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.
[Illustration: FIG. 89. FIG. 90.
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._]
[Illustration:
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.
[Illustration: FIG. 92.
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._]
[Illustration: FIG. 93.
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).
[Illustration: FIG. 94.
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._]
[Illustration: FIG. 95.
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).
[Illustration: FIG. 96.
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._]
[Illustration: FIG. 97.
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.