criterion of animal as well as vegetable perfection is the ability
to continue the species[69];" and in their progress to this state
certain changes take place in the parts and organs of _all_ animals
and vegetables: there is, therefore, an analogy in this respect
between them; and this analogy also furnishes another argument
against Dr. Herold's hypothesis, as we shall presently see. These
changes are of three kinds: In the vegetable kingdom, at least in
the phænogamous classes, there is a succession of developments
terminating in the appearance of the generative organs, inclosed
in the flower; in this kind the integuments, or most of them,
are usually _persistent_. In insects and other annulose and some
vertebrate animals, there is a succession of spoliations, or
simultaneous changes of the whole integument, till the animal appears
in its perfect form with powers of reproduction; in this kind the
integuments are _caducous_.--In man and most of the vertebrate
animals there is a gradual action of the vital forces in different
organs till they are fitted for reproduction; accompanied, as
progress is made to the adult state, by the acquisition of certain
organs, &c. as of teeth, horns, pubes, feathers, &c.[70] Let us
now consider a little in detail the analogies that appear to exist
between the second and the first and third kinds. I shall first
consider the latter as the least obvious. That able, judicious, and
learned physiologist, Dr. Virey, has pointed out no inconsiderable
resemblance between the metamorphosis of the insect, and the changes,
which he denominates a metamorphosis by metastasis, to which most
vertebrate animals are subject. In them, he observes, a state
analogous to the _larva_ state begins at the exclusion of the fœtus
from the womb; it is deprived of teeth, and its viscera are only
accommodated to milk: in the cornute species the horns are in embryo:
the digestive system now preponderates, and the great enjoyment is
_eating_. A second state, in a degree analogous to that of _pupa_,
commences at the period of dentition--the teeth now produce another
modification in the intestinal canal, which becomes capable of
receiving and digesting solid food: during this period the vital
forces are all tending to produce the perfect state of the animal;
and in this state, in man especially, the individual is _educated_
and fitted to discharge the duties of active life. Again, analogous
to the _imago_ state is the age of puberty, in which the complete
development of the sexual powers takes place in both sexes, and the
animal has arrived at its acme, and can continue its kind[71]: now
the digestive powers diminish in their activity, and love reigns
paramount. When this state is fully attained, no further or higher
change is to be expected, and the progress is soon towards decay and
the termination of the animal's mortal career. So we see that in fact
man and other _mammalia_, though they do not simultaneously cast
their skins like the insect; or pass into a state of intermediate
repose, before they attain the perfection of their nature, like the
caterpillar; have their three _states_, in each of which they acquire
new parts, powers, and appetites.
But a more striking analogy has been traced between the insects
that undergo a complete metamorphosis and the _vegetable_ kingdom;
for though the _primary_ analogy seems to be between the _Polypus_
and the _Plant_, yet the _secondary_ one with the _Insect_ is not
by any means remote. There are circumstances to which I shall have
occasion hereafter to call your attention, which afford some ground
for supposing, that the substance of the insect and the vegetable
partakes of the same nature, at least approximates more nearly, than
that of the insect and the vertebrate animal; and every one who
has observed these little creatures with any attention, will have
observed amongst them forms and organs borrowed as it were from the
kingdom of Flora; and _vice versâ_ the Botanist, if he makes the
comparison, will find amongst his favourite tribes many striking
resemblances of certain insects. But the analogy does not stop here;
for the butterfly and the plant appear to have been created with
a particular reference to each other, both in the epoch of their
appearance and the changes that take place in them. Thus, as Dr.
Virey has observed, the caterpillar is simultaneous with the leaf
of the tree or plant on which it feeds, and the butterfly with the
flowers of which it imbibes the nectar[72]. Swammerdam, I believe,
was the first who noticed the analogy between the _changes_ of
the insect and the vegetable, and has given a table in which he
has contrasted their developments, including other animals that
undergo a metamorphosis[73]: an idea which has been generalized
by Bonnet[74], and adopted and enlarged by Dr. Virey[74]. A state
analogous to that of the _larva_ in the insect begins in the plant
when it is disclosed from the seed, or springs from its hybernaculum
in the bulb, &c., or is evolved from the gemma; integument after
integument, often in various forms, as cotyledon, radical, cauline,
or floral leaves, expands as the stem rises, all which envelopes
incase the true representative of the plant, the fructification, as
the various skins do the future butterfly. When these integuments
are all expanded, the fructification appears inclosed by the calyx
or corolla as the case may be, in which the generative organs are
matured for their office--this is the bud, which is clearly analogous
to the _pupa_ state of the insect. Next the calyx and corolla expand,
the impregnation of the germen takes place, and the seed being
ripened, and dispersed by the opening of the seed-vessel or ovary of
the plant, the individual dies: thus the _imago_ state of the insect
has its representative in the plant. "If we place," says Dr. Virey,
"here the egg of the insect, next its caterpillar, a little further
the chrysalis, and lastly the butterfly--what is this but an animal
stem--an elongation perfectly similar to that of the plant issuing
from the seed to attain its blossoming and propagation?"[75]
There being, therefore, this general analogy in their progress to
that state in which they can continue their species between every
part of animated nature, it holds good, I think, that the same
analogy should take place in their developments. If the adult man
or quadruped, &c. is evidently an evolution of the fœtus, as from
microscopical observations it appears that they are[76], if the
teeth, horns, and other parts, &c. to be acquired in his progress to
that state are already in him in their embryos, we may also conclude
that the butterfly and its organs, &c. are all in the newly-hatched
caterpillar. Again, if the blossom and its envelopes are contained
in the gemma, the bulb, &c. where they have been discovered[77],
it follows analogically that the butterfly and its integuments all
preexist in its forerunner.
Perhaps after this view of the objections to Dr. Herold's hypothesis,
it will not be necessary to say much with regard to the argument
he draws from the change of organs--the loss of some and the
acquisition of others--since this may readily be conceived to be the
natural consequence of the vital forces tending more and more to the
formation of the butterfly, and the withdrawing of their action more
and more from the caterpillar; I shall not, therefore, enter further
into the question, especially since the change of organs will come
more regularly under our notice upon a future occasion.
Winged insects, many branchiopod _Crustacea_, and the Batracian
reptiles, have been observed by Dr. Virey to bear some analogy to the
_mammalia_, _aves_, &c. in another respect. In leaving their egg,
they only quit their first integument, answering to the _chorion_ or
external envelope of the human fœtus; they therefore still continue
a kind of fœtus, so to speak, more or less enveloped under other
tunics, and principally in their _amnios_, or the covering in which
the fœtus floats in the _liquor amnii_[78]. This the butterfly does
in the pupa case; and its birth from this, under this view, will be
the true birth of the animal. In the human subject, the ova upon
impregnation are said to pass from the ovary through the Fallopian
tube into the uterus. In the insect world, upon impregnation, the
eggs pass first from the ovaries into the oviduct, answering to the
Fallopian tube, which in them terminates in the _ovipositor_, or
the instrument by which the parent animal conveys the eggs to their
proper station: there is, therefore, nothing properly analogous to
the uterus in the insect, and the substance upon which the larva
feeds upon exclusion answers the purpose of a placenta.
After this general view of the most modern theories with regard to
the _metamorphosis_ of insects, I shall in the present and some
following letters, treat separately of the different states through
which these little beings successively pass.
* * * * *
The first of these is the _Egg state_, the whole class of insects
being strictly oviparous. Some few tribes indeed bring into the
world living young ones, and have on that account been considered
as _viviparous_, but incorrectly, for the embryos of none of these
are nourished, as in the true viviparous animals, within a uterus
by means of a placenta, but receive their development within true
eggs which are hatched in the body of the mother. This is proved
by the observations of Leeuwenhoek, who found eggs in the abdomen
of a female scorpion[79]; and of Reaumur, with regard to the
flesh-fly (_Musca carnaria_) and other viviparous flies as they
have been called[80]. A similar mode of production takes place in
vipers and some other reptiles, which have hence been denominated
_ovo-viviparous_, to distinguish them from the true viviparous
animals--the class _Mammalia_. By far the larger portion of
insects is oviparous in the ordinary acceptation of the term. The
ovo-viviparous tribes at present known are scorpions; the flesh-fly
and several other flies; a minute gnat belonging to Latreille's
family of _Tipulariæ_[81]; some species of _Coccus_; some bugs
(_Cimicidæ_)[82]; and most _Aphides_, which last also exhibit the
singular fact of individuals of the same species being some oviparous
and others ovo-viviparous, the former being longer in proportion than
the latter.--Bonnet, however, is of opinion that the eggs of the
first are not perfect eggs, but a kind of cocoon, which defends the
larva, already formed in some degree, from the cold of winter[83].
When excluded from the body of the mother, or from the egg, as has
been before observed, some insects appear nearly in the form of
their parents, which, with a very slight alteration, they always
retain; others, and the greater number, assume an appearance totally
different from that of their parents, which they acquire only after
passing through various changes. It is to these last, which have
chiefly engaged the attention of Entomologists, that the title of
metamorphoses has been often restricted. As, however, those insects
which undergo the slightest change of form, as spiders do, undergo
_some_ change, and almost all insects cast their skins several
times[84] before they attain maturity, Linné and most Entomologists,
till very recently, have regarded the whole class as undergoing
metamorphoses, and as passing through _four_ different states, viz.
the Egg--the Larva--the Pupa--and the Imago.
It is obvious, however, that in ovo-viviparous species _three_ states
of their existence only come under our cognizance, as these, being
hatched in the body of the mother, come forth first under the form of
larvæ. There is even one tribe of insects which presents the strange
anomaly of being born in the _pupa_ state. This is the Linnean
genus _Hippobosca_ (_Pupipara_ fam. Latr.), to which our forest-fly
belongs, the females of which lay bodies so much resembling eggs,
that they were long considered as such until their true nature was
ascertained by Reaumur (most of whose observations were confirmed
by De Geer), who, from their size, which nearly equals that of
the parent fly--from their slight motion when first extruded--from
spiraculiform points which run down each side of them--and lastly,
from their producing not a larva, as all other insects' eggs do,
but perfect flies in the winged state--inferred, and doubtless with
reason, that they are not real eggs, but pupæ, or larvæ just ready
to assume the pupa state, which, however strange it may seem, have
passed the egg and larva states in the body of the mother[85].
Insects, therefore, as to their mode of birth, may be divided into--
I. _Ovo-viviparous_, subdivided into--
1. _Larviparous_, coming forth from the matrix of the mother in
the state of larvæ, as the Scorpion (_Scorpio_), the Flesh-fly
(_Musca_), the Plant-louse (_Aphis_), &c.
2. _Pupiparous_, continuing in the matrix of the mother during
the larva state, and coming forth in that of _pupa_, as the
Forest-fly (_Hippobosca equina_), the Sheep-louse (_Melophagus
ovinus_), the Bat-louse (_Nycteribia Vespertilionis_), &c.
II. _Oviparous_. All other insects.
Our business for the remainder of this letter will be with the latter
description of these little animals.
The unerring foresight with which the female deposits her eggs
in the precise place where the larvæ, when excluded, are sure to
find suitable food; and the singular instruments with which, for
this purpose, the extremity of their abdomen is furnished, have
been noticed in a former letter[86], and those last mentioned will
be adverted to in a future one. I shall now, therefore, confine
myself to other circumstances connected with the subject, arranged
for the sake of order under several distinct heads, as--their
_exclusion_--_situation_--_substance_--_number_--_size_--_figure_--
_colour_--and _period of hatching_.
i. _Exclusion._ The exclusion or extrusion of the impregnated eggs
takes place, when, passing from the ovary into the oviduct, they are
conducted by means of the ovipositor, in which it terminates, to
their proper situation. By far the greater number of insects extrude
them _singly_, a longer interval elapsing between the passage of each
egg in some than in others. In those tribes which place their eggs
in groups, such as most butterflies and moths, and many beetles,
they pass from the ovaries usually with great rapidity; while in the
_Ichneumonidæ_, _Sphegidæ_, _Œstri_, and other parasitic genera,
which usually deposit their eggs singly, an interval of some minutes,
hours, or perhaps even days, intervenes between the extrusion of
each egg. One remarkable instance of the former mode I noticed in
my letter on the _Perfect Societies_ of Insects[87]; another may
be cited, to which you may yourself be a witness--I allude to that
common moth, vulgarly called the _Ghost (Hepialus Humuli)_, which
lays a large number of minute black eggs, resembling grains of
gunpowder, and ejects them so fast that, according to De Geer, they
may be said to _run_ from the oviduct, and are sometimes expelled
with the force of a popgun[88]. A Tetrapterous insect, the genus of
which is uncertain, is said, when it is taken, to discharge its eggs
like shot from a gun[89]. And a friend of mine, who had observed with
attention the proceedings of a common crane-fly (_Tipula_), assured
me that several females which he caught projected their eggs to the
distance of more than ten inches.
A few _Diptera_ extrude them in a sort of chain or necklace, each
egg being connected by a glutinous matter with that which precedes
and follows it. In a small species of a genus allied to _Psychoda_
(a kind of midge), which one season was abundant in a window of my
house, this necklace is composed of eggs joined by their sides, not
unlike those strung by children of the seeds of the mallow[90]. Other
_Tipulidæ_ on the contrary extrude their eggs joined end to end,
so as to resemble a necklace of oval beads. _Beris clavipes_ and
_Sciura Thomæ_, two other flies, produce a chain about an inch long,
consisting of oval eggs connected, in an oblique position, side by
side; an arrangement very similar prevails in the ribband of eggs
which drop from some of the _Ephemeræ_[91].
These eggs, like those of the insects first mentioned, though
connected, are expelled in succession; but other tribes, as the
_Libellulidæ_, with the exception of _Agrion_, many _Ephemeræ_,
_Trichopterous_ insects, &c. expel the whole at once, as it were
in a mass. In those first mentioned they are gummed together in an
oblong cluster[92]. In one _Ephemera_ mentioned by Reaumur[93], they
formed two oblong masses, each containing from three to four hundred
eggs, and three and a half or four lines long. These animals as soon
as their wings are developed eject these masses by two orifices,
and are aided in the process by two vesicles full of air, wherever
they happen to alight or to fall; in most instances it is the water,
their proper element, that receives them, but the animal does not
appear to know the difference between a solid and a liquid, and
seems only anxious how to free herself from a burthen that oppresses
her; all has been contrived that an insect so short-lived may finish
her different operations with the utmost celerity: the term of her
existence would not have admitted the leisurely extrusion of such a
number of eggs in succession[94]. Some _Trichoptera_, or May-flies,
as _Phryganea grandis_ L., exclude their eggs in a double packet,
enveloped in a mass of jelly, (a circumstance often attending the
eggs that produce aquatic larvæ,) upon the leaves of willows[95].
A similar double packet in the year 1810 I observed appended to
the anus of a black species with long antennæ, probably _Phryganea
atrata_ F.[96] Upon taking several of the females I was surprised to
find in the above situation a seemingly fleshy substance of a dirty
yellow. At first, from its annular appearance, I conceived it to be
some parasitic larva, but was not a little surprised upon pulling
it away that it was full of globular transparent dusky eggs: it was
about two lines and a quarter in length and nearly one in breadth.
Being bent double it was attached to the animal by the intermediate
angle, and when unfolded was constricted in the middle[97]. Each
half, which was roundish, had about ten sharp transverse ridges, the
interstices of which appeared as if crenated, an appearance produced
by the eggs which it contained. Upon more than gentle pressure it
burst and let out the eggs. Though resembling the packet of _P.
grandis_ in shape and other circumstances, it was nothing like
jelly, but had rather a waxy appearance, and seems to have been
covered by a membrane: so that the excluded larvæ must probably have
eaten their way out of it. I have still by me, in 1822, specimens of
these egg-packets, which, after the lapse of so many years, retain
their original form and colour. It is not improbable that other
species extrude their eggs in a similar case. Scopoli says of _P.
bicaudata_ L., that the female carries about under her belly her eggs
united into a globe, like _Lycosa saccata_[98]. The eggs of _Geometra
Potamogata_ F. are also enveloped in a gelatinous substance, and the
mass is covered with leaves[99].
Insects of the _Diptera_ order also, like frogs and toads, commit
their eggs to the water imbedded in masses of jelly. Dr. Derham
describes two different kinds of them, in one of which the eggs were
laid in parallel rows end to end, and in another in a single row,
in which the sides were parallel[100]. But the most remarkable and
beautiful specimen of this kind that I ever saw was one that, many
years ago, I took out of a pond at Wittersham in Kent, from which I
requested a young lady to make the drawing I send you[101]. The mass
of jelly, about an inch and a quarter long, and rather widest in the
middle, was attached by one end to some aquatic grass, and from one
end to the other ran a spiral thread of very minute eggs, the turns
of the screw being alternately on each side.
The mode of exclusion of the eggs of the _Blattæ_, which are engaged
for a whole week in the business of oviposition, is very singular:
the female deposits one or two large suboviform capsules, as large as
half their abdomen, rounded on one side, and on the other straight
and serrated, which at first is white and soft, but soon becomes
brown and hard. This egg-case, as it may be called, contains sixteen
or eighteen eggs arranged in a double series, and the cock-roaches
when hatched make their escape through a cleft in its straight
side, which shuts so accurately when they have quitted it, that at
first it appears as entire as before[102]. The insects of the genus
_Mantis_ also, or what are called the _praying insects_, when they
deposit their eggs, eject with them a soft substance, which hardens
in the air and forms a long kind of envelope resembling parchment,
in which the eggs are arranged also in a double series. And the
Locusts (_Gryllus Locusta_ L.) are said by Morier[103] to deposit in
the ground an oblong substance, of the shape of their abdomen, which
contains a considerable number of eggs arranged neatly in rows. The
peristaltic motion observed in the females of some insects during
oviposition has been before described[104].
ii. _Situation._ Under this head I include the situation in which the
female insect places her eggs when extruded, whether she continues her
care of them and carries them about till they hatch, or whether she
entirely deserts them, placing them either without a covering within
reach of their food, or enveloping them in hair or otherwise protecting
them from accident or the attack of enemies. I shall consider them
under two views: _first_, as depositing their eggs in _groups_, whether
covered or naked; and _secondly_, as depositing them _singly_.
1. Those that deposit their eggs in _groups_ are first to be
considered. I shall begin with those that protect them with some kind
of _covering_.
I have already mentioned in a former letter[105] the silken bag with
which _Lycosa saccata_ Latr., a kind of spider, surrounds her eggs, and
in which she constantly carries them about with her, defending them to
the last extremity. Many other spiders, indeed nearly the whole tribe,
fabricate similar pouches, but of various sizes, forms, texture, and
colours. Some are scarcely so big as a pea, others of the size of a
large gooseberry; some globular, some bell-shaped; others, the genus
_Thomisus_ Walck. in particular, depressed like a lupine; some of a
close texture like silk; others of a looser fabric resembling wool:
some consisting of a single pellicle, but most of a double, of which
the interior is finer and softer[106]; some white; others inclining
to blue; others again yellow or reddish; most of them are of a whole
colour, but that of _Epeira fasciata_ is gray varied with black[107].
And while the parent spider of some kinds (the _Lupi_) always carries
her egg-bag attached to her anus, others hold them by their palpi and
maxillæ; and others suspend them by a long thread, or simply fasten
them in different situations, either constantly remaining near them
(the _Telariæ_), or wholly deserting them (the _Retiariæ_). The eggs of
one of these last Lister describes as often fixed in a very singular
situation--the cavity at the end of a ripe cherry; and thus, as he
expresses it--"_Stomachi maxime delicatuli quoties hanc innocuam buccam
non minus ignoranter quam avide devorarunt_[108]."
Herman informs us, that the species of the genus _Chelifer_ carry
their eggs in a mass under their belly[109].
Madam Merian gives an account of two species of _Blatta_, which
she affirms carry an egg-pouch about with them--one species (_B.
gigantea?_) she describes as carrying its eggs in a globular pouch of
web like certain spiders, and the other in a brown bag, which, when
alarmed, it drops and makes off[110]. But this admirable paintress of
natural objects was not always correct in her statements[111]: it seems
very improbable, from the habits of those species of which we know the
history, that any of them should spin a pouch of web for their eggs.