margins of the membranous wings are directed
outwards and slightly downwards, the posterior or thin margins
inwards and slightly upwards. During extension the positions of
the margins are reversed by the wings twisting and rotating upon
their long axes, the anterior margins, as in bats and birds, being
directed upwards and forwards, and making a very decided angle with
the horizon. The wings in the beetles are insignificantly small
when compared with the area of the body. They are, moreover, finely
twisted upon themselves, and possess great power as propellers and
elevators.--_Original._

FIG. 90.--The Water-Bug (_Genus belostoma_). In this insect
the superior wings (elytra or wing covers _r_) are semi-membranous.
They are geared to the membranous or under wings (_a_) by a hook, the
two acting together in flight. When so geared the upper and under
wings are delicately curved and twisted. They moreover taper from
within outwards, and from before backwards.--_Original._]

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_, _f f_), and least so at the tips (_b
b_), and along the posterior margins (_a a_). The anterior pair (_e
e_) are analogous in every respect to the posterior (_f f_). 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 (_f f_), 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._]

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 investing 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.[82] 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, 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.[83] The function of the nervures is not ascertained; but as they contain spiral vessels which apparently communicate with the tracheæ 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).

[82] The wings of the May-fly are folded longitudinally and
transversely, so that they are crumpled up into little squares.

[83] Kirby and Spence, vol. ii. 5th ed., p. 352.

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 (_f b_) to the phalanges of the bat (fig. 94, _f b_). The
parts marked _f b_ 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 (_d f_) 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._]

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 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).

FIG. 94.--Right wing of the Bat (_Phyllorhina gracilis_),
dorsal surface. _d e f_, 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 (_d f_) 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).

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; _d e
f_, 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, _r_, p. 36) is short and powerful, and twisted upon itself to the extent of something less than a quarter of a turn. As a consequence, the long axis of the shoulder-joint is nearly at right angles to that of the elbow-joint. Similar remarks may be made regarding the radius (the principal bone of the forearm) (_d_), and the second and third metacarpal bones with their phalanges (_e f_), all of which are greatly elongated, and give strength and rigidity to the anterior or thick margin of the wing. The articular surfaces of the bones alluded to, as well as of the other bones of the hand, are spirally disposed with reference to each other, the long axes of the joints intersecting at nearly right angles. The object of this arrangement is particularly evident when the wing of the living bat, or of one recently dead, is extended and flexed as in flight.

In the flexed state the wing is greatly reduced in size, its under surface being nearly parallel with the plane of progression. When the wing is fully extended its under surface makes a certain angle with the horizon, the wing being then in a position to give the down stroke, which is delivered _downwards_ and _forwards_, as in the insect. When extension takes place the elbow-joint is depressed and carried forwards, the wrist elevated and carried backwards, the metacarpo-phalangeal joints lowered and inclined forwards, and the distal phalangeal joints slightly raised and carried backwards. The movement of the bat’s wing in extension is consequently a spiral one, the spiral running alternately from below upwards and forwards, and from above downwards and backwards (compare with fig. 79, p. 147). As the bones of the arm, forearm, and hand rotate on their axes during the extensile act, it follows that the posterior or thin margin of the wing is rotated in a downward direction (the anterior or thick one being rotated in an opposite direction) until the wing makes an angle of something like 30° with the horizon, which, as I have already endeavoured to show, is the greatest angle made by the wing in flight. The action of the bat’s wing at the shoulder is particularly free, partly because the shoulder-joint is universal in its nature, and partly because the scapula participates in the movements of this region. The freedom of action referred to enables the bat not only to rotate and twist its wing as a whole, with a view to diminishing and increasing the angle which its under surface makes with the horizon, but to elevate and depress the wing, and move it in a forward and backward direction. The rotatory or twisting movement of the wing is an essential feature in flight, as it enables the bat (and this holds true also of the insect and bird) to balance itself with the utmost exactitude, and to change its position and centre of gravity with marvellous dexterity. The movements of the shoulder-joint are restrained within certain limits by a system of check-ligaments, and by the coracoid and acromian processes of the scapula. The wing is recovered or flexed by the action of elastic ligaments which extend between the shoulder, elbow, and wrist. Certain elastic and fibrous structures situated between the fingers and in the substance of the wing generally take part in flexion. The bat flies with great ease and for lengthened periods. Its flight is remarkable for its softness, in which respect it surpasses the owl and the other nocturnal birds. The action of the wing of the bat, and the movements of its component bones, are essentially the same as in the bird.

THE WINGS OF BIRDS.

_The Bones of the Wing of the Bird--their Articular Surfaces, Movements, etc._--The humerus, or arm-bone of the wing, is supported by three of the trunk-bones, viz. the scapula or shoulder-blade, the clavicle or collar-bone, also called the _furculum_,[84] and the coracoid bone,--these three converging to form a _point d’appui_, or centre of support for the head of the humerus, which is received in _facettes_ or depressions situated on the scapula and coracoid. In order that the wing may have an almost unlimited range of motion, and be wielded after the manner of a flail, it is articulated to the trunk by a somewhat lax universal joint, which permits vertical, horizontal, and intermediate movements.[85] The long axis of the joint is directed vertically; the joint itself somewhat backwards. It is otherwise with the elbow-joint, which is turned forwards, and has its long axis directed horizontally, from the fact that the humerus is twisted upon itself to the extent of nearly a quarter of a turn. The elbow-joint is decidedly spiral in its nature, its long axis intersecting that of the shoulder-joint at nearly right angles. The humerus articulates at the elbow with two bones, the radius and the ulna, the former of which is pushed from the humerus, while the other is drawn towards it during extension, the reverse occurring during flexion. Both bones, moreover, while those movements are taking place, revolve to a greater or less extent upon their own axes. The bones of the forearm articulate at the wrist with the carpal bones, which being spirally arranged, and placed obliquely between them and the metacarpal bones, transmit the motions to the latter in a curved direction. The long axis of the wrist-joint is, as nearly as may be, at right angles to that of the elbow-joint, and more or less parallel with that of the shoulder. The metacarpal or hand-bones, and the phalanges or finger-bones are more or less fused together, the better to support the great primary feathers, on the efficiency of which flight mainly depends. They are articulated to each other by double hinge-joints, the long axes of which are nearly at right angles to each other.

[84] The furcula are usually united to the anterior part of the
sternum by ligament; but in birds of powerful flight, where the
wings are habitually extended for gliding and sailing, as in
the frigate-bird, the union is osseous in its nature. “In the
frigate-bird the furcula are likewise anchylosed with the coracoid
bones.”--Comp. Anat. and Phys. of Vertebrates, by Prof. Owen,
vol. ii. p. 66.

[85] “The os humeri, or bone of the arm, is articulated by a small
rounded surface to a corresponding cavity formed between the coracoid
bone and the scapula, in such a manner as to allow great freedom of
motion.”--Macgillivray’s Brit. Birds, vol. i. p. 33.

“The arm is articulated to the trunk by a ball-and-socket joint,
permitting all the freedom of motion necessary for flight.”--Cyc. of
Anat. and Phys., vol. iii. p. 424.

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 composing the wing, particularly those of the forearm, rotating on their axes 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 primary and secondary feathers which they bear, the precise angles necessary for flight; it in fact insures that the wing, and the curtain or fringe of the wing which the primary and secondary feathers form, shall be screwed into and down upon the air in extension, and unscrewed or withdrawn from it during flexion. The wing of the 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 primary and secondary feathers (fig. 63, p. 138, and fig. 97, p. 176).

_Traces of Design in the Wing of the Bird--the arrangement of the Primary, Secondary, and Tertiary Feathers, etc._--There are few things in nature more admirably constructed than the wing of the bird, and perhaps none where design can be more readily traced. Its great strength and extreme lightness, the manner in which it closes up or folds during flexion, and opens out or expands during extension, as well as the manner in which the feathers are strung together and overlap each other in divers directions to produce at one time a solid resisting surface, and at another an interrupted and comparatively non-resisting one, present a degree of fitness to which the mind must necessarily revert with pleasure. If the feathers of the wing only are contemplated, they may be conveniently divided into three sets of three each (on both sides of the wing)--an upper or dorsal set (fig. 61, _d_, _e_, _f_, p. 136), a lower or ventral set (_c_, _a_, _b_), and one which is intermediate. This division is intended to refer the feathers to the bones of the arm, forearm, and hand, but is more or less arbitrary in its nature. The lower set or tier consists of the primary (_b_), secondary (_a_), and tertiary (_c_) feathers, strung together by fibrous structures in such a way that they move in an outward or inward direction, or turn upon their axes, at precisely the same instant of time,--the middle and upper sets of feathers, which overlap the primary, secondary, and tertiary ones, constituting what are called the “coverts” and “sub-coverts.” The primary or rowing feathers are the longest and strongest (_b_), the secondaries (_a_) next, and the tertiaries third (_c_). The tertiaries, however, are occasionally longer than the secondaries. The tertiary, secondary, and primary feathers increase in strength from within outwards, _i.e._ from the body towards the extremity of the wing, and so of the several sets of wing-coverts. This arrangement is necessary, because the strain on the feathers during flight increases in proportion to their distance from the trunk.

FIGS. 98, 99, 100, and 101 show the muscles and elastic ligaments,
and the arrangement of the primary and secondary feathers on the
ventral aspects of the wing of the crested crane. The wing is in the
extended condition.

_y_ (fig. 98), Great pectoral muscle which depresses the wing.

_a b_, Voluntary muscular fibres terminating in elastic band _k_.
This band splits up into two portions (_k_, _m_). A somewhat similar
band is seen at _j_. These three bands are united to, and act in
conjunction with, the great fibro-elastic web _c_, to flex the
forearm on the arm. The fibro-elastic web is more or less under the
influence of the voluntary muscles (_a_, _b_).

_o_, _p_, _q_, Musculo-fibro-elastic ligament, which envelopes the
roots of the primary and secondary feathers, and forms a symmetrical
network of great strength and beauty, its component parts being
arranged in such a manner as to envelope the root of each individual
feather. The network in question supports the feathers, and limits
their peculiar valvular action. It is enlarged at figs. 99 and
101, and consists of three longitudinal bands, _r s_, _t u_, _v
w_. Between these bands two oblique bands, _g_ and _h_, run; the
oblique bands occurring between every two feathers. The marginal
longitudinal band (_v_, _w_) splits up into two processes, one of
which curves round the root of each feather (_x_) in a direction from
right to left (_c_, _b_, _a_), the other in a direction from left to
right (_d_, _e_, _f_). These processes are also seen at _m_, _n_ of
fig. 100.--_Original._]

The manner in which the roots of the primary, secondary, and tertiary feathers are geared to each other in order to rotate in one direction in flexion, and in another and opposite direction in extension, is shown at figs. 98, 99, 100, and 101, p. 181. In flexion the feathers open up and permit the air to pass between them. In extension they flap together and render the wing as air-tight as that of either the insect or bat. The primary, secondary, and tertiary feathers have consequently a valvular action.

_The Wing of the Bird not always opened up to the same extent in the Up Stroke._--The elaborate arrangements and adaptations for increasing the area of the wing, and making it impervious to air during the down stroke, and for decreasing the area and opening up the wing during the up stroke, although necessary to the flight of the heavy-bodied, short-winged birds, as the grouse, partridge, and pheasant, are by no means indispensable to the flight of the long-winged oceanic birds, unless when in the act of rising from a level surface; neither do the short-winged heavy birds require to fold and open up the wing during the up stroke to the same extent in all cases, less folding and opening up being required when the birds fly against a breeze, and when they have got fairly under weigh. All the oceanic birds, even the albatross, require to fold and flap their wings vigorously when they rise from the surface of the water. When, however, they have acquired a certain degree of momentum, and are travelling at a tolerable horizontal speed, they can in a great measure dispense with the opening up of the wing during the up stroke--nay, more, they can in many instances dispense even with flapping. This is particularly the case with the albatross, which (if a tolerably stiff breeze be blowing) can sail about for an hour at a time without once flapping its wings. In this case the wing is wielded in one piece like the insect wing, the bird simply screwing and unscrewing the pinion on and off the wind, and exercising a restraining influence--the breeze doing the principal part of the work. In the bat the wing is jointed as in the bird, and folded during the up stroke. As, however, the bat’s wing, as has been already stated, is covered by a continuous and more or less elastic membrane, it follows that it cannot be opened up to admit of the air passing through it during the up stroke. Flight in the bat is therefore secured by alternately diminishing and increasing the area of the wing during the up and down strokes--the wing rotating upon its root and along its anterior margin, and presenting a variety of kite-like surfaces, during its ascent and descent, precisely as in the bird (fig. 80, p. 149, and fig. 83, p. 158).

FIG. 102.--Shows the upward inclination of the body and the flexed
condition of the wings (_a b_, _e f_; _a´ b´_, _e´ f´_) in the
flight of the kingfisher. The body and wings when taken together
form a kite. Compare with fig. 59, p. 126, where the wings are fully
extended.]

_Flexion of the Wing necessary to the Flight of Birds._--Considerable diversity of opinion exists as to whether birds do or do not flex their wings in flight. The discrepancy is owing to the great difficulty experienced in analysing animal movements, particularly when, as in the case of the wings, they are consecutive and rapid. My own opinion is, that the wings are flexed in flight, but that all wings are not flexed to the same extent, and that what holds true of one wing does not necessarily hold true of another. To see the flexing of the wing properly, the observer should be either immediately above the bird or directly beneath it. If the bird be contemplated from before, behind, or from the side, the up and down strokes of the pinion distract the attention and complicate the movement to such an extent as to render the observation of little value. In watching rooks proceeding leisurely against a slight breeze, I have over and over again satisfied myself that the wings are flexed during the up stroke, the mere extension and flexion, with very little of a down stroke, in such instances sufficing for propulsion. I have also observed it in the pigeon in full flight, and likewise in the starling, sparrow, and kingfisher (fig. 102, p. 183).

It occurs principally at the wrist-joint, and gives to the wing the peculiar quiver or tremor so apparent in rapid flight, and in young birds at feeding-time. The object to be attained is manifest. By the flexing of the wing in flight, the “_remiges_,” or rowing feathers, are opened up or thrown out of position, and the air permitted to escape--advantage being thus taken of the peculiar action of the individual feathers and the higher degree of differentiation perceptible in the wing of the bird as compared with that of the bat and insect.

In order to corroborate the above opinion, I extended the wings of several birds as in rapid flight, and fixed them in the outspread position by lashing them to light unyielding reeds. In these experiments the shoulder and elbow-joints were left quite free--the wrist or carpal and the metacarpal joints only being bound. I took care, moreover, to interfere as little as possible with the action of the elastic ligament or alar membrane which, in ordinary circumstances, recovers or flexes the wing, the reeds being attached for the most part to the primary and secondary feathers. When the wings of a pigeon were so tied up, the bird could not rise, although it made vigorous efforts to do so. When dropped from the hand, it fell violently upon the ground, notwithstanding the strenuous exertions which it made with its pinions to save itself. When thrown into the air, it fluttered energetically in its endeavours to reach the dove-cot, which was close at hand; in every instance, however, it fell, more or less heavily, the distance attained varying with the altitude to which it was projected.

Thinking that probably the novelty of the situation and the strangeness of the appliances confused the bird, I allowed it to walk about and to rest without removing the reeds. I repeated the experiment at intervals, but with no better results. The same phenomena, I may remark, were witnessed in the sparrow; so that I think there can be no doubt that a certain degree of flexion in the wings is indispensable to the flight of all birds--the amount varying according to the length and form of the pinions, and being greatest in the short broad-winged birds, as the partridge and kingfisher, less in those whose wings are moderately long and narrow, as the gulls, and many of the oceanic birds, and least in the heavy-bodied long and narrow-winged sailing or gliding birds, the best example of which is the albatross. The degree of flexion, moreover, varies according as the bird is rising, falling, or progressing in a horizontal direction, it being greatest in the two former, and least in the latter.

It is true that in insects, unless perhaps in those which fold or close the wing during repose, no flexion of the pinion takes place in flight; but this is no argument against this mode of diminishing the wing-area during the up stroke where the joints exist; and it is more than probable that when joints are present they are added to augment the power of the wing during its active state, _i.e._ during flight, quite as much as to assist in arranging the pinion on the back or side of the body when the wing is passive and the animal is reposing. The flexion of the wing is most obvious when the bird is exerting itself, and may be detected in birds which skim or glide when they are rising, or when they are vigorously flapping their wings to secure the impetus necessary to the gliding movement. It is less marked at the elbow-joint than at the wrist; and it may be stated generally that, as flexion _decreases_, the twisting flail-like movement of the wing at the shoulder _increases_, and _vice versâ_,--the great difference between sailing birds and those which do not sail amounting to this, that in the sailing birds the wing is worked from the shoulder by being alternately rolled on and off the wind, as in insects; whereas, in birds which do not glide, the spiral movement travels along the arm as in bats, and manifests itself during flexion and extension in the bending of the joints and in the rotation of the bones of the wing on their axes. The spiral conformation of the pinions, to which allusion has been so frequently made, is best seen in the heavy-bodied birds, as the turkey, capercailzie, pheasant, and partridge; and here also the concavo-convex form of the wing is most perceptible. In the light-bodied, ample-winged birds, the amount of twisting is diminished, and, as a result, the wing is more or less flattened, as in the sea-gull (fig. 103).

FIG. 103.--Shows the twisted levers or screws formed by the wings of
the gull. Compare with fig. 53, p. 107; with figs. 76, 77, and 78,
p. 147, and with figs. 82 and 83, p. 158.---_Original._]

_Consideration of the Forces which propel the Wings of Insects._--In the thorax of insects the muscles are arranged in two principal sets in the form of a cross--_i.e._ there is a powerful vertical set which runs from above downwards, and a powerful antero-posterior set which runs from before backwards. There are likewise a few slender muscles which proceed in a more or less oblique direction. The antero-posterior and vertical sets of muscles are quite distinct, as are likewise the oblique muscles. Portions, however, of the vertical and oblique muscles terminate at the root of the wing in jelly-looking points which greatly resemble rudimentary tendons, so that I am inclined to believe that the vertical and oblique muscles exercise a direct influence on the movements of the wing. The shortening of the antero-posterior set of muscles (indirectly assisted by the oblique ones) elevates the dorsum of the thorax by causing its anterior extremity to approach its posterior extremity, and by causing the thorax to bulge out or expand laterally. This change in the thorax necessitates the descent of the wing. The shortening of the vertical set (aided by the oblique ones) has a precisely opposite effect, and necessitates its ascent. While the wing is ascending and descending the oblique muscles cause it to rotate on its long axis, the bipartite division of the wing at its root, the spiral configuration of the joint, and the arrangement of the elastic and other structures which connect the pinion with the body, together with the resistance it experiences from the air, conferring on it the various angles which characterize the down and up strokes. The wing may therefore be said to be depressed by the shortening of the antero-posterior set of muscles, aided by the oblique muscles, and elevated by the shortening of the vertical and oblique muscles, aided by the elastic ligaments, and the reaction of the air. If we adopt this view we have a perfect physiological explanation of the phenomenon, as we have a complete circle or cycle of motion, the antero-posterior set of muscles shortening when the vertical set of muscles are elongating, and _vice versâ_. This, I may add, is in conformity with all other muscular arrangements, where we have what are usually denominated extensors and flexors, pronators and supinators, abductors and adductors, etc., but which, as I have already explained (pp. 24 to 34), are simply the two halves of a circle of muscle and of motion, an arrangement for securing diametrically opposite movements in the travelling surfaces of all animals.

Chabrier’s account, which I subjoin, virtually supports this hypothesis:--

“It is generally through the intervention of the proper motions of the dorsum, which are very considerable during flight, that the wings or the elytra are moved equally and simultaneously. Thus, when it is elevated, it carries with it the internal side of the base of the wings with which it is articulated, from which ensues the depression of the external side of the wing; and when it approaches the sternal portion of the trunk, the contrary takes place. During the depression of the wings, the dorsum is curved from before backwards, or in such a manner that its anterior extremity is brought nearer to its posterior, that its middle is elevated, and its lateral portions removed further from each other. The reverse takes place in the elevation of the wings; the anterior extremity of the dorsum being removed to a greater distance from the posterior, its middle being depressed, and its sides brought nearer to each other. Thus its bending in one direction produces a diminution of its curve in the direction normally opposed to it; and by the alternations of this motion, assisted by other means, the body is alternately compressed and dilated, and the wings are raised and depressed by turns.”[86]

[86] Chabrier, as rendered by E. F. Bennett, F.L.S., etc.

In the _libellulæ_ or dragon-flies, the muscles are inserted into the roots of the wings as in the bat and bird, the only difference being that in the latter the muscles creep along the wings to their extremities.

In all the wings which I have examined, whether in the insect, bat, or bird, the wings are recovered, flexed, or drawn towards the body by the action of elastic ligaments, these structures, by their mere contraction, causing the wings, when fully extended and presenting their maximum of surface, to resume their position of rest, and plane of least resistance. The principal effort required in flight would therefore seem to be made during extension and the down stroke. The elastic ligaments are variously formed, and the amount of contraction which they undergo is in all cases accurately adapted to the size and form of the wings, and the rapidity with which they are worked--the contraction being greatest in the short-winged and heavy-bodied insects and birds, and least in the light-bodied and ample-winged ones, particularly in such as skim or glide. The mechanical action of the elastic ligaments, I need scarcely remark, insures a certain period of repose to the wings at each stroke, and this is a point of some importance, as showing that the lengthened and laborious flights of insects and birds are not without their stated intervals of rest.

_Speed attained by Insects._--Many instances might be quoted of the marvellous powers of flight possessed by insects as a class. The male of the silkworm-moth (_Attacus Paphia_) is stated to travel more than 100 miles a day;[87] and an anonymous writer in Nicholson’s Journal[88] calculates that the common house-fly (_Musca domestica_), in ordinary flight, makes 600 strokes per second, and advances twenty-five feet, but that the rate of speed, if the insect be alarmed, may be increased six or seven fold, so that under certain circumstances it can outstrip the fleetest racehorse. Every one when riding on a warm summer day must have been struck with the cloud of flies which buzz about his horse’s ears even when the animal is urged to its fastest paces; and it is no uncommon thing to see a bee or a wasp endeavouring to get in at the window of a railway car in full motion. If a small insect like a fly can outstrip a racehorse, an insect as large as a horse would travel very much faster than a cannon-ball. Leeuwenhoek relates a most exciting chase which he once beheld in a menagerie about 100 feet long between a swallow and a dragon-fly (_Mordella_). The insect flew with incredible speed, and wheeled with such address, that the swallow, notwithstanding its utmost efforts, completely failed to overtake and capture it.[89]

[87] Linn. Trans. vii. p. 40.

[88] Vol. iii. p. 36.

[89] “The hobby falcon, which abounds in Bulgaria during the summer
months, hawks _large dragonflies_, which it seizes with the foot and
devours whilst in the air. It also kills swifts, larks, turtle-doves,
and bee-birds, although more rarely.”--Falconry in the British Isles,
by Francis Henry Salvin and William Brodrick. Lond. 1855.