of insects; for I think no one will object to that name being given
to their nutritive fluid, especially in the larva, though it
does not circulate by means of a vascular system. The chyle that
is produced in the intestines of animals from the food, is that
fluid substance from which their blood is formed: in insects it is
not absorbed by the lacteals, but transpires through the pores of
the intestinal canal into the general cavity of the body, where,
being exposed to the influence of the oxygen in the air-vessels, it
becomes, though retaining its colour, a different fluid from what
it was before, and analogous to blood in its use and office[407];
only that in these animals, as Cuvier has observed, at least in
their perfect state, the blood, for want of a circulating system,
not being able to seek the air, the air goes to seek the blood[408].
The dispersion of this fluid appears to be universal, so that all
the parts and organs contain it in a greater or less degree[409]. In
many insects, if you break only an antenna or a leg, a drop of fluid
flows out at the wound. In larvæ, the fluid which bathes[410], or
visits, all the internal parts and organs is not only sufficient for
their nutriment, but a large quantity of seemingly superfluous blood
remains that is not wanted for this purpose. This is expended in the
production of the caul or _epiploon_ (_Corps graisseux_ Reaum.),
which laps over and defends all the viscera of the animal, and goes
principally to the formation of the imago[411]. I have said that
Cuvier conceives nutrition in insects to take place by _imbibition_
or immediate absorption; that is, I suppose, the different parts
and organs thus constantly bathed in the blood, imbibe from it the
particles necessary for their constant accretion. M. Chabrier seems
to think that it is the compression and dilatation of the trunk
that duly distributes the nutritive fluid[412]; Lyonet compares the
nutrition of insects by their fibres from this fluid, when formed
into the _corps graisseux_, to that of plants that draw their
support by their roots from the earth[413]. Much obscurity, however,
at present rests upon this subject--much for future investigation
to explore; but in all the works of the MOST HIGH there is always
something inscrutable, something beyond the reach of our senses and
faculties, which teaches us humbly to adore his infinite perfections.
II. The circulation of the _Arachnida_ is next to be considered;
and the term applied to these becomes strictly proper. Two great
tribes, in our view of the subject, constitute this Class,--the
spiders (_Araneidea_) and scorpions (_Scorpionidea_): I shall give
you some account of the circulating vessels of each.--In _spiders_,
the heart in general is a long dorsal vessel as in insects, but
supposed to be confined to the _abdomen_, growing slenderer towards
each extremity, particularly the anal. In some also, as in _Aranea
domestica_, like that of insects, it has lateral muscular appendages;
but in others, as in _Clubiona atrox_, it is without them[414]. It
exhibits a pair of vessels that appear to connect with the gills,
by which the oxygenation of the blood takes place, and a number of
others that ramify minutely and are lost in the analogue of the
_epiploon_, supposed to be their _liver_[415]. Whether these last are
to be regarded merely as _veins_, has not been ascertained; they seem
rather to convey the blood outwards, than to return it back to the
heart: but this question must be left for future investigation. I may
observe, however, that though the heart of the spider has been traced
only in the _abdomen_, it may probably extend into the _trunk_.
The heart of the _scorpion_ has been examined both by Treviranus
and Marcel de Serres; but I shall principally confine myself to the
description of the latter, as the most clear and intelligible. The
heart, then, of these animals is elongated, almost cylindrical,
but attenuated at each end; it is extended from the head to the
extremity of the tail, and appears to have four pairs of lateral
muscles. On each side are four pairs of principal vessels, which go
to the pulmonary pouches, and there ramify. These may be assimilated
to _veins_. Besides these, there are four other vessels that cross
them, forming with them an acute angle, and which, with four branches
of smaller size, receive the blood from the pulmonary pouches, and
distribute it to the different parts of the body,--these are the
_arteries_. Before it enters the tail, the heart throws out two
vascular branches which do not go to the gills, but distributing the
blood to different parts, ought to be considered as arteries[416].
Treviranus mentions bunches of reticulated vessels, concerning the
use and origin of which he seems uncertain[417]; but as they approach
the gills they are probably the branching extremities of what M. de
Serres considers as the veins.
I am, &c.
FOOTNOTES:
[348] _Genes._ ix. 4.
[349] _N. Dict. d'Hist. Nat._ xxx. 130.
[350] Cuv. _Anat. Comp._ iv. 167.
[351] Herold _Schmetterl._ 25. note *. VOL. III. p. 53.
[352] _N. Dict. d'Hist. Nat._ vii. 313. Cuv. _Anat. Comp._ iv. 411.
[353] _Ibid._ 419, 407.
[354] _Ibid._
[355] _Ibid._ 410.
[356] PLATE XXII. FIG. 15.
[357] Lyonet _Anat._ 105.
[358] _Ibid._ 425.
[359] _Ibid._ 105--.
[360] _De Bombyc._ 15--.
[361] Reaum. i. 160--.
[362] Cuv. _Anat. Comp._ iv. 418.
[363] Marcel de Serres _Mem. du Mus._ 1819. 69.
[364] Swamm. _Bibl. Nat._ _t._ xl. _f._ 4. _t._ xv. _f._ 4.
[365] _De Bombyc._ _t._ iii. _f._ 4.
[366] _Ubi supr._ 414.
[367] _Ibid._ 425--.
[368] _Ibid._ 419.
[369] _Ibid._ 412.
[370] Lyonet _Anat._ 413.
[371] Lyonet _Ibid._ 426. Cuv. _Anat. Comp._ iv. 419.
[372] Lyonet says (426), "au-delà de trois millions de fois plus
petits qu'un grain de sable"!!
[373] _Ibid._
[374] His words are--"In silkworms I have clearly seen various small
vessels spring from and approaching to the heart, which I have
even filled with a coloured liquid. But whether they were veins or
arteries I cannot yet affirm." i. 112. a. 176. a. According to Cuvier
(_Anat. Comp._ iv. 418), but I cannot find the passage, Swammerdam
also mentions having seen a red fluid issue from small vessels in
grasshoppers.
[375] Reaum. v. 103.
[376] Bonnet ii. 309. Perhaps in both cases the alimentary canal was
the organ seen.
[377] Reaum. iv. 171--.
[378] Lesser L. ii. 84. note.
[379] De Geer ii. 505--. vi. 287.
[380] _On the Microscope._ i. 130.
[381] _Ibid._
[382] _Sur le Vol des Ins._ 325--.
[383] Lyonet _Anat._ 427--.
[384] Cuv. _Anat. Comp._ iv. 418--.
[385] _Mem. du Mus._ 1819. 71.
[386] _N. Dict. d'Hist. Nat._ xvi. 208.
[387] Marcel de Serres, in his _Observations on the Dorsal Vessel of
Insects_[388], endeavours to prove that the principal use of that
vessel is the more perfect animalization of the chyle that, transuding
through the pores of the intestinal canal, is imbibed by it. In
insects, he observes, that undergo metamorphoses, in which the growth
or development of parts is often very rapid, it is requisite that
a considerable portion of the chyle should be in reserve for this
purpose. On this account it is that the _Epiploon_ or adipose tissue
is so abundant in larvæ to what it is in the perfect insect. That the
importance also of this part to insects is proved by the circumstance,
that all their interior parts communicate by fibrils with this tissue,
and that probably their various organs derive the nutriment from
it by their means. He then asks by which of the viscera is the fat
elaborated, or by what means does the chyle which transudes from the
intestinal canal pass to the state of fat? Facts seem to indicate, says
he, that the function of the dorsal vessel is to pump up the chyle,
and to cause it then to transude through the meshes of the adipose
tissue, where it finishes by elaborating that mass of fat so abundant
in larvæ and certain perfect insects, which are thus enabled to sustain
the effects of a long fast. So that this vessel is only a _secretory_
organ, analogous to so many others that exist in insects; but the
secretion which it has to produce is the most important of all, since
the support of the vital powers depends upon it: it is, in effect,
that vessel which completes the function of animalization, and which
itself prepares the nutritive fluid[389]. He observes, amongst other
reasons he brings to support his theory, that the colour of the fluid
which it contains is always analogous to that of the adipose tissue
that surrounds it, and that the colour of that tissue never changes
without that of the fluid undergoing a corresponding alteration,--that
when, as in many perfect insects, the quantity of fat diminishes, the
dorsal vessel also diminishes in size, and that the same reagents which
coagulate the fat, coagulate equally the fluid in the dorsal vessel,
which seems to indicate an identity between them[390].
But there are circumstances that militate against this hypothesis.
The analysis which Lyonet has given of the fluid contained in the
dorsal vessel of the _Cossus_[391], seems to prove that it is more
analogous to gum or varnish. He saw indeed a few globules, which
appeared ten times as big as the others, which swam upon the water,
but which he did not regard as component parts of the fluid, but as
little drops of grease extravasated by dissection. The fluid of the
vessel itself easily mixed with water, and appeared to sink in it to
the bottom[392]. This proves that it is not of a fatty or oleaginous
nature. But the strongest objection is stated by M. Carus, who
judiciously observes[393], That it is contradictory to suppose that
a canal should absorb or exude fluids by its parietes in a different
form. Further experiments however seem necessary to ascertain the
nature of the fluid and its object.
[388] _Mem. du Mus._ 1819.
[389] _Ibid._ 68--.
[390] _Ibid._ 69--.
[391] See above, p. 85.
[392] Lyonet _Anat._ 426--.
[393] _Introd. to Comp. Anat._ ii. 277. Engl. Trans.
[394] This seems some confirmation of Dr. Virey's opinion, that
insects in their first states are still a kind of _fœtus_. See above,
VOL. III. p. 61--.
[395] _Introd. to Comp. Anat._ ii. 393--. Engl. Trans.
[396] _Introd. to Comp. Anat._ ii. 395--. Engl. Trans.
[397] _Introd. to Comp. Anat._ ii. 396--. Engl. Trans.
[398] _Ibid._ 398.
[399] _Ibid._ 399.
[400] _Ibid._ 398.
[401] _Introd. to Comp. Anat._ ii. 399--. Engl. Trans.
[402] Reaum. i. 409, 643--. Malpigh. _De Bombyc._ 38.
[403] Lesser L. ii. 87 note *.
[404] _Ubi supra._
[405] Reaumur iv. 264.
[406] Ibid. 260--.
[407] Herold _Schmetterl_. 24.
[408] _Anat. Comp._ iv. 165.
[409] Marcel de Serres (p. 67.) speaks of this fluid as being, after
it has transuded through the intestinal canal, a fluid in _repose_,
which seems to indicate that it is perfectly _stagnant_; but when we
consider that it is not only incessantly entering the body and making
its way to every part, but is also, by means of the various secretory
organs, constantly converted into new products, and so going out
again in many cases, it will appear evident that it cannot be
considered as a stagnant fluid, since there must be a constant though
probably slow motion towards the points of absorption or imbibition.
[410] Dr. Kidd (_Philos. Trans._ 1825. 236.) did not find the
abdominal viscera of the mole-cricket thus circumstanced, nor more
lubricated than the intestines of the higher animals.
[411] Cuv. _Anat. Comp._ iv. 158. Herold _Schmetterl._ 28.
[412] _Sur le Vol des Ins._ c. iv. 88. note 1.
[413] _Anat._ 428.
[414] Treviranus _Arachnid._ 28. _t._ iii. _f._ 28, 29.
[415] _Ibid._ 29. _t._ iii. _f._ 30, 31.
[416] _N. Dict. d'Hist. Nat._ xxx. 420. Comp. Treviran. _Arachnid._
10--.
[417] _Ibid._ 9--.
LETTER XL.
_INTERNAL ANATOMY AND PHYSIOLOGY
OF INSECTS, CONTINUED._
DIGESTION.
"The immense Class of insects," says the immortal Cuvier, "in the
structure of its alimentary canal exhibits as many variations as
those of all the vertebrate animals together: there are not only the
differences that strike us in going from family to family and from
species to species; but one and the same individual has often a canal
quite different, according as we examine it in its larva or imago
state; and all these variations have relations very exact, often easily
estimable, with the temporary or constant mode of life of the animals
in which it is observable. Thus the voracious larvæ of the _Scarabæi_
and butterflies have intestines ten times as large as the winged and
sober insects--if I may use such an expression--to which they give
birth[418]."
In the natural families of these creatures, the same analogy takes
place with respect to this part that is observable in the rest of the
Animal Kingdom; the length and complication of the intestines are
here, as in the other Classes, often an index of a less substantial
kind of nutriment; while their shortness and slenderness indicate
that the insect lives by prey[419].
In considering therefore the parts connected with the digestive
functions of the insect world, it will not be amiss to have reference
to their _food_, and their mode of taking it; but first it will be
proper to state and define the parts of this important organ.
In general the alimentary canal[420] is composed of the same
essential _tunicks_ as that of the vertebrate animals, consisting
of an interior epidermis, a papillary and cellular tunick, and an
exterior muscular one[421]. The first is usually tender, smooth,
and transparent; but not always discoverable, perhaps on account of
its tender substance[422]. Ramdohr does not notice the papillary
and cellular tunicks; they are probably synonymous with what he
denominates--the _flocky layer_ (_Die flockige lage_), and which
he describes, when highly magnified, as appearing to consist of
very minute globules or dark points, and as being of a cellular
structure[423]. The _exterior_ tunick is thicker and stronger than
the _interior_, and composed of muscular fibres, running either
longitudinally, or transversely so as to form rings round the canal.
This tunick mostly begins at the mouth, and goes to the anus,
changing its conformation in different parts of the above intestine.
Sometimes however it originates only at the beginning of the
stomach[424]. With respect to its general disposition, that canal--in
its relative length, in the size of its different parts, in the
number and form of its dilatations, and particularly of its stomachs
and its cœcums, and in the folds of its interior--exhibits variations
altogether analogous to those of vertebrate animals, and which
produce similar effects[425]. As to its _parts_, it may be considered
as consisting of _two_ larger portions, between which the biliary or
hepatic vessels form the point of separation. In the first, the most
universal parts are the gullet and the stomach; and in the second,
the small intestine and the large intestine[426].
1. The _gullet_ (_Œsophagus_[427]) is that portion of the intestinal
canal which, receiving the food from the pharynx, or immediately
from the mouth, conveys it to the stomach. Though it often ends just
behind the _head_[428], it is usually continued through the _trunk_,
and sometimes even extends into the middle of the _abdomen_[429];
it therefore seldom much exceeds in length half the body. It is
constantly long when the head is connected with the trunk by a narrow
canal--as in the _Hymenoptera_, _Neuroptera_, _Lepidoptera_, &c.; but
is frequently short when these parts are more intimately united[430].
It often ends in a kind of sac analogous to the crop of birds.
Under this head I must mention a part discovered by Ramdohr, which
he calls the _food-bag_ (_Speisesack_), as he thinks, peculiar to
_Diptera_[431]. From the mouth in these proceeds a narrow tube into
the abdomen, where it expands into a blind sac having no connexion
with the stomach; so that the fluid food, as blood, &c. stored in
it, must be regurgitated into the mouth before it can pass into
that organ[432]. Thus these animals, besides their stomach, have a
_reservoir_ in which to store up their food; the product therefore of
a single meal will require several days to digest it.
2. The _stomach_ (_Ventriculus_[433]) is that part of the intestinal
canal immediately above the bile-vessels, which receives the food
from the gullet for digestion, and transmits it when digested to
the lower intestines[434]. By its admixture with the gastric juice,
the food acquires in the stomach a quite different colour from
what it had in the gullet. In herbivorous _insects_ it contains no
acid, but, like the gastric juice of herbivorous _quadrupeds_, is
of an alkaline nature[435]. The chyle is forced through this organ,
probably in part by the pressure of the muscular fibres during the
peristaltic motion; and being pressed through the _inner_ skin, is
first collected in the intermediate cellular part, and ultimately
forced through the _outer_ skin[436]. At its posterior end it
terminates in the _pylorus_, a fleshy ring or sphincter formed of
annular muscular fibres[437]. The stomach often consists of two or
more successive divisions, which are separated from each other, and
are often of an entirely different conformation and shape[438]. In
the _Orthoptera_, Predaceous _Coleoptera_, and several other insects,
an organ of this kind precedes the ordinary stomach, which from its
structure Cuvier denominates a second stomach or _gizzard_[439];
Posselt improperly calls it _Cardia_[440]; and by Ramdohr it is
named the _plaited-stomach_ (_Falten-magen_[441]). It is a short
fleshy part consisting of two skins, placed above the opening of the
stomach, and perhaps rather belongs to the gullet. The _inner_ skin
is formed into longitudinal folds, and sometimes armed with horns,
teeth, or bristles. Its cavity is very small and compressed, so as
to admit only small masses of food, and yet present them to a wide
surface for the action of the teeth or bristles;--in this stomach
therefore, as in the gizzard of birds, to which it seems clearly
analogous[442], the food is more effectually comminuted and rendered
fit for digestion. The muscles, by which its action upon the food is
supported, in some species amount to many thousands[443]. Rudiments
of a gizzard are sometimes found concealed in the gullet of many
insects[444]. The idea of Swammerdam, Cuvier, &c. that grasshoppers
and other insects that have this kind of stomach, chew the cud[445],
Ramdohr affirms is entirely erroneous[446]. Besides its divisions,
the stomach has other _appendages_ that require notice. In most
_Orthoptera_, a pair or more of blind intestines or _cœca_ may be
found at the point of union of the gizzard with the stomach[447],
which have been regarded as forming a _third_ stomach: they also
begin the stomach in the louse[448]; they form a coronet round the
apex of that organ, in the grub of the cockchafer[449]; and in that
of the rose-beetle, there is one at the apex, one in the middle,
and a third at the base[450]. Besides these appendages, which are
formed of the skin of the stomach, there are others that are not
so. In the Predaceous and some other beetles, the whole external
surface of this organ is covered with small blind appendages opening
into the space between its two skins, which cause it to resemble a
shaggy cloth; these Ramdohr calls _shags_ (_zotte_[451]), and Cuvier,
_hairs_[452] (_villi_). These appendages the latter author seems to
regard as organs that secrete the gastric juice and render it to the
stomach[453]; but the former thinks their use uncertain[454].
3. The _small intestines_ (_Intestina parva_) are the portion
of intestines next the stomach, and consist often of _three_
distinct canals;--the first is supposed to be analogous to the
_duodenum_; it is found only in the Coleopterous genera _Silpha_
L. and _Lampyris_ L., and is distinguished from the succeeding
intestine by being perfectly smooth[455]. Next follows the _thin
intestine_ (_Dünndarm_), which in the above insects is wrinkled;
it most commonly immediately follows the stomach. Sometimes it is
wholly wanting, as in _Agrion_, the _Hemiptera_[456], &c. Ramdohr
conjectures that it is not solely destined for conveying the
excrement, but that probably some juices are separated in it from
the food especially for the nutrition of the gall-vessels, as their
principal convolutions are mostly near this intestine[457]; which
perhaps may in some cases be regarded as analogous to the _jejunum_
in vertebrate animals. The third pair of the small intestines, which
perhaps represents the _ileum_, Ramdohr distinguishes by the name
of _club-shaped_ (_Keulförmigen Darm_[458]). It may generally be
regarded as only a continuation of the former thickened at the end so
as to resemble a club reversed. It is however sometimes _separated_
from the thin intestine, as in _Cerambyx moschatus_[459].
4. The _large intestines_ (_Intestina magna_) consist sometimes
of two portions. The _thick intestine_ (_Dicken-darm_), which may
be regarded as a kind of _cœcum_, is found only in the larvæ of
the Lamellicorn beetles, but never in the perfect insect. In shape
it is oval and folded; whence it is thicker than the rest of the
intestinal canal, and is constantly filled with excrement[460]. The
second portion of these intestines is the _rectum_ (_Mastdarm_),
which terminates in the anal passage. This part is scarcely ever
wanting, except when the insect evacuates no excrement, which is the
case with the grubs of bees, wasps, and the antlion (_Myrmeleon_).
In the imago of _Telephorus_, at least in _T. fuscus_, it is also
obsolete[461]: in most cases, however, it is very distinct from
the preceding intestine. Sometimes it consists of only one tunick
composed of muscular fibres[462]. When the gullet is wide, the
_rectum_ is usually so likewise; but when it follows a club-shaped
or thick intestine, it is narrow[463]. It generally may be termed
_short_[464]. When wide, it often contains a great quantity of
excrement, as the gullet does of undigested food; but when narrow,
the excrement seldom remains long in it. This intestine also in a few
cases has a lateral enlargement or _cœcum_ (_Blind-darm_), being a
continuation of the same skin; but perhaps this enlargement is really
analogous to what Ramdohr calls the _thick intestine_, though in
these cases he regards it as an appendage of the rectum[465].
I must now call your attention to the _bile-vessels_ of insects.
These, by Malpighi[466] and the earlier physiologists, who regarded
them as a kind of _lacteals_, were denominated _varicose_ vessels:
but Cuvier--and his opinion after some hesitation has been adopted by
Ramdohr--considers them as vessels for the secretion of _bile_, and as
analogous to the _liver_ of animals that have a circulation[467]. As
the want of blood-vessels prevents insects from having any gland, the
bile is produced with them, as all their other secretions, by slender
vessels that float in their nutritive fluid, and from thence secrete
the elements proper to form that important product, which usually
tinges them with its own yellow hue; though in the Lamellicorns and
Capricorns they are of an opaque white, and in the _Dytisci_ of a
deep brown colour[468]. Their bitter taste further proves that they
contain the bile[469]. They are long, slender, filiform, tortuous or
convoluted, and mostly simple vessels; sometimes gradually smaller
toward the base[470], at others towards the apex[471]. In some,
screw-shaped[472]: in one larva, with hemispherical elevations[473]: in
the cockchafer, part of them are fringed on each side with an infinity
of short, blind, minute, setiform tubes, while the rest are naked[474];
they are composed of a single, thin, transparent membrane, according
to Ramdohr[475]; but Cuvier thinks their texture is spongy[476]. They
appear to contain a number of small, irregular, dark granules, which
float in a peculiar fluid, with which, however, they are not always
filled throughout, nor are they constantly permeable from one end to
the other. Thus in the meal-worm beetle (_Tenebrio Molitor_), the
common trunk by which they are attached to the intestinal canal is
composed of gelatinous granules[477]. The place of their insertion is
generally a little below the _pylorus_, but in the common cockroach
they are inserted into the stomach just above that part[478]. Usually
each vessel opens singly into the intestinal canal, which the whole
number surround at an equal distance from each other[479]. Sometimes,
however, they are connected with it by a common tube in which they
all unite, as in the asparagus-beetle (_Lema Asparagi_[480]), and
the mole-cricket (_Gryllotalpa vulgaris_[481]); in the house-fly
(_Musca domestica_), and other _Muscidæ_, each pair unites so as to
form a single branch on each side of the canal previously to their
insertion[482]; in the field-cricket (_Gryllus campestris_) they are
all inserted in one spot[483]; and when numerous, they are generally
attached singly though irregularly[484]. These vessels at their base do
not open into the cavity of the intestinal canal, but merely into the
space between its outer and inner tunicks, the last being constantly
imperforated[485].