our insect air-balloonists, I fear you will think the motions of
those which fly by means of _wings_ less interesting. You will
find, however, that they are not altogether barren of amusement.
Though the wings are the principal instruments of the flight of
insects, yet there are others subsidiary to them, which I shall here
enumerate, considering them more at large under the orders to which
they severally belong. These are wing-cases (_Elytra_, _Tegmina_,
and _Hemelytra_); winglets (_Alulæ_); poisers (_Halteres_); tailets
(_Caudulæ_); hooklets (_Hamuli_); base-covers (_Tegulæ_), &c.
Besides, their _tails_, _legs_, and even _antennæ_, assist them in
some instances, in this motion.
As _wings_ are common to almost the whole class, I shall consider
their structure here. Every wing consists of two membranes, more
or less transparent, applied to each other: the upper membrane
being very strongly attached to the nervures (_Neuræ_), and the
lower adhering more loosely, so as to be separable from them. The
nervures[547] are a kind of hollow tube,--above elastic, horny,
and convex; and flat and nearly membranaceous below,--which take
their origin in the trunk, and keep diminishing gradually, the
marginal ones excepted, to their termination. The vessels contained
in the nervures consist of a spiral thread, whence they appear to
be air-vessels communicating with the tracheæ in the trunk.--The
expansion of the wing at the will of the insect, is a problem that
can only be solved by supposing that a subtile fluid is introduced
into these vessels, which seem perfectly analogous to those in the
wings of birds; and that thus an impulse is communicated to every
part of the organ, sufficient to keep it in proper tension. We see
by this, that a wing is supported in its flight like a sail by its
cordage[548]. It is remarkable that those insects which keep the
longest on the wing, the dragon-flies (_Libellulina_) for instance,
have their wings most covered with nervures. The wings of insects in
flying, like those of other flying animals, you are to observe, move
vertically, or up and down.
In considering the flight of insects, I shall treat of that of each
order separately, beginning with the _Coleoptera_ or beetles. Their
subsidiary instruments of flight are their wing-cases (_Elytra_),
and in one instance, winglets (_Alulæ_). The former[549], which in
some are of a hard horny substance, and in others are softer and
more like leather, though they are kept immoveable in flight, are
probably, by their resistance to the air, not without their use
on this occasion. The winglets are small concavo-convex scales,
of a stiff membranaceous substance, generally fringed at their
extremity[550]. I know at present of only one coleopterous insect
that has them (_Dytiscus marginalis_). They are placed under the
elytra at their base. Their use is unknown; but it may probably be
connected with their flight. The wings of beetles[551] are usually
very ample, often of a substance between parchment and membrane. The
nervures that traverse and extend them, though not numerous, are
stronger and larger than those in the wings of insects of the other
orders, and are so dispersed as to give perfect tension to the organ.
When at rest--except in _Molorchus_, _Atractocerus_, _Necydalis_,
and some other genera--they are folded transversely under the
elytra, generally near the middle, with a lateral longitudinal fold,
but occasionally near the extremity[552]. When they prepare for
flight, their antennæ being set out, the elytra are opened so as to
form an angle with the body and admit the free play of the wings,
and they then fly off, striking the air by the vertical motion of
these organs, the elytra all the while remaining immoveable. During
their flight the bodies of insects of this order, as far as I have
observed them, are always in a position nearly vertical, which gives
to the larger sorts, the stag-beetle for instance, a very singular
appearance. Olivier, probably having some of the larger and heavier
beetles in his eye, affirms that the wings of insects of this order
are not usually proportioned to the weight of their bodies, and that
the muscular apparatus that moves them is deficient in force. In
consequence of which, he observes, they take flight with difficulty,
and fly very badly. The strokes of their wings being frequent, and
their flight short, uncertain, heavy, and laborious, they can use
their wings only in very calm weather, the least wind beating them
down. Yet he allows that others, whose body is lighter, rise into
the air and fly with a little more ease; especially when the weather
is warm and dry, their flights however being short, though frequent.
He asserts also, that no coleopterous insect can fly against the
wind[553]. These observations may hold perhaps with respect to many
species; but they will by no means apply generally. The cockchafer
(_Melolontha vulgaris_), if thrown into the air in the evening, its
time of flight, will take wing before it falls to the ground. The
common dung-chafer (_Geotrupes stercorarius_)--wheeling from side to
side like the humble-bee--flies with great rapidity and force, and,
with all its dung-devouring confederates, directs its flight with
the utmost certainty, and probably often against the wind, to its
food. The root-devourers or tree-chafers (_Melolontha_, _Hoplia_,
&c.) support themselves, like swarming bees, in the air and over
the trees, flying round in all directions. The _Brachyptera_ and
_Donaciæ_, in warm weather, fly off from their station with the
utmost ease;--their wings are unfolded, and they are in the air in
an instant, especially the latter, as I have often found when I have
attempted to take them. None are more remarkable for this than the
_Cicindelæ_, which, however, taking very short flights, are as easily
marked down as a partridge, and affords as much amusement to the
entomologist, as the latter to the sportsman.--It is to be observed
that many insects in this order have no wings, and the female
glow-worms neither wings nor elytra.
Many persons are not aware that the insects of the next order,
the _Dermaptera_, can fly: but earwigs (_Forficula_), their size
considered, are furnished with very ample and curious wings, the
principal nervures of which are so many radii, diverging from a
common point near the anterior margin. Between these are others
which, proceeding from the opposite margin, terminate in the middle
of the wing[554]. These organs, when at rest, are more than once
folded both transversely and longitudinally.
Wings equally ample, forming the quadrant of a circle, and with
five or six nervures diverging from their base, distinguish
the _strepsipterous_ tribe. When unemployed, these are folded
longitudinally[555].
Probably in the next order (_Orthoptera_), the _Tegmina_, or
wing-covers--since they are usually of a much thinner substance
than elytra--assist them in flying. They are however quite covered
by irregular reticulations, produced by various nervures sent forth
by the longitudinal ones, and running in all directions. When at
rest, the inner part of one laps over that of the other[556]: but in
different genera there is a singular variation in this circumstance.
Thus in _Blatta_, _Phasma_, and male _Acridæ_, and generally
speaking, but not invariably, in _Locusta_ and _Truxalis_,--the left
elytrum laps over the right: but in _Mantis_; _Mantispa_; some
female _Acridæ_; _Gryllus_; and _Gryllotalpa_; the right is laid over
the left. The wings in this order, though always ample and larger
than the tegmina, do not invariably form a quadrant of a circle,
falling often short of it. They are extended by means of nervures,
which, like so many rays, diverge from the base of the wing, and
are intersected alternately by transverse ones, which thus form
quadrangular areas, arranged like bricks in a wall. When at rest,
they are longitudinally folded. The flight of these insects, as far
as it has been observed, much resembles, it is said, that of certain
birds. Ray tells us that both sexes of the house-cricket (_Gryllus
domesticus_) fly with an undulating motion, like a woodpecker,
alternately ascending with expanded wings, and descending with folded
ones[557]. The field- and mole-crickets (_Gryllus campestris_ and
_Gryllotalpa vulgaris_), as we learn from Mr. White[558],--and,
since the structure of their wings is similar, probably the other
_Orthoptera_,--fly in the same way.
_Hemipterous_ insects, with respect to their _Hemelytra_, may
be divided into two classes. Those in which they are all of the
same substance--varying from membrane to a leathery or horny
crust[559]--and those in which the base and the apex are of different
substances; the first being generally corneous, and the latter
membranaceous[560]. The former or homopterous division includes the
_Cicadariæ_, Latr.; _Aphis_; _Chermes_; _Thrips_; and _Coccus_;--and
the latter the heteropterous division, comprehending besides the
_Geocorisæ_, Latr., _Notonecta_; _Sigara_; _Nepa_; _Ranatra_; and
_Naucoris_ of Fabricius. The posterior tibiæ of some of this last
division (_Lygæus phyllopus_, _foliaceus_, &c. F.) are furnished
on each side with a foliaceous process--which may act the part of
outriggers, and assist them in their flight[561]. I can give you no
particular information with respect to the aërial movements of the
insects of this order: the British species that belong to it are
generally so minute that it is not easy to trace them with the naked
eye; and unless some kind optician, which is much to be wished, would
invent a telescope by which the proceedings of insects could be
examined at a distance, there is no other way of studying them.
The four wings of the next order, the _Trichoptera_ or case-worm
flies, both in their shape and nervures resemble those of many
moths[562]; only instead of scales they are usually covered with
hairs, and the under wings, which are larger than the upper, fold
longitudinally. Some of these flies, I have observed, move in a
direct line, with their legs set out, which makes them look as if
they were walking in the air. In flying they often apply their
antennæ to each other, stretching them out straight, and thus
probably are assisted in their motion.
The _Lepidoptera_ vary so infinitely in the shape, comparative
magnitude, and appendages of their wings, that I should detain you
too long did I enlarge upon so multifarious a subject. I shall
therefore only observe, that one species is described, both by Lyonet
and De Geer[563] (_Lobophora hexaptera_), as having six wings; for
besides the four ordinary ones, it has a winglet (_Alula_) attached
to the base of the lower one, and placed, when the wings are folded,
between it and the upper. These organs in this order you know are
covered with scales of various shapes[564]. Their nervures are
diverging rays, which issue either from a basal area or from the
base itself, and terminate in the exterior margin[565]. The wings of
many male butterflies, hawk-moths, and moths, are distinguished by
a remarkable apparatus, noticed by De Geer, and since by many other
naturalists[566] for keeping them steady and underanged in their
flight. The upper wings, on their underside near their base, have a
minute process, bent into a hook (_Hamus_), and covered with hairs
and scales. In this hook one or more bristles (_Tendo_), attached to
the base of the under wing, have their play. When the fly unfolds its
wings, the hook does not quit its hold of the bristle, which moves to
and fro in it as they expand or close. The females, which seldom fly
far, often have the bristles but never the hook. The hairy tails of
some insects, _Sesia_, belonging to the hawk-moth tribe, are expanded
when they fly, so as to form a kind of rudder, which enables them to
steer their course with more certainty.
The insects of this, and of every other order, except the
_Coleoptera_, fly with their bodies in a horizontal position,
or nearly so. As their wings are usually so ample, we need not
wonder that the _Lepidoptera_ are excellent fliers. Indeed they
seem to flit untired from flower to flower, and from field to
field; impelled at one while by hunger, and at another by love or
maternal solicitude.--The distance to which some males will fly is
astonishing. That of one of the silk-worm moths (_Attacus Paphia_)
is stated to travel sometimes more than a hundred miles in this
way[567].--Our most beautiful butterfly, the purple emperor (_Apatura
Iris_), when he makes his first appearance fixes his throne on the
summit of some lofty oak, from whence in sunny days, unattended by
his empress, who does not fly, he takes his excursions. Launching
into the air from one of the highest twigs, he mounts often to
so great a height as to become invisible. When the sun is at the
meridian his loftiest flights take place; and about four in the
afternoon he resumes his station of repose[568].--The large bodies of
hawk-moths (_Sphinx_, F.) are carried by wings remarkably strong both
as to nervures and texture, and their flight is proportionably rapid
and direct. That of butterflies is by dipping and rising alternately,
so as to form a zigzag line with vertical angles, which the animal
often describes with a skipping motion, so that each zigzag consists
of smaller ones. This doubtless renders it more difficult for the
birds to take them as they fly; and thus the male, when paired, often
flits away with the female.
Amongst the _Neuropterous_ tribes the most conspicuous insects are
the dragon-flies (_Libellulina_), which--their metamorphosis, habits,
mode of life, and characters considered--form a distinct natural order
of themselves. Their four wings, which are nearly equal in size, are
a complete and beautiful piece of net-work, resembling the finest
lace, the meshes of which are usually filled by a pure, transparent,
glassy membrane. In two of the genera belonging to this tribe, the
wings, when the animal is at rest, are always expanded, so that they
can take flight in an instant, no previous unfolding of these organs
being necessary. In _Agrion_, the other genus of the tribe, the wings
when they repose are not expanded. I have observed of these insects,
and also of several others in different orders, that without turning
they can fly in all directions--backwards, and to the right and left,
as well as forwards. This ability to fly all ways, without having to
turn, must be very useful to them when pursued by a bird. Leeuwenhoek
once saw a swallow chasing an insect of this tribe, which he calls
a _Mordella_, in a menagerie about a hundred feet long. The little
creature flew with such astonishing velocity--to the right--to the
left--and in all directions--that this bird of rapid wing and ready
evolution was unable to overtake and entrap it; the insect eluding
every attempt, and being generally six feet before it[569]. Indeed,
such is the power of the long wings by which the dragon-flies are
distinguished, particularly in _Æshna_ and _Libellula_, and such the
force of the muscles that move them, that they seem never to be wearied
with flying. I have observed one of the former genus (_Anax Imperator_,
Leach) sailing for hours over a piece of water--sometimes to and fro,
and sometimes wheeling from side to side; and all the while chasing,
capturing, and devouring the various insects that came athwart its
course, or driving away its competitors--without ever seeming tired, or
inclined to alight. Another species (_Æshna variegata_), very common
in lanes and along hedges, which flies, like the _Orthoptera_, in a
waving line, is equally alert and active after its prey. This however
often alights for a moment, and then resumes its gay excursive flights.
The species of the genus _Agrion_ cut the air with less velocity;
but so rapid is the motion of their wings, that they become quite
invisible. Hawking always about for prey, the Agrions, from the variety
of the colours of different individuals, form no uninteresting object
during a summer stroll. With respect to the mode of flight of the
other neuropterous tribes I have nothing to remark; for that of the
_Ephemeræ_, which has been most noticed, I shall consider under another
head.
The next order of insects, the _Hymenoptera_, attract also general
attention as fliers, and from our earliest years. The ferocious hornet,
with its trumpet of terror; the intrusive and indomitable wasp;
the booming and pacific humble-bee, the frequent prey of merciless
schoolboys; and that universal favourite, the industrious inhabitant
of the hive,--all belonging to it,--are familiar to every one. And in
summer-time there is scarcely a flower or leaf in field or garden,
which is not visited by some of its numerous tribes. The four wings
of these insects, the upper pair of which are larger than the under,
vary much in their nervures. From the saw-flies (_Serrifera_), whose
wings are nearly as much reticulated as those of some _Neuroptera_, to
the minute _Chalcis_ and _Psilus_, in which these organs are without
nervures, there is every intermediate variety of reticulation that
can be imagined[570]. It has been observed that the nervures of the
wings are usually proportioned to the weight of the insect. Thus the
saw-flies have generally bodies thicker than those of most other
_Hymenoptera_, while those that have fewer nervures are more slender.
This, however, does not hold good in all cases--so that the dimensions
and cut of the wings, the strength of their nervures, and the force
of their muscles, must also be taken into consideration. The wings of
many of these insects when expanded, are kept in the same plane by
means of small hooks (_Hamuli_) in the anterior margin of the under
wing, which lay hold of the posterior margin of the upper[571]. Another
peculiarity also distinguishes them. Base covers (_Tegulæ_), or small
concavo-convex shields, protect the base of the wings from injury[572],
or displacement.
The most powerful fliers in this order are the humble-bees, which,
like the dung-chafers (_Geotrupes_), traverse the air in segments
of a circle, the arc of which is alternately to right and left. The
rapidity of their flight is so great, that could it be calculated,
it would be found, the size of the creature considered, far to
exceed that of any bird.--The aërial movements of the hive-bee are
more direct and leisurely. When leaving the hive for an excursion,
I have observed that as soon as they come out they turn about as
if to survey the entrance, and then wheeling round in a circle,
fly off. When they return to the hive, they often fly from side to
side, as if to examine before they alight. When swarming, the heads
of all are turned towards the group at the mouth of their dwelling;
and upon rising into the air these little creatures fly so thick in
every direction, as to appear like a kind of net-work with meshes of
every angle. The queen also, upon going forth, when her object is to
pair, after returning to reconnoitre, begins her flight by describing
circles of considerable diameter, thus rising spirally with a rapid
motion[573]. The object of these gyrations is probably to increase
her chance of meeting with a drone.--I have not much to tell you
with respect to the flight of other insects of this order, except
that a spider-wasp (_Pompilus viaticus_) whose sting is redoubtable,
and which often, when we are in the vicinity of sandy sunny banks,
accompanies our steps, has a kind of jumping movement when it flies.
The next order, the _Diptera_, consists altogether of two-winged
flies:--but to replace the under wings of the tetrapterous insects,
they are furnished with poisers, and numbers of them also with
winglets. The poisers (_Halteres_) are little membranaceous threads
placed one under the origin of each wing, near a spiracle, and
terminated by an oval, round, or triangular button, which seems
capable of dilatation and contraction. The animal moves these organs
with great vivacity, often when at rest, and probably when flying.
Their winglets (_Alulæ_) are different from those of _Dytiscus
marginalis_, and the moth before noticed. Like them, they are of
rigid membrane, and fringed; but they consist generally of two
concavo-convex pieces (sometimes surrounded by a nervure), situated
between the wing and the poisers, which, when the insect reposes,
fold over each other like the valves of a bivalve shell; but when it
flies they are extended. The use of neither of these organs seems
to have been satisfactorily ascertained. Dr. Derham thinks they are
for keeping the body steady in flight; and asserts, that if either
a poiser or winglet be cut off, the insect will fly as if one side
overbalanced the other, till it falls to the ground; and that if
both be cut off, they will fly awkwardly and unsteadily, as if they
had lost some very necessary part[574]. Shelver cut off the winglets
of a fly, leaving both wings and poisers, but it could no longer
fly. He next cut off the poisers of another, leaving the wings and
winglets, and the same result followed. He found, upon removing one
of these organs, that they were not properly compared to balancers.
Observing that a common crane-fly (_Tipula crocata_) moved the knee
of the hinder tibia in connexion with the wing and poiser, he cut
it off, and it could no longer fly: this last experiment, however,
seems contradicted by the fact, which has been often observed, that
the insects of this genus will fly when half their legs are gone. He
afterwards cut off both its poisers, when it could neither fly nor
walk. Hence he conjectures that the poisers are connected with the
feet, and are air-holders[575]. I have often seen flies move their
poisers very briskly when at rest, particularly _Seioptera vibrans_,
before mentioned. This renders Shelver's conjecture--that they are
connected with respiration--not improbable. Perhaps by their action
some effect may be produced upon the spiracle in their vicinity,
either as to the opening or closing of it.
There are three classes of fliers in this order, the form of whose
bodies, as well as the shape and circumstances of their wings, is
different. First are the slender flies--the gnats, gnat-like flies,
and crane-flies (_Tipulariæ_). The bodies of these are light,
their wings narrow, and their legs long, and they have no winglets.
Next are those whose bodies, though slender, are more weighty--the
_Asilidæ_, _Conopsidæ_, &c.; these have larger wings, shorter legs,
and very minute and sometimes even obsolete winglets. Lastly come
the flies, the _Muscidæ_, &c., and their affinities, whose bodies
being short, thick, and often very heavy, are furnished not only
with proportionate wings and shorter legs, but also with conspicuous
winglets. From these comparative differences and distinctions,
we may conjecture in the first place--since the lightest bodies
are furnished with the longest legs, and the heaviest with the
shortest--that the legs act as poisers and rudders, that keep
them steady while they fly, and assist them in directing their
course[576]; and in the next--since the winglets are largest in the
heaviest bodies, and altogether wanting in the lightest--that one of
their principal uses is to assist the wings when the insect is flying.