Orders in which they are covered by elytra, tegmina, or hemelytra,
are generally naked, except that the spots in those of _Fulgora
laternaria_, _serrata_, &c., and the whole wing in _Flata_,
_Aleyrodes_, and others, are covered with a kind of farinaceous
powder; but in all the remaining Orders, hairs or scales are more
or less implanted in these organs: as the _Lepidoptera_ are the
most remarkable for the clothing of their wings, I shall leave them
till the last, and begin with the _Neuroptera_. If you lightly
pass your finger over the wing of any dragon-fly (_Libellula_ F.,
_Æshna_ F.), from the apex towards the base, you will find that the
longitudinal nervures are, as it were, serrulated with very minute
bristles, which point towards the extremity; if you next move the
finger across the wing, from the posterior to the anterior margin, a
similar circumstance will strike you. M. Chabrier conjectures that,
amongst other uses[1954], these hairs may contribute to fix the
atmospheric fluid when the wings are depressed in flight, while it
glides over them as they rise[1955]; in _Ascalaphus_, _Myrmeleon_,
_Nemoptera_, _Hemerobius_, &c., the nervures are more visibly
bristled; the bristles diverging on each side from the longitudinal
ones, but all pointing towards the apex from the connecting or
transverse ones; in _Panorpa_, besides these bristles, short hairs,
pointing the same way, are thickly planted in the membrane of the
wing; and in _Hemerobius_ the margins of the wing are fringed; in
the _Ephemerina_, _Corydalis_, &c., the wings are naked. In the
_Trichoptera_ Order, as their name imports, they are covered with
minute decumbent hairs, less easily seen but still existing in the
secondary pair. In the _Hymenoptera_ in general the wings are covered
with minute hairs or bristles; but in _Tiphia_, _Scolia_--with the
exception of _S. Radula_ and affinities in which they are hairy--and
others, the wings are nearly naked; in _Pompilus_, _Pepsis_, &c.,
the hairs are infinitely numerous and very short; in the _Sphecidæ_,
_Mutilla_, &c., they are more distinct, longer, and less numerous;
in the humble-bee (_Bombus_) and many others the apex of the wing is
darkened by a large number of more conspicuous hairs, each of which
seems to spring from a minute tubercle: as these tubercles are in
a part of the wing that is strengthened by few nervures, they may
probably be intended to supply their place, in giving firmness and
tension to this part. The wings of _Diptera_, under the present head,
may be viewed with regard to the hairs that are implanted in the
_membrane_ of the wing, in its _nervures_, and in its _margin_. In
the first view, in _Stratyomis_ and immediate affinities the wing is
nearly naked; but in _Xylophagus_, _Beris_, and the great majority
of the Order, the membrane of the wings is thickly planted with
innumerable very minute bristles, not to be seen but under a powerful
lens, often black, and seemingly crowning a little prominence, and
giving the wing an appearance of the finest net-work. As to the
clothing of the nervures, the costal, in _Anthrax_, _Bombylius_,
&c., is often remarkably bristly at the base, with hairs intermixed;
in _Œstrus Ovis_, in the inner margin or edge of this nervure, is
a single series of bristles, or rather short spines, like so many
black points; in _Œ. Equi_ the whole _costa_ is covered with short
decumbent hairs or bristles; in _Musca pagana_ F., just at the apex
of the costal areolet, that nervure is armed with a spur or diverging
bristle larger than the rest, which is also to be found in many
others of the _Muscidæ_, some of which have two and others more of
these spurs. The little moth-like midges (_Psychoda_ Latr., _Hirtæa_
F.) at first appear to have the whole surface of their wings covered
with hairs; but upon a closer examination it will be seen that they
are planted in the nervures, from each of which they diverge, so
as under a lens to give it a very elegant appearance[1956]. This
fly has its wings beautifully fringed with fine hairs, the _third_
circumstance to be attended to under this head; in the Tipulidans,
and many others of this Order, the apex and posterior margin are
also finely fringed with short hairs. Some Dipterous insects make a
near approach to the _Lepidoptera_ in the _covering_ of their wings:
in the common gnat, when the wings are not rubbed, the nervures are
adorned by a double series of _scales_, and the marginal fringe also
consists of them[1957]; and in a Georgian genus, which appears in
some degree to connect _Culex_ with _Anthrax_ &c., there are scales
scattered upon the membrane as well as upon the nervures; besides,
its antennæ[1958] and abdomen are also covered with them.
The Order, the clothing of whose organs of flight excites the
admiration of the most incurious beholder, is that to which the
excursive butterfly belongs, the _Lepidoptera_. The gorgeous wings of
these universal favourites, as well as those of the hawk-moths and
moths, owe all their beauty, not to the _substance_ of which they are
composed, but to an infinite number of little _plumes_ or _scales_
so thickly planted in their upper and under surface, as in the great
majority entirely to conceal that substance. Whether these are really
most analogous to plumes or scales has been thought doubtful. De
Geer is inclined to think, from their terminating at their lower
end in little quills and other circumstances, that they resemble
_feathers_ as much as scales[1959]; Reaumur on the contrary suspects
that they come nearer to _scales_[1960]. Their substance, approaching
to membrane, seems to make further for the former opinion, and their
shape and the indentations that often occur in their extremity,
furnish an additional argument for the latter. Their numbers are
infinite; Leeuwenhoek found more than 400,000 on the wings of the
silk-worm moth (_Bombyx Mori_)[1961]; and in those of some of the
larger moths and butterflies the number must greatly exceed this.
You will observe however that in many _Lepidoptera_ the wings are
_partially_, and in some instances _generally_, transparent: thus in
_Hesperia Proteus_, a butterfly before noticed for the long tail that
distinguishes its secondary wings, there are many transparent spots;
in _Attacus Atlas_, one of the largest of moths, and its affinities,
there is as it were a _window_ in each wing formed by a transparent
triangular space; in _A. Polyphemus_, _Paphia_, &c., the pupil of
the ocellus is transparent, which in the former is divided by a
nervure. In several of the Heliconian butterflies, and in _Zygæna_
F., &c., the greater part of both wings is transparent, with scales
only upon their nervures, round their margin, or forming certain
bands or spots upon them; in _Parnassius Apollo_, _Mnemosyne_, &c.,
the scales are so arranged as not wholly to cover the wings, which
renders them semidiaphanous; and in some (_Nudaria_) the wings are
intirely denuded. With regard to _size_, the scales vary often
considerably in different tribes; in _Heliconia_ they appear to be
more minute than in the rest; and in _Castnia_ they are the largest
and coarsest; the extremity of the wings of Lepidopterous insects in
general is fringed with longer scales than their surfaces, and even
those of the last in the same wing; sometimes vary in magnitude.
The little seeming tooth that projects from the middle of the
posterior margin in the upper wings of _Notodonta_, a subgenus of
_Bombyx_ L., is merely produced by some longer diverging hairs. The
_shape_ and _figure_ also of scales are very various--some being
long and slender; others short and broad; some nearly round; others
oval, ovate, or oblong; others spathulate; others panduriform or
parabolical; some again almost square or rhomboidal; many triangular;
some representing an isosceles triangle, and others an equilateral
one; lastly, some are lanceolate and others linear; again, some have
a very short pedicle and others a very long one: with regard to their
_extremity_; some are intire, without projecting points or incisions,
while others are furnished with them: of these some terminate in a
single long mucro, others have several shorter ones; some are armed
with teeth, varying in number from two to thirteen in different
species[1962]. Many other forms might be enumerated, but these are
sufficient to give you a general notion of the infinite variety of
this part of the works of the CREATOR. I must next say a word or two
upon their _arrangement_ on the wing. In most instances this is in
transverse lines, which sometimes vary a little from a rectilinear
course, and the extremity of the scales of one row reposes on the
base of those of the succeeding one, so that in this respect their
arrangement is like that of tiles in a roof: in some cases it is
not so regular: thus the minute scales on the wings of _Parnassius
Apollo_, and others with subdiaphanous wings, are arranged without
order; in _Pieris_ and other Diurnal _Lepidoptera_, and many of the
Crepuscular and Nocturnal, there appears to be a _double_ layer of
scales on both sides of the wing; the under layer usually consisting
of _white_ ones. If you denude the wings of any butterfly, which you
may easily do by scraping it lightly on both sides with a penknife,
you will be amused to trace the lines in which the scales were
planted, consisting of innumerable minute dots: the lines of the
under side, in some cases, so cut those of the upper side, as by
their intersection to form lozenges. With regard to the position of
the scales on the wing, they usually lie flat, but sometimes their
extremity is incurved: in the beautiful _Argynnis Vanillæ_ a very
singular appearance of numerous transverse ridges is produced by the
extremity of those scales that cover the longitudinal nervures of the
primary wings, except at the base, being recurved.
But though the general clothing of the wings of _Lepidoptera_ consists
of these little scales, yet in some cases they are either replaced by
_hairs_ or mixed with them. Thus, in the _clear_ parts of the wings of
Heliconians, _Attaci_, &c., short inconspicuous hairs are planted; in
a large number of the Orders the _upper_ side of the Anal Area of the
secondary wings is hairy; in several Crepusculars (_Sphinx Phœnix_,
&c.), where there is a double layer as before mentioned, the upper
one consists of dense hairs, except at the apex, and the lower one
of scales; and in most of them the scales of the primary wings are
piliform, and the secondary are covered by what approach very near to
real hairs; many of the _Attaci_ are similarly circumstanced: the four
wings of _A. Cytherea_ are also covered externally with hair.
7. Before I conclude this long diatribe on the organs of flight of
insects, I must not omit some notice of the infinite diversity of
_colours_ with which their wings are often variegated and adorned
by the CREATOR, who loves to delight us by the beauty, as well as
to astonish and awe us by the immensity and grandeur of his works.
Though the wings in every Order exhibit instances of brilliant and
beautiful colouring, yet those of the _Lepidoptera_ in this respect
infinitely excel them all, and to these, under this head, after
noticing a few in the less privileged Orders, my observations will
be confined. Although in the _Coleoptera_ the wings are seldom
distinguished by their splendour; yet those of some _Cetoniadæ_,
as _Cetonia africana_, are extremely brilliant, and resemble those
of many _Xylocopæ_ in the lovely violet hue that adorns them:
amongst the _Orthoptera_ some _Pterophyllæ_, and in the Homopterous
_Hemiptera_ some _Fulgoræ_, emulate the _Lepidoptera_ in the _ocelli_
that give a kind of life to these organs[1963]; and a vast number of
the destructive tribe of locusts (_Locusta_ Leach) are remarkable for
the fine colours and gaiety of their wings[1964]; in the _Neuroptera_
numerous _Libellulinæ_ emulate the Heliconian butterflies by their
maculation; and in the genus _Ascalaphus_, which represents the
_Lepidoptera_ by its clubbed antennæ[1965], many also have the
resemblance increased by the painting of their wings, so that some
Entomologists have actually considered some of them as belonging to
that Order[1966]; the wings of the _Xylocopæ_, before alluded to,
sometimes add to the deep tints of the violet--which also prevail
in the wings of several _Diptera_--towards their extremity the most
brilliant metallic green or copper varying,
"As the site varies in the gazer's hand,"
and even those wings that consist of clear colourless membrane
are often rendered extremely beautiful from the reflection of the
prismatic colours. I should undertake an endless task did I attempt
to specify all the modes of marking, clouding, and spotting, that
variegate a wing, and all the shades of colour that paint it, amongst
the Lepidopterous tribes; I shall therefore confine myself to a
few of the principal, especially those that distinguish particular
tribes and families. Of whole coloured wings--I know none that dazzle
the eye of the beholder so much as the upper surface of those of
_Morpho Menelaus_ and _Telemachus_: Linné justly observes that there
is scarcely any thing in nature that for brightness and splendour
can be paralleled with this colour; it is a kind of rich ultramarine
that vies with the deepest and purest azure of the sky; and what
must cause a striking contrast in flight, the prone surface of the
wings is as dull and dark as the supine is brilliant, so that one
can conceive this animal to appear like a planet in full radiance,
and under eclipse, as its wings open and shut in the blaze of a
tropical sun: another butterfly, _Papilio Ulysses_, by its radiating
cerulean disk, surrounded on every side by a margin intensely black,
gives the idea of light first emerging from primeval obscurity; it
was probably this idea of light shining in darkness that induced
Linné to give it the name of the wisest of the Greeks in a dark and
barbarous age. I know no insect upon which the sight rests with
such untired pleasure, as upon the lovely butterfly that bears the
name of the unhappy Trojan king (_P. Priamus_); the contrast of the
rich green and black of the velvet of its wings with each other,
and with the orange of its abdomen, is beyond expression regal and
magnificent. But peculiar beauties of colour sometimes distinguish
whole _tribes_ as well as _individuals_. What can be more lovely
than that tribe of little butterflies that flit around us every
where in our summer rambles, which are called _blues_, and which
exhibit the various tints of the sky? _Lycæna Adonis_ of this tribe
scarcely yields to any exotic butterfly in the celestial purity of
its azure wings: our native _coppers_ also, _Lycæna dispar_[1967],
_Virgaureæ_, &c., are remarkable for the fulgid colour of these
organs; in _Argynnis_ the upper side of their wings is tawny,
spotted with black, while the under side of the secondary ones is
very often adorned by the appearance of _silver_ spots. How this
remarkable effect of metallic lustre, so often reflected by spots in
the wings of butterflies, is produced, seems not to have occupied
the attention of Entomologists. M. Audebert is of opinion that the
similar lustre of the plumes of the humming birds (_Trochilus_) is
owing to their density, to the polish of their surface, and to the
great number of little minute concave mirrors which are observable
on their little beards[1968]. But these observations will not apply
to the scales of the wings of butterflies, which are always very
thin and generally very flat: in some instances, as in _Morpho
Menelaus_, there appears more than one very slight channel upon a
scale; but this takes place also in others that reflect no lustre.
Their metallic hues must therefore principally be occasioned by
the high polish of their surface and the richness of their tints.
It is the purity of the white, in conjunction with their shining
surface, contrasted with the dull opaque colour of the under side
of the secondary wings, that causes the spots that decorate those
of the Fritillaries (_Argynnis_) to emulate the lustre of silver.
In _Papilio_ the Trojans are distinguished by the black wings with
sanguine spots, and the Greeks by the same with yellow spots; but
these have proved in some instances only sexual distinctions[1969].
In the _Danai candidi_ L. the colour of the tribe may be described
as sacred to the _day_, since every shade, from white or the palest
yellow to full orange, is exhibited by them. The yellows prevail also
in those _Noctuæ_, the trivial names of which Linné made to end in
_ago_, as _N. Fulvago_, _Citrago_, &c. I must not conclude this part
of my subject without noticing one of the most striking ornaments of
the wings of _Lepidoptera_, the many-coloured _eyes_ which decorate
so large a number of them. Some few birds, as the Peacock and Argus
Pheasant, have been decked by their CREATOR very conspicuously with
this almost dazzling glory; but in the insects just named it meets us
every where. Some, as one of our most beautiful butterflies, _Vanessa
Io_[1970], have them both on the primary and secondary wings;
others, as _Noctua Bubo_[1971], only on the primary; others again,
as _Smerinthus ocellata_[1972], only on the secondary: in some also
they are on both sides of the wing, as in _Hipparchia Ægeria_[1973],
and in others only on the upper side, as in _Vanessa Io_; in others
again only on the under side, as in _Morpho Teucer_[1974]: in some
likewise they are very large, as in the secondary wings of the same
butterfly: and in others very small, as in those in the wings of the
blues (_Lycæna_). Once more, in some they consist only of _iris_ and
_pupil_, as in _Hipparchia Semele_, and in others of many concentric
circles besides, as in _Morpho Teucer_, &c.
v. _Legs_[1975]. We are next to consider those organs of motion
affixed to the trunk, by which insects transport themselves
from one place to another on the earth or in the water, and by
which also they perform various operations connected with their
economy[1976]. In treating of them we should consider their _number_;
_kind_; _substance_; _articulation_ with the _trunk_; _position_;
_proportions_; _clothing_; _composition_; _folding_; and _motions_.
1. _Number._ Having before very fully explained to you the number
and kind of the legs of insects in their _preparatory_ states[1977],
I shall now confine myself to the consideration of these organs
in their _perfect_ or last state; beginning with their _number_.
Insects, properly so called, as I formerly observed[1978], in this
state, including the anterior pair or arms, have only _six_ legs,
none exceeding or falling short of this number; but in several of
the Diurnal _Lepidoptera_ (_Vanessa_, &c.) the anterior pair are
_spurious_, or at least not used as legs, the tarsi having neither
joints nor claws[1979]; this in some cases is said to be only a
_sexual_ distinction[1980]. In _Onitis_, _Phanæus_, and some other
_Scarabæidæ_ M^cL., the arm has either none or a spurious tarsus or
_manus_[1981]; which in the first of these genera is also a sexual
character. From both these instances we see that walking is only a
_secondary_ use of forelegs in the insect tribes. Besides insects
proper, a whole tribe of mites (_Caris_ Latr., _Leptus_ Latr., _Astoma_
Latr., _Ocypete_ Leach) have only _six_ legs; the rest, and the
_Arachnida_ in general, have _eight_; in the _Myriapods_, _Pollyxenus_
has _twelve pairs_; _Scutigera_ has _fifteen_; the terrestrial
_Glomerides_ (_G. Armadillo_, &c.) _sixteen_; and the oceanic (_G.
ovalis_) _twenty_; the oriental _Scolopendræ_ Leach, _twenty-one_;
_Polydesmus_ has usually about _thirty pairs_; _Craspedosoma_,
_fifty_; _Geophilus electricus_ at least _sixty_; in _Iulus terrestris_
there are more than _seventy_; in _I. sabulosus_ nearly _one hundred_;
in _I. fuscus_, 124; and in _I. maximus_ 134 pairs or 268 single legs.
But with respect to the _Geophili_, _Iuli_, &c., it is to be observed,
that the number of pairs varies in different individuals; and the
circumstance that has been before mentioned[1982], that these animals
keep acquiring legs in their progress to the perfect state, instead
of losing them, renders it difficult to ascertain what is the natural
number of pairs in any species.
2. _Kinds._ Upon a former occasion I gave you a sufficiently full
account of the _kinds_ of legs[1982], and I have also assigned my
reasons for giving a different denomination to the _anterior_ legs
under certain circumstances[1983]; I shall not therefore enlarge
further upon this head.
3. _Substance._ The _substance_ of the legs is generally regulated
more or less by that of the rest of the body, only in soft-bodied
insects they seem usually more firm and unbending. Each joint is a
tube, including the moving muscles, nerves, and air vessels.
4. _Articulation with the Trunk._ M. Cuvier has observed that the
hip (_coxa_), which is the joint that unites the leg with the body,
rather inosculates, in its acetabulum, than articulates in any
precise manner[1984]; but this observation, though true of a great
many, will not apply universally, for the legs of _Orthopterous_
insects, and of most of the subsequent Orders, are _suspended_
rather than inosculating. Even in many _Coleoptera_ a difference
is observable in this respect. I have before mentioned that what
are called the _puncta ordinaria_, which distinguish the sides of
the _prothorax_ of many _Scarabæidæ_ and _Geotrupidæ_, form a base
for an elevation of the interior surface with which the extremity
of the base of the clavicle, which plunges deep into the breast,
ginglymates[1985]; this structure may also be found in other
Lamellicorns, as the stag-beetle (_Lucanus_) and _Dynastes_, that
have not those excavations; in these last it is an elevated ridge
forming a segment of a circle with, it should seem, a posterior
channel, receiving a corresponding cavity and protuberance of the
clavicle. With regard to the mid-leg, in _Copris_, the _coxa_ is
emboxed in a nearly longitudinal cavity of the _medipectus_, and the
_coxa_ of the hind-leg anteriorly is suspended to a transverse cavity
of the _postpectus_, but posteriorly it is received by a cavity of
the first segment of the abdomen; so that it may be regarded as
suspended anteriorly, and inosculating posteriorly.
In some tribes of this Order, as the Weevils (_Curculio_ L.) and
Capricorns (_Cerambyx_), the coxæ of the four anterior legs are
subglobose[1986] and extremely lubricous, and are received each by
a socket that fits it, and is equally lubricous. In the bottom of
this externally, and in the head of the coxa, is an orifice for
the transmission of muscles, nerves, and bronchiæ; but the coxa
is suspended by ligament in the socket. This structure approaches
as near the ball and socket as the nature of the insect skeleton
will permit; the high polish of the articulations acts the part of
_synovia_, and the motion is in some degree rotatory or versatile,
whereas in _Copris_, &c., lately mentioned, it seems to be more
limited, and is probably, at least in the mid- and hind-legs, only
in two directions; in the middle pair, probably, from the _coxæ_
being in a position parallel with the breast, opposite to that of the
hind pair. In _Dytiscus_ L., _Carabus_ L., and some other beetles,
the coxæ, especially the posterior pair, appear to be fixed and
incapable of motion. In many insects these coxæ seem to belong as
much to the abdomen as to the trunk. We have just seen this to be the
case in _Copris_, &c.; and in the _Lepidoptera_, if the former be
separated from the latter, the legs will be detached with it.
4. _Location._ We are now to consider the _location_ and _position_
of the legs, both in general and with respect to each other. And
first, as has been before stated, we may observe that, in the
hexapods with wings, the _arms_ belong to the _manitrunk_, and are
attached to the _antepectus_ on each side the _prosternum_; and the
two pair of _legs_ to the _alitrunk_, the _mid-legs_ being attached
to the _medipectus_, between the _scapularia_ and _mesosternum_; and
the _hind-legs_ to the _postpectus_, between the _parapleura_ and
the _posternum_; and further, that the _arms_ are opposed to the
_prothorax_: the _mid-legs_ to the _mesothorax_ and the _primary_
organs of flight; and the _hind-legs_ to the _metathorax_ and the
_secondary_ organs of flight; though in some cases the wings appear
to be _behind_ the legs and in others _before_ them: thus, in
_Panorpa_, the former are nearer the head than the latter; but in
the _Libellulina_ the reverse of this takes place, the legs being
much nearer the head than the wings: in both cases, however, the
_scapularia_ and _parapleuræ_ run from the legs to the wings, but
in an oblique direction; and in _Panorpa_ these pieces assume the
appearance of articulations of the legs. In most of the _apterous_
hexapods they appear to be attached laterally between the _thorax_
and the _pectus_[1987]; but in the flea (_Pulex_) they are _ventral_.
In this tribe the arms are usually stated to be inserted in the
_head_[1988]: but I once succeeded in separating the head of a
flea from the trunk, and these organs remained attached to the
latter[1989]. As to the _Octopods_ and _Arachnidæ_, in the mites
(_Acarus_ L.) they are lateral, and in their analogues, the spiders
(_Aranea_ L.), they emerge between the thorax and the breast,
which last they nearly surround; in the _Phalangidæ_ the bases of
the coxæ approach near to each other, being separated only by a
narrow _sternum_; in their antagonists, _Chelifer_ and _Scorpio_,
they apply to each other, the anterior ones acting as _maxillæ_.
In the _myriapods_ the legs of the _Chilopoda_ Latr., and some
_Chilognatha_, as _Glomeris_, are inserted laterally, a single pair
in a segment; but in _Iulus_ L. their attachment is ventral, the coxæ
seem to spring from a common base, and there are two pair to each
segment[1990], except the three first, which bear each a single pair.
I shall next consider how the legs are located with respect to each
other. To render this clear to you I shall represent each of the
variations, which amount in all to twelve in the hexapods that have
fallen under my notice, by six dots.
1. [Illustration: dots] In this arrangement the legs are all planted
near to each other, there being little or no interval between the
pairs, and between the legs of each pair. It is exemplified in the
_Lepidoptera_, _Blatta_, and many _Diptera_.
2. [Illustration: dots] Similar to the preceding, but the anterior pair
are distant from the two posterior; exemplified in the bees (_Apis_)
and most _Hymenoptera_; _Chironomus_; _Scutellera_; _Pachysoma_ K.[1991]
3. [Illustration: dots] Like the last, but the posterior pair is
distant from the two anterior. _Examples_: _Silpha_, _Necrophorus_,
_Telephorus_, &c.
4. [Illustration: dots] Similar to the last, but the legs of the
posterior pair are more distant from each other than the four
anterior. _Ex. Curculio_ L.
5. [Illustration: dots] The legs of each pair near each other, but
the pairs distant. _Ex. Gibbium._
6. [Illustration: dots] Both the legs of each pair and the pairs
distant. _Ex. Blaps_, &c.