[620] _Bibl. Nat._ _t._ xix. _f._ 3. β.
[621] Reaum. v. 377. _t._ xxix. _f._ 7. _s._
[622] _N. Dict. d'Hist. Nat._ xxx. 388.
[623] _Ibid._ 427--.
[624] _Arachnid._ 31. _t._ ii. _f._ 21. _p._ 9.
[625] _N. Dict. d'Hist. Nat._ xxii. 114. 117. comp. VOL. I. p. 127.
[626] _Ibid._ xxviii. 6.
[627] _Osservazioni_, &c. 13--.
[628] VOL. II. p. 243--. _N. Dict. d'Hist. Nat._ iv. 308.
[629] _Ibid._ iv. 309.
[630] _Ibid._ v. 252.
[631] De Geer iv. 358. _t._ xiii. _f._ 9. _m._
[632] VOL. II. p. 241--. III. p. 147--.
[633] De Geer iii. 41.
[634] VOL. I. p. 451, where by mistake it is represented as the work
of _Aphis Abietis_.
[635] De Geer iii. 111.
[636] Reaum. iii. _t._ xxvi. _f._ 4-6.
[637] _N. Dict. d'Hist. Nat._ vi. 305.
[638] VOL. III. p. 221.
[639] Treviran. _Arachnid._ 44. In Paraguay a spider is found which
makes spherical cocoons of yellow silk, which are spun because of the
permanence of the colour. This operation occasions a flow of water
from the eyes and nose of the spinners. Azara _Voyag._ 212. See also
Murray in _Werner. Trans._ 1823. 8--.
[640] Reaum. v. 24.
[641] VOL. II. p. 244--.
[642] Ramdohr _Anat._ _t._ ii.-vi.
[643] _Ibid._ 20. See above, p. 107. As some of the _Sialisteria_
render to the _stomach_ (see above, p. 131), there seems no small
affinity between these shags and those organs.
[644] Cuv. _Anat. Comp._ iv. 132, 136.
[645] Reaum. vi. Pref. xxviii. 177--.
[646] _Ibid._ 253--.
[647] _Ibid._ iii. 375.
[648] _Anat._ _t._ xii. _f._ 6.
[649] _Ibid._ xxi. _f._ 3. _I I._
[650] Reaum. iii. 230.
[651] VOL. III. p. 79--.
[652] Reaum. iii. 215. Bonnet ix. 182.
[653] VOL. III. p. 68--.
[654] Marcel de Serres _Mem. du Mus._ 1819. 133, 141.
[655] De Geer, v. 6.
[656] Rai. _Hist. Ins._ 62.
[657] VOL. II. p. 242--. 248. Rai. _Hist. Ins._ 94, 382.
[658] Reaum. v. 448.
[659] _Ibid._ v. 722.
[660] VOL. I. p. 196. II. p. 176.
[661] _Encyclop. Britan._ viii. 205. from _Journ. de Phys._
[662] VOL. II. p. 174.
[663] Reaum. iii. 318--. _t._ xxvi. _f._ 1-6.
[664] _Ibid._ 396--. _t._ xxxi. _f._ 20-29.
[665] _Insect. Suec._ i. 257.
[666] VOL. I. p. 327.
[667] _N. Dict. d'Hist. Nat._ xvii. 189.
[668] Nicholson's _Journ._ i. 298--.
[669] VOL. III. 281.
[670] _Philos. Trans._ 1670.
[671] _Philos. Trans._ _Ibid._ Ray's Lett. 74.
[672] Amoreux _Ins. Venim._ 236--.
[673] _N. Dict. d'Hist. Nat._ xii. 94.
[674] Southey's _Brazil_, i. 645.
[675] _N. Dict. d'Hist. Nat._ ubi supr.
[676] VOL. II. p. 67.
[677] _Syst. of Chemist._ 533.
[678] Germar _Mag. der Ent._ iii. 445--.
[679] _Mem. Dijon_ 1783. ii. 70.
[680] Reaum. v. 354.
[681] _On Poisons_, i. 265--.
[682] _Ibid._ 269.
[683] Reaum. _ubi supr._
[684] VOL. I. p. 124. III. p. 716--.
[685] _N. Dict. d'Hist. Nat._ xxx. 427.
[686] I use the term _odorous_, not in the same sense as
_odoriferous_, but to include both sweet and fetid scents.
[687] VOL. II. p. 238--. III. p. 147--.
[688] A Brazilian wood so called, but differing from the common cedar.
[689] Dotharding _Insect. Coleopt. Danic._
[690] Sturm _Deutsch. Fn._ i. 27.
[691] Reaum. iii. 494.
[692] _Mon. Ap. Angl._ i. 136.
[693] _Osservaz. sullo Iulus, &c._ 14.
[694] VOL. III. p. 297--.
[695] _N. Dict. d'Hist. Nat._ xv. 487.
[696] _N. Dict. d'Hist. Nat._ iv. 308.
[697] Dated Tripoli in the West, January 21, 1819.
[698] VOL. II. p. 418--.
[699] Reaum. i. 145. Lyonet _Anat._ 106--. _N. Dict. d'Hist. Nat._
xvi. 224. PLATE XXI. FIG. 5. _a._
[700] See above, p. 90. note^a.
[701] See above, p. 78.
[702] Huber i. 273.
LETTER XLII.
_INTERNAL ANATOMY AND PHYSIOLOGY
OF INSECTS, CONTINUED._
REPRODUCTION.
The reproductive organs of insects in their _general_ denominations
and functions correspond with those of the higher classes of animals;
but as to _number_, _proportions_, and other _particular_ details of
their structure, they differ from them very considerably. I shall not
now, however, enter at large upon this subject, but confine myself
principally to the consideration of those organs in the female which
are appropriated to the formation, fecundation, maturation, exclusion
and deposition of their eggs, and other circumstances relating
to that subject. The organs connected with this function are the
_Sperm-reservoir_; the _Oviduct_; the _Ovaries_; and the _Ovipositor_.
I. The Sperm-reservoir (_Spermatheca_) is an organ connecting the
vagina with the oviduct, which, according to Herold, receives the
male sperm as into a reservoir[703], and fecundates the eggs in
their transit through that passage. This vessel, which consists of
a double tunic, in the cabbage-butterfly terminates the vagina,
and is connected with the oviduct by a lateral undulating tube: in
shape it is a rather irregular oblong, and is surmounted by a small
orbicular vesicle, connected by a short tubular footstalk with the
main reservoir[704]. A similar organ was discovered by Malpighi
in the imago of the silkworm, who denominates it the _uterus_; to
which indeed it seems analogous, and which he also regards as a
reservoir for the sperm for the gradual fecundation of the eggs[705].
But in that fly the organ is of a rather different shape, and the
interior vessel terminates in several spherical vesicles[706].
John Hunter by the most decisive experiments, such as covering the
eggs of the unimpregnated moth, after exclusion, with the liquor
taken from the spermatheca in those which had been impregnated,
and rendering them fertile, he demonstrated that this organ was a
reservoir for the spermatic fluid, to impregnate the eggs as they
were ready for exclusion, and that coition and impregnation were
not simultaneous[707]. It is not improbable that in all insects
whose eggs are gradually laid, this provision for their gradual
fecundation, if carefully sought for, might be detected[708].
Rifferschweils is of opinion, that in these cases the eggs are
fertilized in their transit through the oviduct by sperm adhering
to the folds of the _cloacæ_[709]: but this opinion seems less
analogous to what takes place in other cases, with regard to the due
preparation of the eggs for a safe and effectual transit[710].
II. The Oviduct (_Oviductus_) is the canal, always separate from the
vagina, which receives the eggs from the ovary, transmitting them,
often by a peculiar and complex instrument in which it terminates,
to their proper station. This canal sometimes opens into the anal
passage or _cloaca_, and at others, as in the cabbage-butterfly[711],
is distinct, and lies between the sexual organ and the anus. In the
_Arachnida_ there are _two_ oviducts[712].
III. The Ovaries (_Ovaria_) in insects are the viscera in which the
eggs are generated and grow till they arrive at maturity, when they
pass through the oviduct, and are extruded or deposited in their
appropriate station. They vary considerably in their structure. In
all however, except the _Iulidæ_, in which there is only a _single_
ovary[713], the oviduct at its upper or inner extremity terminates in
_two_ branches, usually further subdivided into a number of smaller
conical ones, which several ramifications constitute the _ovaries_,
or egg-tubes as they are sometimes called: these tubes generally
consist of a single membrane, and are joined to the oviduct by
membranous rugose _cloacæ_[714]: in the _Phalangia_, however,
there are _two_ tunics; the outer one of a cellular substance, and
the inner one consisting of spiral fibres like _tracheæ_--a kind
of structure which renders them capable of great extension[715].
Rifferschweils considers the ovaries as formed upon _two_ primary
types.--_First_, _flagelliform_ ovaries, consisting of conical
tubes equal in length, and inserted at the same place at the end
of the primary branches as in the _Lepidoptera_, the Bee, &c.
_Secondly_, _racemose_ ovaries, consisting of short conical tubes,
so proceeding from the primary branches as to render the ovary
racemose or pinnated, as in certain _Neuroptera_, _Coleoptera_, and
_Diptera_[716]: but perhaps their structure will be better understood
if they are divided into _agglomerate_ ovaries and _branching_
ovaries: in the _first_ the egg-tubes form _two_ bundles, in
which the branches are not discernible, as in the _Ephemera_, the
chamæleon-fly, and spiders[717]: and in the _second_ the branches are
distinct, as in the _Lepidoptera_ and the majority of insects.
The number of branches varies in different genera and species. In
_Echinomyia grossa_, a large fly, there are only the _two_ primary
branches[718]; in the common dung-beetle (_Geotrupes stercorarius_)
these appear divided at their apex into fingers[719]: in _Scolia_, a
Hymenopterous genus, and the butterfly of the nettle, there are _three_
secondary branches on each side[720]: in many other _Lepidoptera_ and
the humble-bee there are _four_[721]; in the common louse there are
_five_[722]; in the rhinoceros-beetle and the cockchafer, _six_[723];
in the wasp, _seven_[724]; _eight_ in the cockroach[725]; _twelve_
in the _Carabi_ and the mealworm-beetle[726]; _thirty_ in the large
green grasshopper (_Acrida viridissima_[727]); _thirty-two_ in the
cheese-maggot-fly[728]; and in the hive-bee more than _a hundred and
fifty_[729].
The number of _eggs_ also contained in the ovaries varies. In
_Echinomyia grossa_ there is only _one_ egg in each, and only
_two_ at once in the matrix[730]: in another fly produced by
the cheese-maggot there are _four_[731]; in the louse there are
_five_; in the cockchafer _six_[732]; in the hive-bee _sixteen_
or _seventeen_ are visible at the same time[733]; and in the
silkworm-moth _sixty_ or _seventy_[734]. Besides the eggs, the
tubes contain a pellucid mucus, and at their upper extremity the
eggs are lost in a granular mucous mass, in which, however, they
may still be discovered with a microscope[735]. With regard to the
termination of the ovaries or egg-tubes internally,--in those that
have agglomerated ones it is not to be traced, the whole appearing
like an oblong obtuse or acute body[736]: but in the branching ones
it is more easily traced; at first they converge in most cases
to a point; this is seen to advantage in the caterpillar of some
butterflies, when near assuming the pupa, in which they are readily
discovered, and represent with great truth and elegance the bud of
some blossom[737]; but in time they diverge, and sometimes become
convoluted[738]; they generally terminate in a slender simple
filament, but in the louse in a fork[739]; they are sometimes
extremely long, as in the wasp and _Lepidoptera_[740]; in the
hive-bee they appear to be shorter[741].
IV. We are next to consider the _Ovipositor_, or instrument by which
numerous insects are enabled to introduce their eggs into their
appropriate situations, and where the new-born larva may immediately
meet with its destined food. As this instrument is one of the most
striking peculiarities with which the wisdom of the CREATOR has
gifted these little animals, and in many cases is extremely curious
and wonderful, both in its structure and modes of operation--though
on a former occasion I gave you a brief account of several kinds of
them[742], I shall now enter more at large into the subject, and
describe these often complex machines, as they are exhibited in most
of the different Orders of insects.
With regard to the _Coleoptera_ Order, there are doubtless numerous
variations in the structure of this organ; but very few have been
noticed, and those chiefly belong to insects whose grubs feed on
timber. In these it is usually retractile one part within another,
like the pieces of a telescope: in _Buprestis_ it consists of three
long and sharp _laminæ_, the two lateral ones forming a sheath to
the intermediate one, which probably conveys the eggs[743]: in
_Elater_ it is a cylindrical organ, terminating in a pair of conical
joints, which seem to form a forceps, and including a tube probably
conveying the egg to the forceps, which perhaps introduces it[744].
The Ovipositor of _Prionus coriarius_ differs from that of _Callidium
violaceum_, and many Capricorns before described[745]: it consists
merely of a long bivalve piece ending in a kind of forceps, and
hollowed above into a channel for the passage of the eggs[746].
In the _Orthoptera_ the instrument of oviposition is more simple;
in _Locusta_ consisting merely of four robust three-sided pieces,
two above and two below, the former pair at the end curving upwards
and the latter downwards[747], these pieces seem calculated when
they have entered the earth to enlarge the burrow, and the animal
appears able to separate them very widely from each other[748].
The ovipositor of _Acrida viridissima_, which like that of many
Hymenopterous insects forms a kind of appendage or tail to the body,
has been described both by De Geer and Latreille as consisting of
_two_ valves only[749]; but in reality it consists of _six_, two
upper and four lower, as you may ascertain by means of a pin or
the point of a penknife, which will readily separate them. This is
confirmed by a figure of Stoll's of a species which seems to connect
_Conocephalus_ with _Gryllus_. In this the ovipositor is considerably
longer than the body of the animal, and is composed of _six_ distinct
pieces; viz. _two_ external ones stouter than the rest, and within
these _four_ others finer than a hair and convolute at the apex[750].
There is a considerable variety in the shape of the ovipositors of
the _Acridæ_ and the cognate genera:--thus in _A. viridissima_
this organ is straight, in _A. verrucivora_ bent like a sabre, and
in _Pterophylla citrifolia_ and some others, the whole machine is
short and boat-shaped; in _Scaphura Vigorsii_ it is also rough with
sharp little tubercles[751]. I had an opportunity of observing,
with respect to the first of these insects, that in boring, as is
the case with the _Cicadæ_ and saw-flies, the motion of the valves
was alternately backwards and forwards. It appeared also to me that
the two outer pieces of each of the apparent valves were fixed in a
groove in the margin of the intermediate one. I saw this clearly with
respect to the _upper_ pieces, and it is most probable that the lower
are similarly circumstanced. In the cricket tribe (_Gryllus_) the
ovipositor is as long as the abdomen, very slender, terminating in a
knob[752]. It is _apparently_ bivalve like that of _Acrida_, but I
believe is resolvable into the same number of pieces.
In the _Homopterous Hemiptera_ there seems to be more than one type
on which the ovipositor is constructed. In an insect very common with
us, the froth froghopper (_Cercopis spumaria_), some approach is made
to the ovipositors last described, at least the number of pieces is
the same--for it has a pair of external valves forming a sheath, which
includes three sharp _laminæ_ resembling the blades of a lancet, the
middle one of which can be separated into two; this instrument De Geer
had reason to think was scored transversely like a file[753]. In the
insects of this Order so noted for their song[754] (_Cicada_), there
are only _five_ pieces; namely, two valves forming the sheath, two
augers or borers, and an intermediate piece upon which they slide,
each being furnished with an internal groove for that purpose, and the
middle piece with a ridge to fit; a contrivance of Divine Wisdom, to
prevent their dislocation when employed in boring; the augers terminate
in a knob which is externally toothed[755]. This structure approaches
that of the _Hymenoptera_, especially the saw-flies. With regard to the
_Heteropterous_ section of this Order--as they usually do not introduce
their eggs _into_ any substance, they have no call for any remarkable
ovipositor, and therefore are not so furnished. A remark which will
also apply to the _Lepidoptera_ Order.
In the _Libellulina_ amongst the _Neuroptera_, an organ of this kind
is sometimes discoverable. In _Agrion_, Reaumur noticed a part which
he conjectured to be an _ovipositor_; it consists of four _laminæ_
or lancets, the interior pair slender, the exterior wider, and all
externally serrated[756].
The insects of the _Hymenoptera_ Order have long been celebrated for
the organs we are describing, whether used as _saws_, _augers_, or
_darts_. I formerly gave you a very _general_ account of the _saws_,--I
shall now give you a very interesting one in _detail_ copied from an
admirable little essay of Professor Peck. "This instrument," says
he, "is a very curious object; and in order to describe it it will
be proper to compare it with the _tenon-saw_ used by cabinet-makers,
which being made of a very thin plate of steel, is fitted with a back
to prevent its bending. The back is a piece of iron, in which a narrow
and deep groove is cut to receive the plate, which is fixed: the saw
of the _Tenthredo_ is also furnished with a back, but the groove is
in the plate, and receives a prominent ridge of the back, which is
not fixed, but permits the saw to slide forward and backward as it is
thrown out or retracted. The saw of artificers is _single_, but that
of the _Tenthredo_ is _double_, and consists of two distinct saws with
their backs: the insect in using them, first throws out one, and while
it is returning pushes forward the other; and this alternate motion is
continued till the incision is effected, when the two saws receding
from each other, conduct the egg between them into its place. In the
artificial saw the teeth are alternately bent toward the sides, or
out of the right line, in order that the fissure or kerf may be made
sufficiently wide for the blade to move easily. To answer this purpose
in some measure, in that of the _Tenthredo_ the teeth are a little
twisted, so as to stand obliquely with respect to the right line, and
their point of course projects a little beyond the plane of the blade,
without being laterally bent; and all those in each blade thus project
a little outwards: but the kerf is more effectually made, and a free
range procured for the saws, by small teeth placed on the outer side
of each; so that while their vertical effect is that of a _saw_, their
lateral effect is that of a _rasp_. In the artificial saw the teeth
all point outward (_towards the end_) and are simple; but in the saw
of the _Tenthredo_ they point inward, or toward the handle, and their
outer edge is beset with smaller teeth which point outwards (_towards
the end_)[757]." Valisnieri, Reaumur, and De Geer describe the groove
as being in the back; but in Mr. Peck's insect, if there is no error in
his account, it is, as in the _Cicadæ_, in the saw itself[758]. In the
genus _Cimbex_, belonging to the same tribe, the saw differs in shape,
being somewhat _sigmoidal_ or resembling the letter S, while in that
of other saw-flies it is _cultriform_ with a concave edge: other minor
differences distinguish them, which need not be particularized.
A similar structure, with regard to the organ in question, obtains
in the rest of the _Hymenoptera_, even those that use it as a weapon
of offence; but the backs of the saws in them, composed of a single
piece, become a sheath for the darts. The valves, however, vary. In
most of those with an exerted sting, as _Pimpla_, they are linear,
exerted, and as long as the aculeus itself[759]. In _Proctotrupes_
they appear to be united so as to form a tube for the ovipositor,
and are produced by a prolongation of the last abdominal segment.
The darts usually run in two grooves of the sheath, and at their
apex are retroserrulate[760]. In some cases the sheath itself is
serrated[761]. The shanks of the darts are connected with the valves;
so that when these open they are pushed out: sometimes on their outer
side they have a triangular plate towards the base, which prevents
their being pushed out too far[762].
In _Sirex_ and many ichneumons, in which the ovipositor is too long
to be withdrawn within the abdomen, it remains always exerted; but
in general it is retracted within that part when unemployed. In
the gall-fly (_Cynips_) this instrument is really as long as in
_Pimpla_, &c.; but as it is infinitely more slender, when in repose
it is rolled up spirally and concealed within the abdomen. It is
the puncture of this minute organ that produces the curious galls
formerly described to you[763]. But the most anomalous ovipositor in
this Order appears to be that of _Chrysis_ (_C. ignita_, &c.), which
is covered by several demi-tubes or scales enveloping and sliding
over each other: when these scales are removed, the true ovipositor
appears, which is of a structure similar to that of the rest of
the Order, but the valves are long and slender with their summit
generally visible without the anus[764].
Though the ovipositor of the majority of _Dipterous_ insects is a
tube with retractile joints[765], in the crane-flies this organ is
different, and, like that of _Acrida_ above described, consists of what
at first sight appear two valves, but each of which is formed of two
pieces, the upper ones sharp and longer, and the lower pair blunt. The
upper pair forms the auger that bores a hole in the ground, and the
lower conducts the eggs into it after it is bored[766].
In the _Aptera_ and _Arachnida_ in general there seems no remarkable
instrument of this kind; but Treviranus has described one in spiders
for extruding the eggs of a singular construction. It is an oval
plate lying between the external genitals and spinning organs, and
is composed of a number of small screw-shaped cartilages, connected
together in the most wonderful manner. There are few organs, he
observes, in the animal kingdom which for their artificial mechanism
can be compared with this. Each cartilage inosculates very closely
in the adjoining one, and all are besides bound together by a strong
skin[767].
The manner in which the eggs of insects are _fecundated_ by the
male sperm is one of those mysteries of Nature that are not yet
fully elucidated and understood. We can readily conceive that all
the eggs may be fertilized by a single intercourse in the case of
insects which, like the _Ephemeræ_ and _Trichoptera_, exclude the
whole mass at once; or like many moths and butterflies, in a very
short time afterwards; but the subject becomes much more difficult
to explain when we advert to the female of the hive-bee, the whole
number of whose eggs, deposited in _two years_, are, as Huber has
demonstrated, in like manner fertilized by a single act[768]:--if you
bear in mind, however, what I have lately observed with regard to
Malpighi's discovery of a sperm-reservoir in insects, you will more
readily comprehend how in this case a _gradual_ fecundation may take
place. The principal objection to this solution of the difficulty in
the case before us, is derived from the very small size of the organ
supposed to be destined for this purpose--it being scarcely bigger
than the head of a pin[769]: it seems therefore incredible that it
should retain any portion of an extraneous fluid at the end of twelve
or eighteen months, and still more unlikely that the fluid should in
the interval have sufficed for the slightest moistening of not fewer
than 30,000 or 40,000 eggs. The only hypothesis that seems at all to
square with this fact, is that of Dr. Haighton,--that impregnation
is the result not of any actual contact of the sperm with the eggs,
but of some unknown sympathetic influence[770], or rather perhaps of
some penetrating effluvia or _aura seminalis_, which, though small in
quantity, it may retain the power of emitting for a long period.