been expressed at the enormous rapidity with which some wings are made
to vibrate. The wing of the insect is, as a rule, very long and narrow.
As a consequence, a comparatively slow and very limited movement at
the root confers great range and immense speed at the tip; the speed
of each portion of the wing increasing as the root of the wing is
receded from. This is explained on a principle well understood in
mechanics, viz. that when a rod hinged at one end is made to move in a
circle, the tip or free end of the rod describes a much wider circle
_in a given time_ than a portion of the rod nearer the hinge. This
principle is illustrated at fig. 56. Thus if _a b_ of fig. 56 be made
to represent the rod hinged at _x_, it travels through the space _d b
f_ in the same time it travels through _j k l_; and through _j k l_ in
the same time it travels through _g h i_; and through _g h i_ in the
same time it travels through _e a c_, which is the area occupied by
the thorax of the insect. If, however, the part of the rod _b_ travels
through the space _d b f_ in the same time that the part _a_ travels
through the space _e a c_, it follows of necessity that the portion of
the rod marked _a_ moves very much slower than that marked _b_. The
muscles of the insect are applied at the point _a_, as short levers
(the point referred to corresponding to the thorax of the insect), so
that a comparatively slow and limited movement at the root of the wing
produces the marvellous speed observed at the tip; the tip and body of
the wing being those portions which occasion the blur or impression
produced on the eye by the rapidly oscillating pinion (figs. 64, 65,
and 66, p. 139), But for this mode of augmenting the speed originally
inaugurated by the muscular system, it is difficult to comprehend how
the wings could be driven at the velocity attributed to them. The wing
of the blow-fly is said to make 300 strokes per second, _i.e._ 18,000
per minute. Now it appears to me that muscles to contract at the rate
of 18,000 times in the minute would be exhausted in a very few seconds,
a state of matters which would render the continuous flight of insects
impossible. (The heart contracts only between sixty and seventy times
in a minute.) I am, therefore, disposed to believe that the number of
contractions made by the thoracic muscles of insects has been greatly
overstated; the high speed at which the wing is made to vibrate being
due less to the separate and sudden contractions of the muscles at its
root than to the fact that the speed of the different parts of the wing
is increased in a direct ratio as the several parts are removed from
the driving point, as already explained. Speed is certainly a matter
of great importance in wing movements, as the elevating and propelling
power of the pinion depends to a great extent upon the rapidity with
which it is urged. Speed, however, may be produced in two ways--either
by a series of separate and opposite movements, such as is witnessed
in the action of a piston, or by a series of separate and opposite
movements acting upon an instrument so designed, that a movement
applied at one part increases in rapidity as the point of contact
is receded from, as happens in the wing. In the piston movement the
motion is uniform, or nearly so; all parts of the piston travelling at
very much the same speed. In the wing movements, on the contrary, the
motion is gradually accelerated towards the tip of the pinion, where
the pinion is most effective as an elevator, and decreased towards the
root, where it is least effective--an arrangement calculated to reduce
the number of muscular contractions, while it contributes to the actual
power of the wing. This hypothesis, it will be observed, guarantees
to the wing a very high speed, with comparatively few reversals and
comparatively few muscular contractions.
In the bat and bird the wings do not vibrate with the same rapidity as
in the insect, and this is accounted for by the circumstance, that in
them the muscles do not act exclusively at the root of the wing. In the
bat and bird the muscles run along the wing towards the tip for the
purpose of flexing or folding the wing prior to the up stroke, and for
opening out and expanding it prior to the down stroke.
As the wing must be folded or flexed and opened out or expanded every
time the wing rises and falls, and as the muscles producing flexion
and extension are long muscles with long tendons, which act at long
distances as long levers, and comparatively slowly, it follows that
the great short muscles (pectorals, etc.) situated at the root of the
wing must act slowly likewise, as the muscles of the thorax and wing
of necessity act together to produce one pulsation or vibration of the
wing. What the wing of the bat and bird loses in speed it gains in
power, the muscles of the bat and bird’s wing acting directly upon the
points to be moved, and under the most favourable conditions. In the
insect, on the contrary, the muscles act indirectly, and consequently
at a disadvantage. If the pectorals only moved, they would act as
short levers, and confer on the wing of the bat and bird the rapidity
peculiar to the wing of the insect.
The tones emitted by the bird’s wing would in this case be heightened.
The swan in flying produces a loud whistling sound, and the pheasant,
partridge, and grouse a sharp whirring noise like the stone of a
knife-grinder.
It is a mistake to suppose, as many do, that the tone or note produced
by the wing during its vibrations is a true indication of the number
of beats made by it in any given time. This will be at once understood
when I state, that a long wing will produce a higher note than a
shorter one driven at the same speed and having the same superficial
area, from the fact that the tip and body of the long wing will move
through a greater space in a given time than the tip and body of the
shorter wing. This is occasioned by all wings being jointed at their
roots, the sweep made by the different parts of the wing in a given
time being longer or shorter in proportion to the length of the pinion.
It ought, moreover, not to be overlooked, that in insects the notes
produced are not always referable to the action of the wings, these, in
many cases, being traceable to movements induced in the legs and other
parts of the body.
It is a curious circumstance, that if portions be removed from the
posterior margins of the wings of a buzzing insect, such as the wasp,
bee, blue-bottle fly, etc., the note produced by the vibration of
the pinions is raised in pitch. This is explained by the fact, that
an insect whose wings are curtailed requires to drive them at a much
higher speed in order to sustain itself in the air. That the velocity
at which the wing is urged is instrumental in causing the sound, is
proved by the fact, that in slow-flying insects and birds no note
is produced; whereas in those which urge the wing at a high speed,
a note is elicited which corresponds within certain limits to the
number of vibrations and the form of the wing. It is the posterior or
thin flexible margin of the wing which is more especially engaged in
producing the sound; and if this be removed, or if this portion of the
wing, as is the case in the bat and owl, be constructed of very soft
materials, the character of the note is altered. An artificial wing,
if properly constructed and impelled at a sufficiently high speed,
emits a drumming noise which closely resembles the note produced by the
vibration of short-winged, heavy-bodied birds, all which goes to prove
that sound is a concomitant of rapidly vibrating wings.
_The Wing area Variable and in Excess._--The travelling-surfaces of
insects, bats, and birds greatly exceed those of fishes and swimming
animals; the travelling-surfaces of swimming animals being greatly
in excess of those of animals which walk and run. The wing area of
insects, bats, and birds varies very considerably, flight being
possible within a comparatively wide range. Thus there are light-bodied
and large-winged insects and birds--as the butterfly (fig. 57) and
heron (fig. 60, p. 126); and others whose bodies are comparatively
heavy, while their wings are insignificantly small--as the sphinx moth
and Goliath beetle (fig. 58) among insects, and the grebe, quail, and
partridge (fig. 59, p. 126) among birds.
[Illustration:
FIG. 57.--Shows a butterfly with comparatively very large wings.
The nervures are seen to great advantage in this specimen; and the
enormous expanse of the pinions readily explains the irregular
flight of the insect on the principle of recoil. _a_ Anterior
wing. _b_ Posterior wing. _e_ Anterior margin of wing. _f_ Ditto
posterior margin. _g_ Ditto outer margin. Compare with beetle, fig.
58.--_Original._]
[Illustration:
FIG. 58.--Under-surface of large beetle (_Goliathus micans_), with
deeply concave and comparatively small wings (compare with butterfly,
fig. 57), shows that the nervures (_r_, _d_, _e_, _f_, _n_, _n_, _n_)
of the wings of the beetle are arranged along the anterior margins
and throughout the substance of the wings generally, very much as the
bones of the arm, forearm, and hand, are in the wings of the bat, to
which they bear a very marked resemblance, both in their shape and
mode of action. The wings are folded upon themselves at the point _e_
during repose. Compare letters of this figure with similar letters of
fig. 17, p. 36.--_Original._]
The apparent inconsistencies in the dimensions of the body and wings
are readily explained by the greater muscular development of the
heavy-bodied short-winged insects and birds, and the increased power
and rapidity with which the wings in them are made to oscillate. In
large-winged animals the movements are slow; in small-winged ones
comparatively very rapid. This shows that flight may be attained by a
heavy, powerful animal with comparatively small wings, as well as by a
lighter one with enormously enlarged wings. While there is apparently
no fixed relation between the area of the wings and the animal to be
raised, there is, unless in the case of sailing birds,[70] an unvarying
relation between the weight of the animal, the area of its wings, and
the number of oscillations made by them in a given time. The problem
of flight thus resolves itself into one of weight, power, velocity,
and small surfaces; _versus_ buoyancy, debility, diminished speed,
and extensive surfaces,--weight in either case being a _sine quâ non_.
In order to utilize the air as a means of transit, the body in motion,
whether it moves in virtue of the life it possesses, or because of a
force superadded, must be heavier than the air. It must tread and rise
upon the air as a swimmer upon the water, or as a kite upon the wind.
It must act against gravity, and elevate and carry itself forward at
the expense of the air, and by virtue of the force which resides in it.
If it were rescued from the law of gravity on the one hand, and bereft
of independent movement on the other, it would float about uncontrolled
and uncontrollable, as happens in the ordinary gas-balloon.
[70] In birds which skim, sail, or glide, the pinion is greatly
elongated or ribbon-shaped, and the weight of the body is made to
operate upon the inclined planes formed by the wings, in such a
manner that the bird when it has once got fairly under weigh, is in
a measure self-supporting. This is especially the case when it is
proceeding against a slight breeze--the wind and the inclined planes
resulting from the upward inclination of the wings reacting upon each
other, with this very remarkable result, that the mass of the bird
moves steadily forwards in a more or less horizontal direction.
[Illustration:
FIG. 59.--The Red-legged Partridge (_Perdix rubra_) with wings fully
extended as in rapid flight, shows deeply concave form of the wings,
how the primary and secondary feathers overlap and support each other
during extension, and how the anterior or thick margins of the wings
are directed upwards and forwards, and the posterior or thin ones
downwards and backwards. The wings in the partridge are wielded with
immense velocity and power. This is necessary because of their small
size as compared with the great dimensions and weight of the body.
If a horizontal line be drawn across the feet (_a_, _e_) to represent
the horizon, and another from the tip of the tail (_a_) to the root
of the wing (_d_), the angle at which the wing strikes the air is
given. The body and wings when taken together form a kite. The
wings in the partridge are rounded and broad. Compare with heron,
fig. 60.--_Original._]
[Illustration:
FIG. 60.--The Grey Heron (_Ardea cinerea_) in full flight. In the
heron the wings are deeply concave, and unusually large as compared
with the size of the bird. The result is that the wings are moved
very leisurely, with a slow, heavy, and almost solemn beat. The
heron figured weighed under 3 lbs.; and the expanse of wing was
considerably greater than that of a wild goose which weighed over
9 lbs. Flight is consequently more a question of power and weight
than of buoyancy and surface. _d_, _e_, _f_ Anterior thick strong
margin of right wing. _c_, _a_, _b_ Posterior thin flexible margin,
composed of primary (_b_), secondary (_a_), and tertiary (_c_)
feathers. Compare with partridge, fig. 59.--_Original._]
That no fixed relation exists between the area of the wings and the
size and weight of the body, is evident on comparing the dimensions
of the wings and bodies of the several orders of insects, bats, and
birds. If such comparison be made, it will be found that the pinions in
some instances diminish while the bodies increase, and the converse.
No practical good can therefore accrue to aërostation from elaborate
measurements of the wings and trunks of any flying thing; neither
can any rule be laid down as to the extent of surface required for
sustaining a given weight in the air. The wing area is, as a rule,
considerably in excess of what is actually required for the purposes of
flight. This is proved in two ways. First, by the fact that bats can
carry their young without inconvenience, and birds elevate surprising
quantities of fish, game, carrion, etc. I had in my possession at one
time a tame barn-door owl which could lift a piece of meat a quarter of
its own weight, after fasting four-and-twenty hours; and an eagle, as
is well known, can carry a moderate-sized lamb with facility.
The excess of wing area is proved, secondly, by the fact that a large
proportion of the wings of most volant animals may be removed without
destroying the power of flight. I instituted a series of experiments
on the wings of the fly, dragon-fly, butterfly, sparrow, etc., with a
view to determining this point in 1867. The following are the results
obtained:--
_Blue-bottle Fly._--_Experiment 1._ Detached posterior or thin half of
each wing in its long axis. Flight perfect.
_Exp. 2._ Detached posterior _two-thirds_ of either wing in its long
axis. Flight still perfect. I confess I was not prepared for this
result.
_Exp. 3._ Detached one-third of anterior or thick margin of either
pinion obliquely. Flight imperfect.
_Exp. 4._ Detached one-half of anterior or thick margin of either
pinion obliquely. The power of flight completely destroyed. From
experiments 3 and 4 it would seem that the anterior margin of the wing,
which contains the principal nervures, and which is the most rigid
portion of the pinion, cannot be mutilated with impunity.
_Exp. 5._ Removed one-third from the extremity of either wing
transversely, _i.e._ in the direction of the short axis of the pinion.
Flight perfect.
_Exp. 6._ Removed _one-half_ from either wing transversely, as in
experiment 5. Flight very slightly (if at all) impaired.
_Exp. 7._ Divided either pinion in the direction of its long axis
into three equal parts, the anterior nervures being contained in the
anterior portion. Flight perfect.
_Exp. 8._ Notched two-thirds of either pinion obliquely from behind.
Flight perfect.
_Exp. 9._ Notched anterior third of either pinion transversely. The
power of flight destroyed. Here, as in experiment 4, the mutilation of
the anterior margin was followed by loss of function.
_Exp. 10._ Detached posterior two-thirds of right wing in its long
axis, the left wing being untouched. Flight perfect. I expected that
this experiment would result in loss of balancing-power; but this was
not the case.
_Exp. 11._ Detached half of right wing transversely, the left one being
normal. The insect flew irregularly, and came to the ground about a
yard from where I stood. I seized it and detached the corresponding
half of the left wing, after which it flew away, as in experiment 6.
_Dragon-Fly._--_Exp. 12._ In the dragon-fly either the first or second
pair of wings may be removed without destroying the power of flight.
The insect generally flies most steadily when the posterior pair of
wings are detached, as it can balance better; but in either case flight
is perfect, and in no degree laboured.
_Exp. 13._ Removed one-third from the posterior margin of the first and
second pairs of wings. Flight in no wise impaired.
If more than a third of each wing is cut away from the posterior or
thin margin, the insect can still fly, but with effort.
Experiment 13 shows that the posterior or thin flexible margins of the
wings may be dispensed with in flight. They are more especially engaged
in propelling. Compare with experiments 1 and 2.
_Exp. 14._ The extremities or tips of the first and second pair of
wings may be detached to the extent of one-third, without diminishing
the power of flight. Compare with experiments 5 and 6.
If the mutilation be carried further, flight is laboured, and in some
cases destroyed.
_Exp. 15._ When the front edges of the first and second pairs of wings
are notched or when they are removed, flight is completely destroyed.
Compare with experiments 3, 4, and 9.
This shows that a certain degree of stiffness is required for the
front edges of the wings, the front edges indirectly supporting the
back edges. It is, moreover, on the front edges of the wings that the
pressure falls in flight, and by these edges the major portions of the
wings are attached to the body. The principal movements of the wings
are communicated to these edges.
_Butterfly._--_Exp. 16._ Removed posterior halves of the first pair of
wings of white butterfly. Flight perfect.
_Exp. 17._ Removed posterior halves of first and second pairs of wings.
Flight not strong but still perfect. If additional portions of the
posterior wings were removed, the insect could still fly, but with
great effort, and came to the ground at no great distance.
_Exp. 18._ When the tips (outer sixth) of the first and second pairs
of wings were cut away, flight was in no wise impaired. When more was
detached the insect could not fly.
_Exp. 19._ Removed the posterior wings of the brown butterfly. Flight
unimpaired.
_Exp. 20._ Removed in addition a small portion (one-sixth) from the
tips of the anterior wings. Flight still perfect, as the insect flew
upwards of ten yards.
_Exp. 21._ Removed in addition a portion (one-eighth) of the posterior
margins of anterior wings. The insect flew imperfectly, and came to the
ground about a yard from the point where it commenced its flight.
_House Sparrow._--The sparrow is a heavy small-winged bird, requiring,
one would imagine, all its wing area. This, however, is not the case,
as the annexed experiments show.
_Exp. 22._ Detached the half of the secondary feathers of either pinion
in the direction of the long axis of the wing, the primaries being left
intact. Flight as perfect as before the mutilation took place. In this
experiment, one wing was operated upon before the other, in order to
test the balancing-power. The bird flew perfectly, either with one or
with both wings cut.
_Exp. 23._ Detached the half of the secondary feathers and a fourth
of the primary ones of either pinion in the long axis of the wing.
Flight in no wise impaired. The bird, in this instance, flew upwards
of 30 yards, and, having risen a considerable height, dropped into a
neighbouring tree.
_Exp. 24._ Detached nearly the half of the primary feathers in the long
axis of either pinion, the secondaries being left intact. When one wing
only was operated upon, flight was perfect; when both were tampered
with, it was still perfect, but slightly laboured.
_Exp. 25._ Detached rather more than a third of both primary and
secondary feathers of either pinion in the long axis of the wing.
In this case the bird flew with evident exertion, but was able,
notwithstanding, to attain a very considerable altitude.
From experiments 1, 2, 7, 8, 10, 13, 16, 22, 23, 24, and 25, it would
appear that great liberties may be taken with the posterior or thin
margin of the wing, and the dimensions of the wing in this direction
materially reduced, without destroying, or even vitiating in a marked
degree, the powers of flight. This is no doubt owing to the fact
indicated by Sir George Cayley, and fully explained by Mr. Wenham, that
in all wings, particularly long narrow ones, the elevating power is
transferred to the anterior or front margin. These experiments prove
that the upward bending of the posterior margins of the wings during
the down stroke is not necessary to flight.
_Exp. 26._ Removed alternate primary and secondary feathers from either
wing, beginning with the first primary. The bird flew upwards of fifty
yards with very slight effort, rose above an adjoining fence, and
wheeled over it a second time to settle on a tree in the vicinity. When
one wing only was operated upon, it flew irregularly and in a lopsided
manner.
_Exp. 27._ Removed alternate primary and secondary feathers from either
wing, beginning with the _second primary_. Flight, from all I could
determine, perfect. When one wing only was cut, flight was irregular or
lopsided, as in experiment 26.
From experiments 26 and 27, as well as experiments 7 and 8, it would
seem that the wing does not of necessity require to present an unbroken
or continuous surface to the air, such as is witnessed in the pinion
of the bat, and that the feathers, when present, may be separated
from each other without destroying the utility of the pinion. In the
raven and many other birds the extremities of the first four or five
primaries divaricate in a marked manner. A similar condition is met
with in the _Alucita hexadactyla_, where the delicate feathery-looking
processes composing the wing are widely removed from each other. The
wing, however, _ceteris paribus_, is strongest when the feathers are
not separated from each other, and when they _overlap_, as then they
are arranged so as mutually to support each other.
_Exp. 28._ Removed half of the primary feathers from either wing
transversely, _i.e._ in the direction of the short axis of the wing.
Flight very slightly, if at all, impaired when only one wing was
operated upon. When both were cut, the bird flew heavily, and came to
the ground at no very great distance. This mutilation was not followed
by the same result in experiments 6 and 11. On the whole, I am inclined
to believe that the area of the wing can be curtailed with least injury
in the direction of its long axis, by removing successive portions from
its posterior margin.
_Exp. 29._ The carpal or wrist-joint of either pinion rendered immobile
by lashing the wings to slender reeds, the elbow-joints being left
free. The bird, on leaving the hand, fluttered its wings vigorously,
but after a brief flight came heavily to the ground, thus showing that
a certain degree of twisting and folding, or flexing of the wings, is
necessary to the flight of the bird, and that, however the superficies
and shape of the pinions may be altered, the movements thereof must not
be interfered with. I tied up the wings of a pigeon in the same manner,
with a precisely similar result.
The birds operated upon were, I may observe, caught in a net, and the
experiments made within a few minutes from the time of capture.
Some of my readers will probably infer from the foregoing, that the
figure-of-8 curves formed along the anterior and posterior margins of
the pinions are not necessary to flight, since the tips and posterior
margins of the wings may be removed, without destroying it. To such
I reply, that the wings are flexible, elastic, and composed of a
congeries of curved surfaces, and that so long as a portion of them
remains, they form, or tend to form, figure-of-8 curves in every
direction.