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.
_Consideration of the Forces which propel the Wings of Bats and
Birds._--The muscular system of birds has been so frequently and
faithfully described, that I need not refer to it further than to say
that there are muscles which by their action are capable of elevating
and depressing the wings, and of causing them to move in a forward and
backward direction, and obliquely. They can also extend or straighten
and bend, or flex the wings, and cause them to rotate in the direction
of their length during the down and up strokes. The muscles principally
concerned in the elevation of the wings are the smaller pectoral
or breast muscles (_pectorales minor_); those chiefly engaged in
depressing the wings are the larger pectorals (_pectorales major_). The
pectoral muscles correspond to the fleshy mass found on the breast-bone
or sternum, which in flying birds is boat-shaped, and furnished with
a keel. These muscles are sometimes so powerful and heavy that they
outweigh all the other muscles of the body. The power of the bird is
thus concentrated for the purpose of moving the wings and conferring
steadiness upon the volant mass. In birds of strong flight the keel
is very large, in order to afford ample attachments for the muscles
delegated to move the wings. In birds which cannot fly, as the members
of the ostrich family, the breast-bone or sternum has no keel.[90]
[90] One of the best descriptions of the bones and muscles of the
bird is that given by Mr. Macgillivray in his very admirable,
voluminous, and entertaining work, entitled History of British Birds.
Lond. 1837.
The remarks made regarding the muscles of birds, apply with very slight
modifications to the muscles of bats. The muscles of bats and birds,
particularly those of the wings, are geared to, and act in concert
with, elastic ligaments or membranes, to be described presently.
_Lax condition of the Shoulder-Joint in Bats, Birds, etc._--The great
laxity of the shoulder-joint in bats and birds, readily admits of their
bodies falling downwards and forwards during the up stroke. This joint,
as has been already stated, admits of movement in every direction, so
that the body of the bat or bird is like a compass set upon gimbals,
_i.e._ it swings and oscillates, and is equally balanced, whatever
the position of the wings. The movements of the shoulder-joint in
the bird, bat, and insect are restrained within certain limits by a
system of check ligaments and prominences; but in each case the range
of motion is very great, the wings being permitted to swing forwards,
backwards, upwards, downwards, or at any degree of obliquity. They are
also permitted to rotate along their anterior margin, or to twist in
the direction of their length to the extent of nearly a quarter of a
turn. This great freedom of movement at the shoulder-joint enables the
insect, bat, and bird to rotate and balance upon two centres--the one
running in the direction of the length of the body, the other at right
angles or across the body, _i.e._ in the direction of the length of the
wings.
In the bird the head of the humerus is convex and somewhat oval (not
round), the long axis of the oval being directed from above downwards,
_i.e._ from the dorsal towards the ventral aspect of the bird. The
humerus can, therefore, _glide up and down_ in the _facettes_ occurring
on the articular ends of the coracoid and scapular bones with great
facility, much in the same way that the head of the radius glides upon
the distal end of the humerus. But the humerus has another motion; it
moves _like a hinge from before backwards, and_ vice versâ. The axis of
the latter movement is almost at right angles to that of the former.
As, however, the shoulder-joint is connected by long ligaments to the
body, and can be drawn away from it to the extent of one-eighth of
an inch or more, it follows that _a third and twisting movement can
be performed_, the twisting admitting of rotation to the extent of
something like a quarter of a turn. In raising and extending the wing
preparatory to the downward stroke two opposite movements are required,
viz. one from before backwards, and another from below upwards. As,
however, the axes of these movements are at nearly right angles to each
other, a spiral or twisting movement is necessary to run the one into
the other--to turn the corner, in fact.
From what has been stated it will be evident that the movements of the
wing, particularly at the root, are remarkably free, and very varied. A
directing and restraining, as well as a propelling force, is therefore
necessary.
The guiding force is to be found in the voluntary muscles which connect
the wing with the body in the insect, and which in the bat and bird, in
addition to connecting the wing with the body, extend along the pinion
even to its tip. It is also to be found in the musculo-elastic and
other ligaments, seen to advantage in the bird.
_The Wing flexed and partly elevated by the Action of Elastic
Ligaments--the Nature and Position of such Ligaments in the Pheasant,
Snipe, Crested Crane, Swan, etc._--When the wing is drawn away from
the body of the bird by the hand the posterior margin of the pinion
formed by the primary, secondary, and tertiary feathers rolls down
to make a variety of inclined surfaces with the horizon (_c b_, of
fig. 63, p. 138). When, however, the hand is withdrawn, even in the
dead bird, the wing instantly folds up; and in doing so reduces the
amount of inclination in the several surfaces referred to (_c b_, _d
e f_ of the same figure). The wing is folded by the action of certain
elastic ligaments, which are put upon the stretch in extension, and
which recover their original form and position in flexion (fig. 98,
_c_, p. 181). This simple experiment shows that the various inclined
surfaces requisite for flight are produced by the mere acts of
extension and flexion in the dead bird. It is not, however, to be
inferred from this circumstance that flight can be produced without
voluntary movements any more than ordinary walking. The muscles,
bones, ligaments, feathers, etc., are so adjusted with reference to
each other that if the wing is moved at all, it must move in the
proper direction--an arrangement which enables the bird to fly without
thinking, just as we can walk without thinking. There cannot, however,
be a doubt that the bird has the power of controlling its wings both
during the down and up strokes; for how otherwise could it steer and
direct its course with such precision in obtaining its food? how fix
its wings on a level with or above its body for skimming purposes? how
fly in a curve? how fly with, against, or across a breeze? how project
itself from a rock directly into space, or how elevate itself from a
level surface by the laboured action of its wings?
The wing of the bird is elevated to a certain extent in flight by the
reaction of the air upon its under surface; but it is also elevated by
muscular action--by the contraction of the elastic ligaments, and by
the body falling downwards and forwards in a curve.
That muscular action is necessary is proved by the fact that the pinion
is supplied with distinct elevator muscles.[91] It is further proved
by this, that the bird can, and always does, elevate its wings prior
to flight, quite independently of the air. When the bird is fairly
launched in space the elevator muscles are assisted by the tendency
which the body has to fall downwards and forwards: by the reaction of
the air; and by the contraction of the elastic ligaments. The air and
the elastic ligaments contribute to the elevation of the wing, but both
are obviously under control--they, in fact, form links in a chain of
motion which at once begins and terminates in the muscular system.
[91] Mr. Macgillivray and C. J. L. Krarup, a Danish author, state
that the wing is elevated by a vital force, viz. by the contraction
of the _pectoralis minor_. This muscle, according to Krarup, acts
with one-eighth the intensity of the _pectoralis major_ (the
depressor of the wing). He bases his statement upon the fact that
in the pigeon the pectoralis minor or elevator of the wing weighs
one-eighth of an ounce, whereas the pectoralis major or depressor
of the wing weighs seven-eighths of an ounce. It ought, however, to
be borne in mind that the volume of a muscle does not necessarily
determine the precise influence exerted by its action; for the
tendon of the muscle may be made to act upon a long lever, and,
under favourable conditions, for developing its powers, while that
of another muscle may be made to act upon a short lever, and,
consequently, under unfavourable conditions.--On the Flight of Birds,
p. 30. Copenhagen, 1869.
That the elastic ligaments are subsidiary and to a certain extent under
the control of the muscular system in the same sense that the air is,
is evident from the fact that voluntary muscular fibres run into the
ligaments in question at various points (_a_, _b_ of fig. 98, p. 181).
The ligaments and muscular fibres act in conjunction, and fold or flex
the forearm on the arm. There are others which flex the hand upon the
forearm. Others draw the wing towards the body.
The elastic ligaments, while occupying a similar position in the wings
of all birds, are variously constructed and variously combined with
voluntary muscles in the several species.
_The Elastic Ligaments more highly differentiated in Wings which
vibrate rapidly._--The elastic ligaments of the swan are more
complicated and more liberally supplied with voluntary muscle than
those of the crane, and this is no doubt owing to the fact that the
wings of the swan are driven at a much higher speed than those of
the crane. In the snipe the wings are made to vibrate very much more
rapidly than in the swan, and, as a consequence, we find that the
fibro-elastic bands are not only greatly increased, but they are also
geared to a much greater number of voluntary muscles, all which seems
to prove that the musculo-elastic apparatus employed for recovering or
flexing the wing towards the end of the down stroke, becomes more and
more highly differentiated in proportion to the rapidity with which
the wing is moved.[92] The reason for this is obvious. If the wing is
to be worked at a higher speed, it must, as a consequence, be more
rapidly flexed and extended. The rapidity with which the wing of the
bird is extended and flexed is in some instances exceedingly great; so
great, in fact, that it escapes the eye of the ordinary observer. The
speed with which the wing darts in and out in flexion and extension
would be quite inexplicable, but for a knowledge of the fact that the
different portions of the pinion form angles with each other, these
angles being instantly increased or diminished by the slightest quiver
of the muscular and fibro-elastic systems. If we take into account the
fact that the wing of the bird is recovered or flexed by the combined
action of voluntary muscles and elastic ligaments; that it is elevated
to a considerable extent by voluntary muscular effort; and that it is
extended and depressed entirely by muscular exertion, we shall have
difficulty in avoiding the conclusion that the wing is thoroughly under
the control of the muscular system, not only in flexion and extension,
but also throughout the entire down and up strokes.
[92] A careful account of the musculo-elastic structures occurring in
the wing of the pigeon is given by Mr. Macgillivray in his History of
British Birds, pp. 37, 38.
An arrangement in every respect analogous to that described in the bird
is found in the wing of the bat, the covering or web of the wing in
this instance forming the principal elastic ligament (fig. 17, p. 36).
_Power of the Wing--to what owing._--The shape and power of the
pinion depend upon one of three circumstances--to wit, the length
of the humerus,[93] the length of the cubitus or forearm, and the
length of the primary feathers. In the swallow the humerus, and in
the humming-bird the cubitus, is very short, the primaries being very
long; whereas in the albatross the humerus or arm-bone is long and the
primaries short. When one of these conditions is fulfilled, the pinion
is usually greatly elongated and scythe-like (fig. 62, p. 137)--an
arrangement which enables the bird to keep on the wing for immense
periods with comparatively little exertion, and to wheel, turn, and
glide about with exceeding ease and grace. When the wing is truncated
and rounded (fig. 96, p. 176), a form of pinion usually associated
with a heavy body, as in the grouse, quail, diver, and grebe, the
muscular exertion required, and the rapidity with which the wing moves
are very great; those birds, from a want of facility in turning, flying
either in a straight line or making large curves. They, moreover, rise
with difficulty, and alight clumsily and somewhat suddenly. Their
flight, however, is perfect while it lasts.
[93] “The humerus varies extremely in length, being very short in
the swallow, of moderate length in the gallinaceous birds, longer
in the crows, very long in the gannets, and unusually elongated in
the albatross. In the golden eagle it is also seen to be of great
length.”--Macgillivray’s British Birds, vol. i. p. 30.
The goose, duck (fig. 107, p. 204), pigeon (fig. 106, p. 203) and crow,
are intermediate both as regards the form of the wing and the rapidity
with which it is moved.
The heron (fig. 60, p. 126) and humming-bird furnish extreme examples
in another direction,--the heron having a large wing with a leisurely
movement, the humming-bird a comparatively large wing with a greatly
accelerated one.
But I need not multiply examples; suffice it to say that flight may be
attained within certain limits by every size and form of wing, if the
number of its oscillations be increased in proportion to the weight to
be raised.
_Reasons why the effective Stroke should be delivered downwards and
forwards._--The wings of all birds, whatever their form, act by
alternately presenting oblique and comparatively non-oblique surfaces
to the air,--the mere extension of the pinion, as has been shown,
causing the primary, secondary, and tertiary feathers to roll down till
they make an angle of 30° or so with the horizon, in order to prepare
it for giving the effective stroke, which is delivered, with great
rapidity and energy, in a _downward_ and _forward_ direction. I repeat,
“downwards and forwards;” for a careful examination of the relations of
the wing in the dead bird, and a close observation of its action in the
living one, supplemented by a large number of experiments with natural
and artificial wings, have fully convinced me that the stroke is
invariably delivered in this direction.[94] If the wing did not strike
downwards and _forwards_, it would act at a manifest disadvantage:--
[94] _Prevailing Opinions as to the Direction of the Down
Stroke._--Mr. Macgillivray, in his History of British Birds,
published in 1837, states (p. 34) that in flexion the wing is drawn
upwards, forwards, and inwards, but that during extension, when the
effective stroke is given, it is made to strike outwards, downwards,
and _backwards_. The Duke of Argyll holds a similar opinion. In
speaking of the hovering of birds, he asserts that “if a bird, by
altering the axis of its own body, can direct its wing stroke in some
degree _forwards_, it will have the effect of _stopping_ instead
of promoting progression;” and that, “Except for the purpose of
_arresting_ their flight, birds can never strike except _directly
downwards_--that is, directly against the opposing force of
gravity.”--Good Words, Feb. 1865, p. 132.
Mr. Bishop, in the Cyc. of Anat. and Phys., vol. iii. p. 425, says,
“In consequence of the planes of the wings being disposed either
_perpendicularly_ or _obliquely backwards_ to the direction of
their motion, a corresponding impulse is given to their centre of
gravity.” Professor Owen, in like manner, avers that “a downward
stroke would only tend to raise the bird in the air; to carry it
forwards, the wings require to be moved in an oblique plane, so as to
_strike backwards_ as well as downwards.”--Comp. Anat. and Phys. of
Vertebrates, vol. ii. p. 115.
The following is the account given by M. E. Liais:--“When a bird is
about to depress its wing, this is a little inclined from before
backwards. When the descending movement commences, the wing does not
descend parallel to itself in a direction from before backwards;
but the movement is accompanied by a rotation of several degrees
round the anterior edge, so that the wing becomes more in front
than behind, and the _descending movement is transferred more and
more backwards_.... When the wing has completely descended, it
is both _further back_ and lower than at the commencement of the
movement.”--“On the Flight of Birds and Insects.” Annals of Nat.
Hist. vol. xv. 3d series, p. 156.
_1st._ Because it would present the back or convex surface of the wing
to the air--a convex surface dispersing or dissipating the air, while a
concave surface gathers it together or focuses it.
_2d._ In order to strike backwards effectually, the concavity of the
wing would also require to be turned backwards; and this would involve
the depression of the anterior or thick margin of the pinion, and the
elevation of the posterior or thin one, during the down stroke, which
never happens.
_3d._ The strain to which the pinion is subjected in flight would, if
the wing struck _backwards_, fall, not on the anterior or strong margin
of the pinion formed by the bones and muscles, but on the posterior or
weak margin formed by the tips of the primary, secondary, and tertiary
feathers--which is not in accordance with the structure of the parts.
_4th._ The feathers of the wing, instead of being closed, as they
necessarily are, by a downward and _forward_ movement, would be
inevitably opened, and the integrity of the wing impaired by a downward
and _backward_ movement.
_5th._ The disposition of the articular surfaces of the wing
(particularly that of the shoulder-joint) is such as to facilitate the
downward and _forward_ movement, while it in a great measure prevents
the downward and _backward_ one.
_6th and lastly._ If the wing did in reality strike downwards and
_backwards_, a result the converse of that desired would most assuredly
be produced, as an oblique surface which smites the air in a downward
and _backward_ direction (if left to itself) tends to depress the body
bearing it. This is proved by the action upon the air of free inclined
planes, arranged in the form of a screw.
_The Wing acts as an Elevator, Propeller, and Sustainer, both during
extension and flexion._--The wing, as has been explained, is recovered
or drawn off the wind principally by the contraction of the elastic
ligaments extending between the joints, so that the pinion during
flexion enjoys a certain degree of repose. The time occupied in
recovering is not lost so long as the wing makes an angle with the
horizon and the bird is in motion, it being a matter of indifference
whether the wing acts on the air, or the air on the wing, so long as
the body bearing the latter is under weigh; and this is the chief
reason why the albatross, which is a very heavy bird,[95] can sail
about for such incredible periods without flapping the wings at
all. Captain Hutton thus graphically describes the sailing of this
magnificent bird:--“The flight of the albatross is truly majestic, as
with outstretched motionless wings he sails over the surface of the
sea--now rising high in air, now with a bold sweep, and wings inclined
at an angle with the horizon, descending until the tip of the lower one
all but touches the crest of the waves as he skims over them.”[96]
[95] The average weight of the albatross, as given by Gould, is 17
lbs.--Ibis, 2d series, vol. i. 1865, p. 295.
[96] “On some of the Birds inhabiting the Southern Ocean,” by Capt.
F. W. Hutton.--Ibis, 2d series, vol. i. 1865, p. 282.
_Birds of Flight divisible into four kinds:--_
_1st._ Such as have heavy bodies and short wings with a rapid movement
(fig. 59, p. 126).
_2d._ Such as have light bodies and large wings with a leisurely
movement (fig. 60, p. 126; fig. 103, p. 186).
_3d._ Such as have heavy bodies and long narrow wings with a decidedly
slow movement (fig. 105, p. 200).
_4th._ Such as are intermediate with regard to the size of body,
the dimensions of the wing, and the energy with which it is driven
(fig. 102, p. 183; fig. 106, p. 203; fig. 107, p. 204).
They may be subdivided into those which float, skim, or glide, and
those which fly in a straight line and irregularly.
The pheasant, partridge (fig. 59, p. 126), grouse, and quail, furnish
good examples of the heavy-bodied, short-winged birds. In these the
wing is rounded and deeply concave. It is, moreover, wielded with
immense velocity and power.
The heron (fig. 60, p. 126), sea-mew (fig. 103, p. 186), lapwing
(fig. 63, p. 138), and owl (fig. 104), supply examples of the second
class, where the wing, as compared with the body, is very ample, and
where consequently it is moved more leisurely and less energetically.
[Illustration:
FIG. 104.--The Cape Barn-Owl (_Strix capensis_, Smith), as seen
in full flight, hunting. The under surface of the wings and body
are inclined slightly upwards, and act upon the air after the
manner of a kite. (Compare with fig. 59, p. 126, and fig. 102,
p. 183.)--_Original._]
The albatross (fig. 105, p. 200) and pelican afford instances of the
third class, embracing the heavy-bodied, long-winged birds.
The duck (fig. 107, p. 204), pigeon (fig. 106, p. 203), crow and
thrush, are intermediate, both as regards the size of the wing and
the rapidity with which it is made to oscillate. These constitute the
fourth class.
The albatross (fig. 105, p. 200), swallow, eagle, and hawk, provide
instances of sailing or gliding birds, where the wing is ample,
elongated, and more or less pointed, and where advantage is taken of
the weight of the body and the shape of the pinion to utilize the air
as a supporting medium. In these the pinion acts as a long lever,[97]
and is wielded with great precision and power, particularly at the
shoulder.