temperature: the hardness or softness of the shell, and possibly
differences in the consistence of the included fluid, intended to
serve this very purpose, cause some eggs to be hatched much sooner
than others exposed to the same degree of heat. Thus the eggs of many
flesh-flies are hatched in twenty-four hours[196]; those of bees
and some other insects in three days; those of a common lady-bird
(_Coccinella bipunctata_) in five or six days; those of spiders in
about three weeks; those of the mole-cricket in a month; while those
of many _Lepidoptera_ and _Coleoptera_ require a longer period for
exclusion. The hard eggs of _Lasiocampa Neustria_ and _castrensis_,
noticed above, remain full nine months before being hatched[197], as
do those of another moth (_Hypogymna dispar_), which, though laid
in the beginning of the warm month of August, do not send forth the
included caterpillar till the April following[198]. We know no more
of the cause of this difference than of that which takes place in
the period of exclusion of the eggs of the different species of birds.
Some eggs change considerably both their form and consistence
previously to being hatched. M. P. Huber found that those of
different species of ants when newly laid are cylindrical, opaque,
and of a milky white; but just before hatching their extremities are
arched, and they become transparent with only a single opaque whitish
point, cloud, or zone, in their interior[199]. An analogous change
takes place in the eggs of many spiders, which just before hatching
exhibit a change of form corresponding with that which the included
spider receives when its parts begin to be developed, the thin and
flexible skin of the egg moulding itself to the body it incloses[200].
In proportion as the germe included in the egg is expanded, it
becomes visible through the shell when transparent: this is
particularly the case with spiders, in which, as was before observed,
every part is very distinctly seen. At length, when all the parts
are consolidated so as to be capable of motion, which in spiders
takes place in four or five days after they begin to be visible in
the egg, the animal breaks the pellicle by the swelling of its body
and the movement of its legs, and then quits it, and disengages
all its parts one after the other[201]. In general, at least where
the shell is harder than that of spiders, insects make their way
out by gnawing an opening with their mandibles in the part nearest
their head, which, when the shell is very strong (as in _Lasiocampa
Neustria_, &c.), it is often several hours in accomplishing[202].
In many instances, however, the larva is spared this trouble, one
end of the egg being furnished with a little lid or trap-door, which
it has but to force up, and it can then emerge at pleasure: such
lids are to be found in the eggs of several butterflies and moths,
as _Satyrus Mæra_, _Saturnia pavonia major_, &c. and the common
louse[203]. In those exquisitely elegant eggs, before described, of
some kind of bird-louse (_Nirmus_) found adhering to the base of the
neck feathers of the golden pheasant[204], there is a lid or cap of
this kind of a hemispherical form terminating in a tortuous style.
Those of a species of bug (_Pentatoma_ Latr.), found by our friend
the Rev. R. Sheppard, besides a convex lid are furnished with a
very curious machine, as it should seem, for throwing it off. This
machine is dark-brown, of a corneous substance, and of the shape of
a cross-bow[205], the bow part being attached to the lid or pushing
against it, and the handle, by means of a membrane, to the upper end
of the side of the egg.
When the included animal has made its way out of the egg, it enters
upon a new state of existence, that of _Larva_, to which I shall
direct your attention in the following letter.
FOOTNOTES:
[61] The word μεταμορφοω, and its derivative μεταμορφωσις, are not
extant in any Greek writer before the date of the New Testament.
They are used to express any _external_ change of form or colour,
and metaphorically an _inward_ change and progressive improvement of
the mind. Comp. Matth. xvii. 2. Ælian. _Var. Hist._ l. i. c. 1. Rom.
xiii. 2. 2 Cor. iii. 18. They are, therefore, not improperly applied,
as some have supposed, to the changes of insects.
[62] _Entwickelungsgeschichte der Schmetterlinge_ 12-27. 105--.
[63] Dr. Virey's observations under the article _Embryo_ (_N.
Dict. d'Hist. Nat._ x. 195.) deserve here to be considered. "Il
y a donc quelque chose au dessus de l'intelligence humaine dans
cette formation des êtres; en vain on veut l'approfondir, c'est
un abime dans lequel on ne voit que la main de Dieu. A quoi bon
s'appesantir sur le mystère de la formation des êtres, sans esperance
de l'expliquer? Ne vaut-il pas mieux observer les opérations de la
nature autant qu'il est permis à l'œil humain de les appercevoir?"
[64] § xiv.
[65] _N. Dict. d'Hist. Nat._ x. 193.
[66] _Œuv._ v. 279. "Il n'est pas exact de dire que le cœur, la tête,
et la moelle épinière, sont formés les premiers dans les fœtus des
animaux à sang rouge et vertébrés," says Dr. Virey; "mais il faut
dire seulement que tel est l'ordre dans lequel ces organes commencent
à devenir visibles." _N. Dict. d'Hist. Nat._ x. 196.
[67] _Ibid._ 193.
[68] _Œuvr._ viii. 315.
[69] _Hor. Entomolog._ 446.
[70] See on this subject _N. Dict. d'Hist. Nat._ xx. article
_Metamorphosis_.
[71] _N. Dict. d'Hist. Nat._ xx. 349--.
[72] _N. Dict. d'Hist. Nat._ xx. 348.
[73] _Bibl. Nat._ Ed. Hill. ii. 138.
[74] _Œuvr._ v. 283--.
[75] _N. Dict. d'Hist. Nat._ xx. 355.
[76] Leeuwenhoek discovered in the incipient fœtus of a sheep, not
larger than the eighth part of a pea, all the principal parts of the
future animal. _Arc. Nat._ I. ii. 165, 173.
[77] Bonnet, _Œuvr._ v. 284.
[78] _N. Dict. d'Hist. Nat._ xx. 352.
[79] Select Works by Hoole, i. 132. The fact is confirmed by M. L.
Dufour, who, having opened the abdomen of a female scorpion, found in
the midst of some eggs nearly mature a little scorpion a quarter of
an inch long; it lay without motion, with its tail folded under the
body. _N. Dict. d'Hist. Nat._ xxx. 426.
[80] Reaum. iv. 425--.
[81] _Ibid._ 428--. _t._ xxix. _f._ 10, 11.
[82] Busch, a German author, affirms that many _Cimicidæ_ are subject
to this law. _Schneid._ i. 206.
[83] Quoted in Huber _Fourmis_, 208. Some reptiles also are at one
time oviparous, and at another ovo-viviparous. _N. Dict. d'Hist.
Nat._ xii. 568.
[84] I say _almost_ all insects, because the larvæ of _Hymenoptera_
and _Diptera_ are supposed not to undergo this change. _N. Dict.
d'Hist. Nat._ xx. 365.
[85] Reaum. vi. _Mém._ xiv. De Geer, vi. 280.
[86] See VOL. I. Lett. xi.
[87] See VOL. II. p. 36.
[88] De Geer i. 494--.
[89] Called by M. l'Abbé Preaux, who observed it near Lisieux in
Normandy, _Mouche Baliste._ _N. Dict. d'Hist. Nat._ xxi. 442.
[90] PLATE XX. FIG. 20.
[91] Reaum. vi. 509. _t._ XLV. _f._ 11, 12.
[92] Reaum. vi. 434.
[93] _Ibid._ vi. 494.
[94] The vesicles, which Reaumur thinks may be pulmonary vesicles,
as well as assisting in the extrusion of the masses of eggs, he has
figured _t._ xliv. _f._ 10. _u u._
[95] De Geer ii. 534. _t._ xiii. _f._ 13.
[96] Coquebert _Illustr. Ic. t._ i. _f._ A. B.
[97] PLATE XX. FIG. 25.
[98] _Ent. Carniol._ 269. _n._ 705.
[99] Reaum. ii. 401.
[100] In Raii _Hist. Ins._ 264.
[101] PLATE XX. FIG. 24.
[102] Goeze _Naturf._ xvii. 183--. _t._ iv. _f._ 16-19. Comp. _N.
Dict. d' Hist. Nat._ iii. 475. and xix. 239. De Geer iii. 533.
[103] _Second Journey through Persia_, 100--.
[104] See VOL. II. p. 36.
[105] VOL. I. p. 359--.
[106] Latr. _Hist. Nat. des Fourmis_, 334. _N. Dict. d'Hist. Nat._
ii. 284.
[107] Lister _De Aran._ Tit. 13, 14. _N. Dict. d'Hist. Nat._ ii. 284.
[108] Lister _Ibid._ 56. Tit. 15.
[109] _N. Dict. d'Hist. Nat._ xxvi. 447.
[110] _Ins. Surinam, t._ i.
[111] A striking instance of this may be seen in her forty-ninth
plate, in which she has clapped the rostrated head of _Fulgora
laternaria_ upon the body of a _Cicada_ Latr., affirming it to be the
former fly in its previous state! This might be a trick upon her.
[112] _N. Dict. d'Hist. Nat._ xv. 489.
[113] _Lesser_ L. i. 300.
[114] _Annales du Muséum_, xiv. 441.
[115] _Lesser_ L. i. _t._ ii. _f._ xvi.
[116] Miger _Ann. du Mus._ ubi supr. Comp. _N. Dict. d'Hist. Nat._
xv. 482--.
[117] De Geer i. 192.
[118] Ibid. ii. 982.
[119] Reaum. ii. 97. 159.
[120] Ibid. 107--. _t._ iii. _f._ 15.
[121] De Geer iii. 48. 51.
[122] Reaum. v. 122.
[123] See above, VOL. I. p. 196. 202.
[124] _Journ. de Phys. Philos. Mag._ ix. 244.
[125] Reaum. iv. _Mem._ i.
[126] Rösel, ix. 157. _t._ 265?
[127] Ibid. iii. 197.
[128] See above, VOL. I. p. 195.
[129] PLATE XX. FIG. 14.
[130] Reaum. i. 95-- _f._ 1-13.
[131] Reaum. iv. 615--. _t._ xliv. _f._ 2-7.
[132] _N. Dict. d'Hist. Nat._ xv. 445. Rös. iii. 156.
[133] _Epist._ 1687.
[134] VOL. I. p. 448--.
[135] See above, VOL. I. p. 344--.
[136] Reaum. iii. 8--.
[137] VOL. I. 349--. 371--.
[138] _N. Dict. d'Hist. Nat._ xxviii. 271.
[139] De Geer vii. 194.
[140] Compare _N. Dict. d'Hist. Nat._ xvi. 246. with xx. 352-; but as
the _Amnios_ immediately envelops the fœtus, the pellicle seems most
analogous to it, and the shell to the _Chorion_.
[141] Swamm. _Bibl. Nat._ ed. Hill. l. 133. a. Comp. _N. Dict.
d'Hist. Nat._ xvi. 246.
[142] Swamm. _Ibid._
[143] Sepp. iv. _t._ iii. _f._ i. _c._ v. _t._ iv. _f._ 2.
[144] See above, VOL. I. p. 208: it is there called an _Aphis_, but
it is a distinct genus.
[145] De Geer iii. 245. _t._ xiii. _f._ 20-22.
[146] Sepp. iv. _t._ xiii. _f._ 2. 3.
[147] The sturgeon is said to lay 1,500,000 eggs, and the cod-fish
9,000,000.
[148] Reaum. iv. 392.
[149] See above, VOL. I. p. 350.
[150] De Geer vii. 159.
[151] See above, VOL. II. p. 109.
[152] Ibid. 159. 166.
[153] Ibid. 36--.
[154] iv. _t._ xviii. _f._ 4. 5.
[155] De Geer ii. 638.
[156] _Bibl. Nat._ i. 132. b.
[157] Gould 36.
[158] Reaum. v. 477.
[159] Ibid. iii. 579. v. 121.
[160] _Fourmis_, 69--.
[161] Rösel iii. 152.
[162] De Geer vii. 145.
[163] Ibid. 123--. See above, VOL. I. p. 393.
[164] Raii _Hist. Ins._ 265.
[165] Eggs of various shapes are given PLATE XX. FIG. 3-23. See
also Brunnich. _Entomologia_ 4. _N. Dict. d'Hist. Nat._ xvi. 245.
Reaum. ii. _t._ iii. iv. xiv. xxvi. xxvii. &c.
[166] PLATE XX. FIG. 18.
[167] PLATE XX. FIG. 23. Swamm. _Bibl. Nat. t._ iii. _f._ 7, 8. In a
specimen I opened of this insect the bristles converged so as to form
a kind of tail to the egg.
[168] Darwin _Phytolog._ 512.
[169] Geoffr. _Ins. Par._ i. 480. _t._ x. _f._ 1. _b. c._
[170] See above, VOL. I. p. 261.
[171] Reaum. iii. 386--. _t._ xxxii. _f._ 1. _t._ xxxiii. _f._ 5.
[172] I allude to _Ophion luteum_ F. (_Ichneumon_ L.) Vol. i.
Ed. 3. p. 269, figured PLATE XX. FIG. 22; and the _Hydrachnæ_ or
_Trombidia_. See above, and De Geer vii. 145.
[173] From this circumstance called πολυποικιλος σοφια by the
Apostle, _Ephes._ iii. 10.
[174] _Nat. Theol._ 11th Ed. 375.
[175] PLATE XX. FIG. 19. _a a._
[176] Reaum. iv. 376--. _t._ xxvii. _f._ 9, 10.
[177] _Hist. Nat. gen. et partic. des Crust. et Ins._ xii. 282.
[178] Reaum. iv. 381. _t._ xxvi. _f._ 19, 20.
[179] Roxburgh in _Linn. Trans._ vii. 34.
[180] Some of the _Noctuæ_ have similar eggs, as _N. Lappa._ Sepp iv.
_t._ iii. _f._ 1. _c._
[181] Reaum. _ubi supr. f._ 22, 23.
[182] PLATE XX. FIG. 6. 8.
[183] PLATE XX. FIG. 5.
[184] Ibid. FIG. 3. 4. 7. 9. &c.
[185] Ibid. FIG. 15.
[186] Bonnet _Œuvr._ ii. 9.
[187] _Naturf._ xiii. 229.
[188] _N. Dict. d'Hist. Nat._ xvi. 245.
[189] Reaum. ii. 286.
[190] PLATE XX. FIG. 11. Sepp _t._ iv. _f._ 2.
[191] Reaum. iv. 617.
[192] _Philos. Ent._ 76.
[193] See above, VOL. I. 358--.
[194] See above, VOL. I. Ibid.
[195] Young's _France_, ii. 34. This author asserts, that no art will
hatch the eggs of the common silk-worms the first year, or that in
which they are laid; but that there is a sort brought from Persia,
which are hatched three times a year, and which will hatch in fifteen
days in the proper heat. In 1765, it is said, the common sort hatched
in the first year. _Ibid._ 226--.
[196] In the _N. Dict. d'Hist. Nat._ xii. 564. the eggs of the
flesh-fly are said to hatch in two hours. This is true I believe in
very warm weather.
[197] Brahm. 310.
[198] Rimrod _Naturf._ xvi. 131.
[199] _Fourmis._ 69.
[200] De Geer vii. 195.
[201] Ibid. 196.
[202] Reaum. ii. 167.
[203] Brahm. 249. Rösel. iv. 130. Swamm. _Bibl. Nat. t._ i. _f._ 2.
[204] By Mr. White, jun. cordwainer at Ipswich.
[205] PLATE XX. FIG. 16. _a._
LETTER XXX.
_STATES OF INSECTS._
LARVA STATE.
The _Larva_ state is that in which insects exist immediately after
their exclusion from the egg (or from the mother in ovo-viviparous
species), in which they usually eat voraciously, change their skin
several times, and have the power of locomotion, but do not propagate.
Almost all larvæ, at their birth, are for a time in a very feeble
and languid state, the duration of which differs in different
species. In most it continues for a very short time, a few minutes
or perhaps hours, after which they revive and betake themselves to
their appropriate food. In others, as in the generality of spiders,
this debility lasts for seven or eight days, and in some species
even a month, during which the young ones remain inactive in the
egg-pouch[206], and it is not till they have cast their first skin
that their active state of existence commences.
All larvæ may be divided into two great divisions:--
I. Those which in general form more or less resemble the
perfect insect.
II. Those which are wholly unlike the perfect insect.
I shall begin by calling your attention to the characters of the
_first_ of these divisions: the second, which is by far the most
numerous, will be afterwards considered.
I. The _first_ division includes the larvæ of _Scorpions_, _Spiders_,
_Cockroaches_, _Grasshoppers_, _Lanthorn-flies_, _Bugs_, &c.; or
generally, with the exception of the _Flea_ and _Crustacea_, the
whole of the _Linnean_ Orders _Aptera_ and _Hemiptera_. All these
larvæ, however remotely allied in other respects, agree in the general
similarity which they bear to the perfect insects which proceed from
them. The most acute entomologist, untaught by experience, could
not even guess what would be the form of the perfect insects to be
produced from larvæ of the _second_ division, while they can recognise
the form of the spider, the cricket, the cockroach, the bug, and the
frog-hopper, in that of the larvæ. There are, however, differences in
the degrees of this resemblance, according to which we may, perhaps,
divide this tribe in their second state as follows:--
i. Those that resemble the perfect insect, except in the
relative proportions and number of some of their parts.
ii. Those which resemble the perfect insect, except that they
are apterous, or not yet furnished with organs of flight.
i. Spiders, _Phalangia_, scorpions, lice, _Poduræ_, sugar-lice
(_Lepisma_), mites, centipedes, millepedes, &c. come under the
_first_ subdivision. The larvæ of the first six tribes here mentioned
differ at their birth from the perfect insect, only in size and the
proportions of their parts. Thus the larvæ of spiders have their
legs of a different relative length from that which they subsequently
acquire; and the palpi in the males, which previously to the
discoveries of Treviranus were regarded as their sexual organs, are
not yet fully developed[207]: and a similar difference takes place in
the legs of _Phalangia_. The general form too of the body undergoes
slight alterations, and the colour very considerable ones, with each
change of the skin--a change to which all these tribes are subject.
The larvæ of the three last-mentioned tribes (the mites, centipedes,
and millepedes) differ from the perfect insect not only in the
proportion but also in the number of their parts. Leeuwenhoeck states
(and De Geer confirms his assertion, extending it to other species of
mites[208]), that the common cheese-mite, which in its perfect state
has _eight_ legs, when first excluded from the egg has but _six_, the
third pair being wanting[209]. Some however are born with _eight_
legs, for instance _A. eruditus_ of Schrank, which he saw come from
the egg itself with that number[210]. Others again have never more
than six legs: this is the case with Latreille's genera--_Caris_,
_Leptus_, _Atoma_, and _Ocypetes_ of Dr. Leach[211]. In the centipedes
(_Scolopendridæ_) and millepedes (_Iulidæ_) differences still more
remarkable, as I have stated in a former letter, have been observed
by De Geer; these animals, in their progress to the perfect state,
not only gain several additional pairs of _legs_, but also several
additional _segments_ of the body. This illustrious Entomologist found
that _Pollyxenus lagurus_ (_Scolopendra_ L.) was born a hexapod,
with but three segments and as many pairs of feet, but successively
acquired five additional segments with other appendages, and nine
more pairs of feet[212]. A species of millepede (_Iulus terrestris_
L.), which he also traced from its birth, and which begins the world
at first with only eight segments and six feet, by a successive
development at length acquires, in its perfect state, 50 segments
and not less than 200 feet[213]. The nature of these very singular
accretions, which Latreille and Mr. Wm. MacLeay have also observed
in the centipedes[214], seems not well understood. If, as is most
probable, though De Geer could not find any exuviæ[215], the larvæ
cast a skin before each change, they do not essentially differ from
the metamorphosis of other insects. The legs that these insects thus
acquire are affixed to the abdomen, the six that they set out with
being attached to the part representing the trunk, so that the former
may be regarded as analogous to the prolegs of caterpillars. These
animals therefore, as I have before intimated, invert the order of
Nature, and from _perfect_ degenerate into _imperfect_ insects.
ii. If you examine the _cockroach_, _cricket_, or _grasshopper_, in
different stages of their growth, you will find that the larva does
not vary essentially from the perfect insect, except in wanting wings
and elytra. The case is the same in almost all the Linnean genera
of the modern order--_Hemiptera_; and with _Raphidia_, _Termes_,
and _Psocus_, in the _Neuroptera_. Some of these, however, exhibit
slighter discrepancies in the proportion of some of their parts,
but without affecting the general resemblance. Thus the larvæ of
the common ear-wig have at first only _eight_, and subsequently
_nine_ joints to their antennæ, whereas the perfect insect has
_fourteen_[216]; and the forceps is quite different, resembling
rather two straight styles than what its name implies. In those
also of many bugs (_Coreus marginatus_ F. &c.), the joints of the
antennæ are of a shape dissimilar to that which obtains in the
perfect insect. In that of the common water-scorpion, the anal
air-tube, which is so long in the imago, is scarcely visible[217]. In
the _Cicada_ tribe, so celebrated for their song[218], neither the
larva nor the imago have the enormous thigh armed below with strong
teeth, the tibiæ terminating in a fixed incurved claw, probably for
the purpose of digging the holes into which they retire till they
disclose the fly, which distinguish the pupæ of some species, and is
particularly conspicuous in one commonly brought from China[219].
These often exhibit also other minor differences.
II. In treating of the second great division of larvæ, those that
are wholly unlike the parent insect,--which includes, with few
exceptions[220], the whole of the Linnean orders, _Coleoptera_,
_Lepidoptera_, _Hymenoptera_, _Diptera_, the majority of the
_Neuroptera_, _Coccus_ and _Aleyrodes_ in _Hemiptera_, and the genus
_Pulex_ in _Aptera_,--I shall advert to their characters, under
several distinct heads; and to avoid unnecessary circumlocution, I
shall in what follows wholly leave out of consideration the _first_
division already explained, and use the term _larvæ_ with reference
only to those of the _second_. The heads under which I propose to
treat of them are: The _substance_ of their body, its _parts_,
_shape_, or figure, _clothing_, _colour_. Also the _Economy_ or mode
of life of these creatures: their _food_, _moultings_, _growth_,
_age_, _sex_, and their preparations for assuming the _Pupæ_.
i. _Substance_, with the exception of the head and six fore-feet,
which are usually corneous, the exterior integument or skin of larvæ
is commonly of a membranous texture, and the body is of a much softer
consistence than in the perfect insect. In those, however, of some
_Staphylinidæ_ and other _Coleoptera_, the dorsal part of the three
first pieces, which represent the trunk of the perfect insect, is
hard and horny. Some also have their whole skin coriaceous, as the
tortoise-shell butterfly (_Vanessa polychloros_); and some few, as
the wire-worm (_Elater segetum_), and other _Elateres_, very hard.
I possess a very remarkable larva from Brazil, from the extreme
flatness of its body, and from its having cavities to receive its
legs when unemployed, probably living under bark, the skin of which
is still harder than that of the grub of the Elaters. Perhaps it
has to resist great pressure; and on that account is gifted with
this quality, so seldom to be met with in other kinds of larvæ.
The interior of the body of these animals is generally of a softer
consistence than in the perfect insect. Their intestines, and other
internal organs, are usually wrapped in a voluminous substance of
a fatty nature, which is regarded as analogous to the _epiploon_,
_omentum_, or _caul_, which envelops the viscera of quadrupeds, &c.,
and is called by Reaumur the _corps graisseux_. The use of this
general flexibility of larvæ is obvious; for, their bodies being
mostly long and narrow, a hard rigid covering would have been very
inconvenient, and a considerable impediment to their motions. When
a caterpillar is feeding, it has occasion to apply its body to any
part of the margin of a leaf so as to support itself by its prolegs,
and when moving it wants to give it all the curves that are necessary
to enable it to avoid obstacles, and thread its way through the
sinuous labyrinths which it must often traverse. On the other hand,
the hardness of the substance of its head affords a strong fulcrum
to the muscles which keep its powerful jaws in constant play. The
larvæ, indeed, of some _Diptera_ have a membranous head; but their
mandibles, which serve also as legs, are not grinders, but merely
claws, the muscles of which require less powerful support[221]. Under
this head it may be proper to observe, that generally larvæ are
opaque; but some, as those of ants, and a few _Lepidoptera_[222],
are diaphanous. That of _Corethra crystallina_ (_Tipula_ De Geer) is
so beautifully transparent as to resemble a piece of crystal, and
scarcely to be distinguished from the water in which it lives[223].
ii. _Parts._ The body of each larva consists of the _head_, including
its different organs, and of the _succeeding segments_, of which the
three first may usually be denominated the _trunk_, and have the
six anterior feet, when present, attached to their under side: the
remainder is the _abdomen_. The latter includes in some species a
variable number of membranous feet, as well as various appendages
affixed usually to its tail and sides. No larva is ever furnished
with wings[224]. Each of these greater divisions, and the organs
which they include, require separate consideration.