the last stage of its existence. It is now termed an _Imago_, or
perfect insect; and is capable of propagation.
Just after its exclusion, it is weak, soft, and languid: all its
parts are covered with moisture; and, if a winged insect, its wings
have so little the appearance, either in shape, size, or colour,
which they are about to assume, that it might be taken for a
mutilated abortion, rather than an animal in the most vigorous stage
of life. If it be a beetle, its elytra, instead of covering the
back of the abdomen, are folded over the breast: their substance is
soft and leathery, and their white colour exhibits no traces of the
several tints which are to adorn them. If the insect be a butterfly
or a moth, the wings, instead of being of their subsequent amplitude,
and variegated and painted with a variety of hues and markings, are
in large species scarcely bigger than the little finger nail, falling
over the sides of the trunk, and of a dull muddy colour, in which no
distinct characters can be traced. If the excluded insect be a bee
or a fly, its whole skin is white and looks fleshy, and quite unlike
the coloured hairy crust which it will turn to in an hour or two; and
the wings, instead of being a thin, transparent, expanded film, are
contracted into a thick, opaque, wrinkled mass.
These symptoms of debility and imperfection, however, in most
cases speedily vanish. The insect, fixing itself on the spoils of
the pupa, or some other convenient neighbouring support, first
stretches out one organ, and then another: the moisture of its skin
evaporates, the texture becomes firm, the colours come forth in all
their beauty; the hairs and scales assume their natural position;
and the wings expanding, extend often to five or six times their
former size--exhibiting, as if by magic, either the thin transparent
membranes of the bee or fly, or the painted and scaly films of
the butterfly or moth, or the coloured shells of the beetle. The
proceedings here described I witnessed very recently with regard
to a very interesting and beautiful butterfly, the only one of its
description that Britain has yet been ascertained to produce--I mean
_Papilio Machaon_. The pupa of this being brought to me by a friend
early in May this year (1822), on the sixteenth of that month I had
the pleasure to see it leave its puparium. With great care I placed
it upon my arm, where it kept pacing about for the space of more than
an hour; when all its parts appearing consolidated and developed, and
the animal perfect in beauty, I secured it, though not without great
reluctance, for my cabinet--it being the only living specimen of this
fine fly I had ever seen. To observe how gradual, and yet how rapid,
was the development of the parts and organs, and particularly of the
wings, and the perfect coming forth of the colours and spots, as the
sun gave vigour to it, was a most interesting spectacle. At first
it was unable to elevate or even move its wings; but in proportion
as the aërial or other fluid was forced by the motions of its trunk
into their nervures, their numerous corrugations and folds gradually
yielded to the action, till they had gained their greatest extent,
and the film between all the nervures became tense. The ocelli, and
spots and bars, which appeared at first as but germes or rudiments
of what they were to be, grew with the growing wing, and shone forth
upon its complete expansion in full magnitude and beauty.
To understand more clearly the cause of this rapid expansion and
development of the wings, I have before explained to you that these
organs, though often exceedingly thin, are always composed of two
membranes, having most commonly a number of hollow vessels, miscalled
nerves, running between them[683]. These tubes, which, after the
French Entomologists, I would name _nervures_, contribute as well
to the development of the wings, as to their subsequent tension. In
the pupa, and commonly afterwards, the two membranes composing the
organs in question do not touch each other's inner surface, as they
afterwards do: there is consequently a space between them; and being
moist, and corrugated into a vast number of folds like those of a
fan, but transverse as well as longitudinal, and so minute as to be
imperceptible to the naked eye, the wings appear much thicker than
in the end. Now as soon as the insect is disclosed, a fluid enters
the tubes, and being impelled into their minutest ramifications,
necessarily expands their folds; for the nervures themselves are
folded, and as they gradually extend in length with them, the moist
membranes attached to them are also unfolded and extended. In
proportion as this takes place, the expanding membranes approach each
other, and at last, being dried by the action of the atmosphere, become
one. To promote this motion of the fluid, seems the object of the
agitations which the animal from time to time gives to its unexpanded
wings. That a kind of circulation, or rather an injection of an aqueous
fluid into these organs, actually takes place, may be ascertained by
a very simple experiment. If you clip the wings of a butterfly during
the process of expansion, you will see that the nervures are not only
hollow, but that, however dry and empty they may subsequently be found,
they at that time actually contain such a fluid[684]. Swammerdam, who
appears to have been the first physiologist that paid attention to
this subject, was of opinion that an aëriform as well as an aquiform
fluid contributes to produce the effect we are considering. He had
observed that, if a small portion be cut off from the wing of a bee,
a fluid of the latter kind exuded from its vessels in the form of
pellucid globules, becoming insensibly drops--which he concluded
proved the action of the latter; and he noticed, also, that the wings
were furnished with tracheæ, which were at that time distended by the
injected air; whence he justly surmised, that the action of the _air_
was also of great importance to produce the expansion of the wing[685].
And Jurine found that every nervure contains a trachea, which,
proceeding from the interior of the trunk in a serpentine direction,
follows all the ramification of the nervure, though it does not fill
it[686]. Though Reaumur attributes the expansion of the wings chiefly
to an _aqueous_ fluid, yet he suspects that the _air_ on some occasions
contributed to it[687].
The wings of the other tribes of insects probably differ from the
_Lepidoptera_ in the manner in which they are folded. It should seem
from Reaumur's description, that those of some flies, instead of
the straight transverse folds of the former, have angular or zigzag
folds[688]; which equally shorten the wing. Many _Hymenoptera_ have
wings without any nervures except the marginal. We may conjecture that
these are more simply folded, so as to render their expansion more
easy; but even in these wings there are often tracheæ, which appear as
spurious nervures, and help to effect the purpose we are considering.
The operation of expanding their wings, in by far the larger number
of insects, takes place gradually as described above; and, according
to their size, is ended in five, ten, or fifteen minutes; in some
butterflies half an hour, in some even an hour. A few species, such
as _Sphinx Œnotheræ_ F., require several hours, or even a day, for
this operation; and, from the distance to which they creep before
it has taken place, a considerable degree of motion seems requisite
for causing the necessary impulse of the expanding fluids[689]. In
a few genera, however, as the gnat, the gnat-like Tipulidæ, and
the Ephemeræ, this process is so rapid and instantaneous, that the
wings are scarcely disengaged from the wing-cases before they are
fully expanded and fit for flying. These genera quit the pupa at the
surface of the water, from which, after resting upon it for a few
moments, they take flight: but this would evidently be impracticable,
and immersion in the fluid, and consequent death, would result, were
not the general rule in their case deviated from.
Some species of the last of these genera, _Ephemera_, are
distinguished by another peculiarity, unparalleled, as far as is
known, in the rest of the insect world. After being released from
the puparium, and making use of their expanded wings for flight,
often to a considerable distance, they have yet to undergo another
metamorphosis. They fix themselves by their claws in a vertical
position upon some object, and withdraw every part of the body, even
the legs and wings, from a thin pellicle which has inclosed them,
as a glove does the fingers; and so exactly do the exuviæ, which
remain attached to the spot where the Ephemera disrobed itself,
retain their former figure, that I have more than once at first
sight mistaken them for the perfect insect. You can conceive without
difficulty how the body, and even legs, can be withdrawn from their
cases; but you must be puzzled to conjecture how the wings, which
seem as thin, as much expanded, and as rigid as those of a fly, can
admit of having any sheath stripped from them; much less how they
can be withdrawn, as they are, through a small opening at the base
of the sheath. The fact seems to be, that though the outer covering
is rigid, the wing inclosed in it, notwithstanding it is sometimes
more than twenty-four hours before the change ensues, is kept moist
and pliable. In proportion, therefore, as the insect disengages
itself from the anterior part of the skin, the interior or real
wings become contracted by a number of plaits into a form nearly
cylindrical, which readily admits of their being pulled through the
opening lately mentioned; and as soon as the insect is released from
its envelope, the plaits unfold, and the wing returns to its former
shape and dimensions. Thus our little animal, having bid adieu to its
shirt and drawers, becomes, but in a very harmless sense, a genuine
_descamisado_ and _sansculotte_. It does not seem improbable, that
the pellicle we have been speaking of is analogous to that which,
in addition to the outer skin, incloses the limbs of _Lepidoptera_,
&c. in the pupa state, but which they cast at the same time with the
puparium, and leave adhering to it[690].
The body of newly-disclosed insects commonly appears at first of its
full size; but the aphidivorous flies (_Syrphus_ F. &c.), and some
others, in about a quarter of an hour after leaving the pupa become
at least twice as large as they were at their first appearance: this
apparent sudden growth, which is also noticed by Goedart, Reaumur
found to depend upon the expansion of the previously compressed
segments of the animal by means of the included air[691]. Both in this
instance and in that of insects whose wings only require expansion, the
size of the _imago_ often so greatly exceeds that of the _pupa_, that
we can scarcely believe our eyes that it should have been included in
so contracted a space. The pupa of one of the beautiful lace-winged
flies (_Hemerobius Perla_) is not so big as a small pea, yet the body
of the fly is nearly half an inch long, and covers, when its wings and
antennæ are expanded, a surface of an inch square[692].
When the development of the perfect insect is complete, and all
its parts and organs have attained the requisite firmness and
solidity[693], it immediately begins to exercise them in their
intended functions; it walks, runs, or flies in search of food; or
of the other sex of its own species, if it be a male, that it may
fulfill the great end of its existence in this state--the propagation
of its kind. Previously to thus launching into the wide world, or at
least immediately afterwards, almost all insects discharge from their
intestines some drops of an excrementitious fluid, often transparent,
and sometimes red. I have before related to you the alarm that this
last circumstance has now and then produced on the minds of the
ignorant and superstitious[694]. Whether this excrement is produced
indifferently both by males and females I cannot positively assert;
but a circumstance related by Jurine affords some ground for a
suspicion that it is peculiar to the latter. A specimen of a female
of _Lasiocampa Rubi_, when killed emitted some of this fluid, which
dropped upon the floor: this appeared to attract the males to the
apartment in which it happened, and to the very spot--from whence it
may be conjectured, that the scent of the fluid brought them there,
and that the use of it is to bring the sexes together soon after
exclusion from the pupa[695].
The colour, sculpture, and other peculiarities which distinguish
insects in this state I shall consider at large in another letter,
when I treat of their external parts and organs. Under the present
head I shall confine myself to pointing out the characters by which
the _sexes_ of many species are distinguished from each other;
as likewise the _duration of their life_ in their perfect state;
together with the circumstances on which this duration depends.
I. _Sexual Distinctions._ The first general rule that may be laid down
under this section is,--That among insects, contrary to what mostly
occurs in vertebrate animals, the _size_ of the female is almost
constantly larger than that of the male. Even in the larva and pupa
states, a practised eye can judge, from their greater size, which
individuals will become females. There are, however, some exceptions
to this rule. Thus amongst the _Coleoptera_, the male _Dynastidæ_,
remarkable for their horns, as you may see in _D. Alocus_, _Antæus_,
_Actæon_, &c., as likewise those of _Lucanus_, are larger than the
unarmed females[696]. In the _Neuroptera_ the female _Libellulidæ_ are
sometimes sensibly smaller, and never larger, than their males[697]. In
the _Hymenoptera_ the male of the hive-bee, but more particularly that
of _Anthidium manicatum_ and other bees of that genus, is much more
robust than the other sex[698]. In the _Diptera_, the same difference
is observable in _Syrphus Ribesii_, and some other aphidivorous
flies, and also in _Scatophaga stercoraria_[699]. And amongst the
_apterous_ tribes, we are informed by De Geer that the male of
_Argyroneta aquatica_, which builds an aërial palace in the bosom of
the waters[700], usually exceeds the female in bulk[701]. The reason of
this rule seems in some degree connected with the office of the female
as a mother, that sufficient space may be allowed for the vast number
of eggs she is destined to produce; and it is when impregnation has
taken place, and the eggs are ready for extrusion, that the difference
is most sensible. In the majority of cases this sexual disproportion
is not very considerable, but in some few it is enormous. Reaumur
mentions a beetle, of which he intended to give the history, the male
of which is so small compared with the female, that a bull not bigger
than a sheep, or even a hare, set by the side of the largest cow, would
aptly contrast with them. This little beetle, he says, has wings and
elytra, while the giant female has no vestige of either, having the
upper surface of its body naked and membranous[702]. The species to
which this illustrious Naturalist here alludes, does not appear to
have been ascertained. The female of many gall-insects (_Cocci_) is so
large in comparison with the male, that the latter traverses her back
as an ample area for a walk[703]. But this is nothing compared with
the prodigious difference between the sexes of _Termes fatale_, and
other species of white ants, whose males are often many thousand times
less than the females, when the latter are distended with eggs[704].
Accidental differences in the size of the sexes sometimes arise: as
when the female larva has, from any cause, been deprived of its proper
supply of food, it will occasionally be less than the male. De Geer
has stated a circumstance with respect to the _Aphides_ that produce
galls, that should be mentioned under this head--the first, or _mother_
female, is larger than any of her progeny ever become[705].
The second observation that may be generally applied to the sexes of
insects is, that, size excepted, there is a close resemblance between
them in other respects. But to this rule the exceptions are very
numerous, and so important that it is necessary to specify examples
of each under distinct heads.
i. In some species the sexes are either partly or wholly of a
different _colour_. Thus, in the order _Coleoptera_, the elytra of
the male of _Rhagium meridianum_ F. are testaceous, and those of the
female black. _Leptura rubra_ of Linné, with red elytra, is the
female of his _L. testacea_, in which they are testaceous. _Cantharis
dermestoides_ of the same author is the other sex of his _Meloe
Marci_; one of which is chiefly testaceous, and the other black:
which seems to have so misled Linné, that he placed them in different
genera. One more instance in this order, the female of _Cicindela
campestris_, as was first observed to me by our friend Sheppard, has
a black dot on each elytrum, not far from its base near the suture,
which the male has not.
Amongst the _Orthoptera_, the male _Locustæ_ F., as Professor
Lichtenstein has informed us[706], have a fenestrated ocellus, which
is not to be found in the other sex. I was once attending to the
proceedings of a Hemipterous species, _Pentatoma oleracea_ Latr.,
which I found in union: the paired insects had white spots, but
another individual was standing by them, in which the spots were
of a sanguine hue. I mention this by the way only--the spots in
the prolific sexes being of the same colour: but might not the red
spotted one be a neuter?
The sexes of many _Lepidoptera_ likewise differ in their colour. I
must single out a few from a great number of instances. The males of
_Lycæna Argus_ F. have the upper surface of their anterior wings of a
dark blue, while in the female it is wholly brown. The wings of the
former sex of _Hypogymna dispar_ are gray, clouded with brown; but
those of the latter are white, with black spots. In the brimstone
butterfly (_Colias Rhamni_), which is one of the first that appear in
the spring, the wings of the male are yellow--of the female whitish.
In the common orange-tip (_Pieris Cardamines_ F.), one sex has not
the orange tip to the upper wings: and, to name no more, the male of
_Lycæna dispar_, one of our rarest and most beautiful butterflies,
has only a single black spot in the disk of its fulgid wings; while
in the other sex, the primary pair have nine, and the secondary are
black, with a transverse orange fascia near the posterior margin.
But the most remarkable difference in this respect observable in the
insects of the order in question, takes place in a tribe, of which
only one species is certainly known to inhabit Britain--I mean the
_Papiliones Equites_ of Linné: what he has called his _Trojani_ and
_Achivi_ in some instances have proved only different sexes of the
same species. Mr. MacLeay's rich cabinet affords a singular instance
confirming this assertion;--a specimen of a Papilio is divided
longitudinally, the right hand side being male, and the left hand
female. The former belongs to _P. Polycaon_, a Grecian, the latter to
_P. Laodocus_, a Trojan. An instance of two _Grecians_ thus united is
recorded in the _Encyclopédie Méthodique_, as exhibited in a specimen
preserved in the Museum of Natural History at Paris; which on the
right hand side is _P. Ulysses_, on the left _P. Diomedes_[707].
In the _Neuroptera_, the _Libellulidæ_ are remarkable for the
differences of colour in the sexes. In the common _Libellula
depressa_, which you may see hawking over every pool, the abdomen
of the male is usually slate-colour, while that of his partner is
yellow, but with darker side-spots. Reaumur, however, noticed some
males that were of the same colour with the females[708]. Schelver
observed, when he put the skins of _Libellula depressa_ into water,
that the colours common to both sexes were in the substance of the
skin, and remained fixed; while those that were peculiar to one
could be taken off with a hair-pencil, and coloured the water: which
therefore were superficial, and, as it were, laid on[709]. The yellow
males, therefore, that Reaumur observed, were probably such as had
the superficial blue colour which distinguishes them washed off. In
_Calepteryx Virgo_ Leach, the former are of a lovely silky blue, and
the latter green. In _Agrions_ F. nature sports infinitely in the
colours of the sexes.
In the order _Hymenoptera_ there are often differences equally great;
the sexes of many of the Ichneumons and Saw-flies are of quite
different colours. The former tribe Linné has divided into sections,
from the white annulus observable in the antennæ of some, and from
the colour of their scutellum: but these are often merely sexual
characters[710]. The male of _Anthophora retusa_ Latr., a kind of
wild bee, is wholly black, the female wholly gray, and of so very
different an aspect that they were long regarded as distinct species;
a mistake which has likewise occurred with regard to the sexes of
_Osmia cærulescens_, another bee, of which the male has a bronzed and
the female a violet abdomen[711]. The nose of male _Andrenæ_ Latr.
is often yellow, or white, as in _A. hæmorrhoidalis_--when that of
the female is black[712]. The _labrum_ also is often of a different
colour in the sexes, as in _Ceratina_ Latr.
In the _Diptera_, _Aptera_, _Arachnida_, &c., I am not aware of any
striking difference in the colours of the sexes.
ii. The sexes of insects vary (but more rarely than in colour) in
their _sculpture_ also, and _pubescence_. Thus the elytra of the
females of many of the larger water-beetles (_Dytiscus_) are deeply
furrowed, while those of the males are quite smooth and level[713].
The thorax of the female in several species of _Colymbetes_ of the
same tribe, as _C. Hybneri_ and _transversalis_, on each side has
several tortuous impressed lines or scratches, like net-work, which
are not to be discovered in the male. _Hyphydrus gibbus_ Latr.,
which differs solely from _H. ovalis_ (_Dytiscus ovalis_ Illig.) in
being thickly covered with minute impressed puncta, is, from the
observation of the Rev. R. Sheppard, the other sex of this last,
with which he has taken it coupled; and it is by no means improbable
that _Hydroporus picipes_ (_Dytiscus punctatus_ Marsh.) and _H.
lineatus_,--between which, as Gyllenhal has justly observed, the same
difference only exists,--are in like manner sexual varieties. With
respect to _pubescence_, I have not much to say. Another aquatic
beetle, _Acilius sulcatus_ Leach, has not only its elytra sulcated,
but the furrows of these, and a transverse one of the thorax, are
thickly set with hair; while the male is smooth, and quite naked.
Particular care seems to have been taken by the Creator, that when
all the above inhabitants of the water are paired, the male should be
able to fix himself so firmly, by means of his remarkable anterior
tarsi, (which I shall afterwards describe,) and these asperities,
&c. in the upper surface of his mate, as not to be displaced by the
fluctuations of that element, the reluctance of the coy female, or
any other slighter cause.