usually nearly or altogether _horizontal_; but in _Notonecta_ and
_Plea_ they are _deflexed_ and cover the sides of the body; and the
apical area of one wing precisely covers that of the other; where
the scutellum does not intervene, as in _Scutellera_, _Pentatoma_,
&c., the vertical angles of the Anal Area meet in the middle of the
back, so as to exhibit the appearance of a cross. In _Notonecta_, in
which the _hemelytra_ are deflexed, at the apex of the _membrana_
is a fissure which permits the two sides to form an angle with each
other, and to apply exactly to the body. In _Plea_, in which there
is no apical area, the posterior margins of the _tegmina_, as they
ought rather to be termed, unite, but do not lap over each other.
With regard to the appearance of something like a _phialum_, if you
examine the _hemelytra_ of most species of bugs on the underside, you
will see that the costal nervure at the base is inflexed and covers a
kind of channel; if you next take one of _Belostoma grandis_, where
the structure is most conspicuous, or even the common _Nepa cinerea_,
you will find in the same situation, adjacent to the inflexed costal
nervure, a hollow tube running from the base of the wing, and
terminating, after proceeding about one-fourth of its length, in a
hollow cavity, which, as it is covered by a membrane, appears to me
to be a collapsed pouch. This circumstance is worthy of further and
more general investigation.
5. In their _shape_, with few exceptions, _hemelytra_ more or less
represent a wedge, being wider at their apex, where they are usually
obliquely truncated, than at the base; but in _Plea_ Leach they are
obtusangular, with the angle in the sutural margin; in _Notonecta_,
on the contrary, an obtusangular sinus distinguishes that part; in
_Naucoris_ they are curvilinear and every where of equal width;
in _Ranatra_ they are linear and straight; in _Aradus_ they are
oblong, usually with an external lobe or dilatation at their base:
a remarkable instance of the intention of this is observable in a
nondescript Brazilian species, in which the head, prothorax, and
abdomen, are edged with a number of broad foliaceous appendages; if
the base of the _hemelytrum_ had not been furnished with a similar
appendage, the symmetry of the whole body would have been destroyed
by the hiatus between the prothorax and abdomen, as may be seen by
removing the _hemelytra_; but by this compensating contrivance of
Providence, the gap is filled, the above lobe fitting exactly into it.
6. The _neuration_ of these organs will not occupy us long, since
the _corium_ or harder part, though in some species there are traces
of nervures, is often without them. Those of the cucullated species
of _Tingis_ resemble many _tegmina_ in being ornamented by them with
a kind of network, which looks like the finest lace; in several
_Lygæi_, _Edessa_, and some _Reduvii_, there are a few diverging
longitudinal nervures which occasionally by a ramification here and
there form an areolet[1845], but there are seldom any traversing
nervures. The Apical Area is usually most distinguished by nervures,
in some forming several areolets, as in _Aradus_, in others running
parallel to each other, nearly to the end of this area, as in
_Belostoma grandis_, where they are met by a traversing nervure; the
object of this is doubtless to strengthen the membrane.
7. Both _tegmina_ and _hemelytra_ are most commonly naked, yet very
short _hairs_ are found on those of some species of _Cercopis_, and
in many more instances in those of the latter description, as in
_Notonecta_, several _Lygæi_ and _Reduvii_, &c.
8. _Colours_ in _hemelytra_ are very various, and in many instances
are peculiar to families; in certain _Lygæi_ (_L. Hyoscyami_, &c.)
black and red; in _Lygæus compressipes_ and affinities a dingy black;
in some _Reduvii_ black with a large white spot;--but it is needless
to enlarge further on this subject.
9. That _hemelytra_ are used in flight is evident not only from
the large space allowed for their muscles[1846], but likewise from
a circumstance noticed by M. Chabrier, that in flight, in the
_Pentatomæ_ Latr., the corium of the _hemelytrum_ is fixed to the
wing[1846]; in which case both must describe the same arc.
iv. _Wings._ We are next to consider organs which are exclusively
appropriated to _flight_, and therefore are properly denominated
_wings_. These in the Orders that have _elytra_, _tegmina_, or
_hemelytra_, are the pair that correspond with the secondary wings of
the other Orders. It may be said, indeed, that in several instances
both _tegmina_ and _hemelytra_ do not differ at all in substance
or use from the wings that they cover. This is true; but as their
structure in other respects is the same with that of those that are
more solid and less apt for flight, it was convenient to consider
them under the same name.
1. To begin with the _articulation of these organs with the trunk_; in
general it may be stated that this, as in _tegmina_ and _hemelytra_,
is usually by the intervention of three axes, formed by the conflux
of the nervures of the three areas at the base of the wing, which
either immediately or by other pieces are implanted in the trunk, so
as to receive from it the aërial and other fluids, necessary for its
expansion and motions[1847]. Having given this general statement, I
shall next apply it to the wings in some of the different Orders. If
you carefully extract one from the stag-beetle (_Lucanus Cervus_)
or any large species of the _Dynastidæ_, in the _Coleoptera_; the
first thing that will strike you, upon examining the base, will be
the plate before mentioned called by Chabrier the _humerus_, which
is a stout transverse corneous piece, with a deep sinus towards
the wing, filled with ligament: if you again follow the costal,
mediastinal, and postcostal nervures, you will find them unite to
form an axis, consisting of three parallel pieces, which connects
by its intermediate internal piece with one end of this plate. The
nervures of the Intermediate Area terminate also in a corneous axis at
a greater distance from the base than the other two, which connects
with Chabrier's _humerus_ by means of the ligament of the sinus just
named. Those of the Anal Area are received by a ligament attached to
a transverse plate, widest at its anterior end, which connects with
the posterior part of the said _humerus_; and at its posterior end is
united to the _postfrænum_[1848], with which it forms a right angle.
In the _Orthoptera_ Order the structure is not very different, but the
axes and other plates of the base of the wing are less distinct and
rather cartilaginous; the nervures of the Anal Area often terminate
in a transverse one that there forms the segment of a circle[1849];
the inner base of this circle is ligament connected with the
_postfrænum_[1850]. In the Homopterous _Hemiptera_ the three axes may
be readily traced, but the _humeral_ plate, with which they all are
connected, is more irregular in shape, and in _Fulgora_ longitudinal,
with an angular surface; in this Order the nervure, in some cases
consisting of cartilaginous rings[1851], in which the _frænum_ and
_postfrænum_ terminate in the _tegmina_ and wings, is attached
posteriorly to the ligament of the Anal Area. In the Heteropterous
section the three axes are evident, but the _humeral_ plate is not
easily made out. In the _Libellulina_ the axes of the Costal and
Intermediate Areas are the coloured broad plates at their base, formed
by the dilatation of their nervures; that, however, of the Anal is not
dilated, but forms one nervure, in the primary wing, with the _frænum_,
and in the secondary with the _postfrænum_. Having given you this clue
to trace the axes in those tribes in which they are most conspicuous,
it will assist you in searching for them in the remaining Orders,
in all of which they may be traced, except perhaps in those minute
_Hymenoptera_ whose wings have solely the costal nervures; probably in
these there is only _one_ axis. In the _Lepidoptera_ and _Hymenoptera_
a circumstance connected with the present head is observable, which is
not to be discovered in the other Orders: these are the _tegulæ_ or
base-covers, which appear intended to defend the base of the anterior
wings. They are concavo-convex scales, which in the _Lepidoptera_ are
large and of an irregular shape[1852], but in the _Hymenoptera_ are
smaller and semicircular[1853].
2. Wings, with regard to their _substance_, may generally be termed
_membranous_; but they vary in this respect, some being much thicker
than others, either _partially_ or _totally_: in spotted wings,
as in those of many _Libellulina_, _Tettigoniæ_ F., &c., the dark
opaque parts are denser than those that are transparent: in several
_Orthopterous_ insects, as in _Phasma_, some _Mantes_, &c., the
Costal Area or covering part of the wing is of a substance equally
firm with that of the _tegmen_. This is a compensating contrivance,
that where the latter is shorter and smaller than the former,
its membranous part, when folded, may be protected from injury.
Another similar contrivance of DIVINE WISDOM is exhibited by those
_Pterophyllæ_ K. (_Locusta_ F.) whose _tegmina_ resemble the leaves
of plants (_Pt. laurifolia_, &c.); in these the tip of the wings when
folded being longer, is not covered by the _tegmina_, and therefore
exposed to injury; to prevent which this small piece, while the
whole wing, as far as covered by those organs, is membranous, is of
the same substance with them[1854]. The wings of most _Coleoptera_,
_Orthoptera_, _Hemiptera_, and _Thereva coleoptrata_, in the
_Diptera_, &c., are of a firmer substance than those of the other
Orders; in many _Locustæ_ Leach, _Fulgoræ_, &c., they are nearly as
firm as the _tegmina_; and in _Ascalaphus italicus_, except at their
base, the _secondary_ wings are less membranous than the _primary_.
M. Chabrier has observed[1855] that the wings of insects in general
diminish in thickness from their base to their apex, and from their
anterior to their posterior margin.
3. I should have had, it is probable, but little original matter
to communicate under the head of the _composition_ and _neuration_
of wings, had M. Jurine, who has written so ably on those of
_Hymenoptera_, undertaken a survey of the organs of flight in every
Order of insects: but as his views were confined to only _two_ of
the Linnean Orders, it is not wonderful that his system and set
of terms should fail where a generalization is necessary; and I
may stand acquitted of presumption and conceit if I attempt to
substitute a system and body of terms more universally applicable.
Had the plan of this able Entomologist led him to pay attention to
_tegmina_ and _hemelytra_, their division into three longitudinal
areas would have immediately struck him; and having acquired this
outline of the greater natural divisions, he would have applied it
to the Orders that have _wings_ only, and having discovered that it
is to be traced in all, the result would have probably superseded
my labors. Had his life been longer spared, perhaps something of
this kind would have been effected by him; but as he, alas! is gone,
and no abler hand seems to have undertaken the task, I will do what
I can to give you satisfaction on this subject[1856]. You have
already got a tolerably good idea of these areas from what has been
said upon the subject under _tegmina_ and _hemelytra_; but I shall
now more particularly state to you how they are circumstanced in
_wings_. I shall first explain the general law as to their limits.
The _Costal Area_[1857] is all that longitudinal portion of the wing
that lies between the anterior margin and the postcostal nervure;
the _Intermediate Area_[1858] is all that longitudinal portion of
the wing that lies between the postcostal and the anal nervures;
and the _Anal Area_[1859] is all that longitudinal portion of the
wing that lies between the anal nervure and the posterior margin.
But there are other helps to enable you to distinguish the areas
in the different Orders. The Anal Area in all Orders forms the
posterior _fold_ of the wing; in _Coleoptera_ turned under when in
repose; in _Orthoptera_ folded like a fan; in _Lepidoptera_, in some
_Papilionidæ_, forming an arch over the abdomen. Again, in _Blatta_,
the Costal Area is distinguished chiefly by _longitudinal_ nervures;
the Intermediate by _oblique_ ones; and the Anal by _radiating_ ones;
and in both this tribe and the _Mantidæ_ this last Area is marked
out from the Intermediate by a marginal notch, which is not present
in _Phasma_, but is found in both sections of the _Hemiptera_. In
_Locusta_ Leach the notch is between the Costal and Intermediate
Areas: in _Phasma_ the nervures of the Intermediate Area are branches
of the externo-medial, while those of the Anal, as they do in all
the _Orthoptera_, diverge from the base of the wing: in many, as
in _Pterophylla_ K., the part of the wing lately alluded to, that
is longer than the _tegmen_, and of the same substance, points out
the limit of the Costal Area; and in others this part terminates
in a segment of a circle and is differently reticulated at the
apex from the Intermediate: in the Homopterous _Hemiptera_ and the
_Libellulina_, in which the areas at first seem indistinct, they
may generally be easily traced by following them from the axes. The
separation of the Costal from the Intermediate in the remaining
Orders seems less easy on account of the branching of the nervures:
in the rest of the _Neuroptera_ and the _Lepidoptera_, if the
posterior branches of the postcostal nervure are not included, you
will have a narrow Postcostal Area, which in most cases forms an
angle more or less prominent, in _Corydalis_ almost a right angle,
with the Intermediate: in _Hemerobius_ and affinities this part is
distinguished by areolets formed by _transverse_ nervures, while
those of the rest of the wing are _longitudinal_[1860]: but if the
posterior branches are included, the Costal Area will be more ample:
a similar observation applies to the _Hymenoptera_ and _Diptera_;
in these, in all cases, the areolets adjoining the anterior margin,
which follow the _stigma_, should be regarded as belonging to the
Area in question[1861]. In those tribes of the former Order, whose
wings are without _nervures_, the areas are often marked by _folds_.
M. Chabrier has observed that in _Coleoptera_ the specific weight of
the margin of the wing, and its means of resistance, are augmented
by a liquid which is introduced, at the will of the animal, into
a long pocket under the _brachial_, here called the _costal_ and
_mediastinal_ nervures, covered by a supple membrane, which in a
state of repose becomes flaccid[1862]: it is easily detected, being
of a paler colour than the nervures between which it lies; this is
what I call the _Phialum_; we have before seen that it exists also in
_Elytra_ and some _Hemelytra_[1863]; but I have not detected it in
any other _wings_.
I have before given you a sufficiently full account of the _alulæ_
or winglets of _Diptera_[1864]; and shall here only observe that
they are not confined to _one_ particular tribe, as has been usually
imagined; but though sometimes extremely minute, simple, and not
easily detected, are an _universal_ distinction of the Order.
Having thus endeavoured to elucidate the _larger_ Areas into
which wings appear to be divided; I shall next say something on
the _smaller_ ones produced by the intersection or ramification
of the nervures; these had been named areolets (_areolæ_) several
years before M. Jurine's work, in which he calls them, I think
_improperly_, cellules (_cellulæ_), was published; I therefore
retain the _prior_ term. The general structure of the nervures of
the wings of insects having been before explained[1865], I shall not
here repeat what I then said; but there is a curious circumstance
connected with it, particularly visible in the wings of certain
_Hymenoptera_, that I must not pass without notice. If you examine
attentively with a microscope against the light the wing of any
_Nomada_ or _Andrena_, you will discover little transparent points
in some of the smaller transverse nervures that form the middle
areolets, in which the nervure becomes white and looks as if it
was interrupted, though in substance it seems continued: these
little points, somewhat resembling minute air bubbles detained in
the tubes, are what M. Jurine, who first discovered them, has, on
that account, named _bullæ_, which he thus further describes:--"When
the tube (of the nervure) arrives at the spot where a _bulla_ is to
be formed, it extends itself on all sides in minute threads in the
upper membrane of the wing, losing its colour and tubular structure,
which it resumes immediately after the formation of the bulla[1866]."
But if you look closely at them you will find that there is always
a slight _fold_ of the wing that cuts the nervure exactly at the
_bullæ_, and if the fold changes its direction they accompany it;
their object, therefore, is clearly to relax the tension so as to
admit a little motion where the fold is; consequently, rather than
_bullæ_ (bubbles), they should be denominated _articulations_. A
similar construction, but on a larger scale, may be observed in the
wings of _Coleoptera_[1867] and some others, as _Psocus_, where the
folds traverse the nervures. I shall next make a few observations
on the principal nervures; and first a word upon their _names_. M.
Jurine, being of opinion that a striking analogy exists between the
wings of _insects_ and those of _birds_, in which M. Chabrier seems
to agree with him, has named the nervures in the anterior margin of
the wings of the former, _radius_ and _cubitus_, as corresponding
with the bones so named in the fore-arm of the latter, and the plate
which often terminates these nervures in _Hymenoptera_, he names
the _carpus_; it may look like presumption to differ from two such
weighty authorities, but as their observations seem to have been
too limited, in one case to the _Hymenoptera_ and _Diptera_ only;
and in various Orders there is nothing analogous to the _stigma_ or
_carpus_, and all the other nervures of an insect's wing have no
analogue in that of a bird, but more especially as M. Latreille seems
to think with me on this subject[1868], I have retained Linné's term
for the marginal nervure, and for most of the others have adopted
those of the great French Entomologist just mentioned. I shall here
only further observe,--and it seems to me an observation of prime
importance, in the determination of the question of the analogy of
the wings of insects,--that they are not, as in birds, the _fore-leg_
converted into an organ of flight, but, like the wing of the _Draco_,
an organ _superadded_ to the legs; and, further, that the connection
is not with the fore-legs, but, as has been before observed[1869],
with the two posterior pairs.
The _Costa_[1870] is usually the strongest of the nervures, and
that upon which the wing seems to be built; but in some cases, as
in _Blatta_, _Scutellera_, _Cynips_, &c., it is represented by
the mere membrane of the anterior margin; in some _Coleoptera_,
as in _Geotrupes_, _Dytiscus_, &c., its structure, except at the
base, appears to be annular or nearly so, at least a vast number
of corrugations, running transversely, are observable on its upper
and lower surfaces; it is thus capable of greater tension and
relaxation, and more flexile. The _stigma_ or _carpus_[1871], though
most conspicuous in the _Hymenoptera_ Order, may be traced in some
_Coleoptera_, Heteropterous _Hemiptera_, the _Libellulina_, &c.;
but it has no representative in the _Orthoptera_, _Lepidoptera_,
_Trichoptera_, &c. The _mediastinal_ is usually a very slender
nervure, placed between the _costa_ and _postcosta_, sometimes
terminating in the former[1872], and at others in the latter[1873]: in
the _Orthoptera_, _Lepidoptera_, &c., however, and some others, it is
a very conspicuous and principal one[1873]; in the _Hymenoptera_ it
is obsolete, merging in those nervures[1874]. The _Postcosta_ is the
principal nervure of the wing in _Scutellera_, but in _Staphylinus_
it is wanting; in _Chalcis sispes_ it is the only true nervure of
that organ, the others being represented by spurious ones[1875]. The
_externomedial_ and _internomedial_ are sometimes distinct at their
origin, but more frequently are branches from a common stem.