singly or connectedly to the intestine merely by a few muscular fibres;
for they do not enter it, their ends having no orifice. This structure
is mostly to be met with in the _Coleoptera_[486]. In caterpillars,
the tops of these vessels perforate the outer skin of the rectum, and
proceeding in dense convolutions to the anus, become at last so fine
that their terminations cannot be discovered[487]. In other cases,
the extremities of a pair of these vessels _unite_ so as to form a
double one: this may be seen in those of _Philonthus politus_[488], and
probably other rove-beetles: and lastly, in others the bile-vessels are
_free_, hanging down by the intestinal canal, without being attached
to it or to each other. This structure is constantly found in the
_Orthoptera_ and _Hymenoptera_ Orders, &c.[489].
With regard to their _number_, the bile-vessels vary from two to
upwards of one hundred and fifty, yet so that their whole amount is
constantly the product of the number two,--at least as far as they
have been counted: and even when those on one side are not alike,
a similar variation takes place in the other, as may be seen in
_Galleruca Vitellinæ_, where on each side are two long ones and one
shorter[490]; the most usual numbers are, _four_--_six_--or _many_,
that is, more than _twenty_--
_Two_ bile-vessels are found in the larva of _Cetonia aurata_[491].
_Four_ most _Coleoptera_, _Diptera_,
and _Hemiptera_[492].
_Six_ _Lepidoptera_, some _Coleoptera_[493],
&c.
_Eight_ _Myrmeleon_, _Hemerobius_[494].
_Fourteen_ _Formica rufa_[495].
_Twenty_ larva of _Clavellaria Amerinæ_[496].
_Many_ _Libellulina_, _Orthoptera_, and
_Hymenoptera_[497].
The bile-vessels vary considerably in _length_: in many cases where
they are _free_ they are _short_[498]; they are often very long,
and perhaps those that are _fixed_ may be generally stated as the
longest. In the Lamellicorn beetles they are remarkable for their
great length[499].
Having given you this general account of the intestinal canal and its
parts and appendages, I shall now state some of the peculiarities
that in this respect distinguish particular tribes and families.
The _Coleoptera_ alone, exhibit as many variations in the
structure of the alimentary tube as all the other Orders of
insects together:--to particularize these would occupy too large
a portion of this letter, I shall therefore only notice a few
of the most remarkable. In general they may be stated as having
universally a stomach, a small intestine and rectum, and not more
than _three_ pairs of _fixed_ or _united_ bile-vessels. In the
Predaceous beetles, the _gullet_ mostly widens at the base into a
considerable _crop_, followed by a _gizzard_, a shaggy _stomach_,
and two pairs of _united_ bile-vessels. The whole alimentary canal
in these, is never less than _double_, and sometimes _treble_ the
length of the body[500]. In the _carnivorous_ beetles, at least the
_Staphylinidæ_ and _Silphidæ_, there is little or no _crop_, and
the _gizzard_ is hidden: in the former, the whole length of the
intestinal canal is not _twice_, while in the latter it is more than
_four_ times that of the body[501]. In these also the intermediate
portion of the large intestine is singularly annulated[502]. In the
_Petalocera_ the _stomach_ is usually longer than all the rest of
the intestines together, and often convoluted: in the cockchafer
the whole intestinal canal is nearly _five_ times the length of the
body, _four_ parts of which is occupied by the stomach[503]. In
the grub the canal scarcely exceeds the length of the animal[504].
In _Lampyris_ the stomach exhibits a remarkable appearance, having
on each side a series of spherical _folds_ or _vesicles_[505].
Have these any thing to do with the secretion of its phosphoric
matter? _Tenebrio_ has a _gizzard_ armed internally with calluses,
and a shaggy stomach, and _Blaps_ does not differ materially;
their entire canal is more than twice the length of the body[506].
In the _vesicatory_ beetles (_Cantharis_, _Meloe_, &c.) there is
no _gizzard_, and the canal is less than twice the length of the
body[507]. Little is known with regard to the alimentary canal of
the beetles distinguished by a _rostrum_ (_Rhyncophora_). In the
only two that appear to have been examined, _Rhynchites Betuleti_ and
_Cryptorhynchus Lapathi_, that canal is moderately long, the stomach
partially shaggy, and the small intestine inversely claviform; but
in other respects they differ materially[508]. In the former there
is no crop or gizzard, the stomach is fringed on each side, except
at its upper extremity, with a series of small _cœca_ or shags, and
there are _three_ pairs of bile-vessels[509]; while in the latter
the gullet is dilated into a crop which includes a gizzard in which
the skill of a DIVINE artist is singularly conspicuous:--though
so minute as scarcely to exceed a large pin's head in size, it is
stated to be armed internally with more than 400 pairs of teeth,
moved by an infinitely greater number of muscles[510]. A transverse
section of this gizzard represents two concentric stars, with nine
rays each[511]: the object of this structure is, the comminution
of the timber which this beetle has to perforate and probably
devour[512]. The stomach is very slender, but dilates in the middle
into a spherical vesicle[513], and there are only two pairs of
bile-vessels[514]. In the _Capricorn_ beetles, the part we are
considering varies much: in general we may observe that it is more
than _double_ the length of the body, that the stomach is long
and slender, and usually naked, that the gullet terminates in a
crop without a distinct gizzard, and that there are _three_ pairs
of bile-vessels[515]. In the Herbivorous beetles (_Chrysomela_,
_Cassida_, &c.) the canal is more than double the length of the
body, and in some much longer[516], the stomach is long, and commonly
naked; but in _Chrysomela violacea_ it is covered with hemispherical
prominences[517], and in _Chrysomela Populi_ it is shaggy[518]; in
the insect last named and _Galleruca Vitellinæ_ the rectum consists
of _two_ pieces[519]. In this tribe the intestines of the larva
resemble those of the perfect insect[520].
In the _Orthoptera_ the alimentary canal, which continues the same in
every state, is short, or only moderately long; the gullet has one
or two lateral pouches or crops[521], and terminates in a gizzard
of curious construction, with singular folds and teeth[522]; then
follows a short stomach, usually with a pair or more of _cœca_ at its
upper extremity[523]; the lower intestines are not distinct, and the
bile-vessels numerous, short and free[524].
In the _Neuroptera_, many of the genera are distinguished by the
remarkable length of the gullet, and by the lower intestines forming
one short piece[525]. In the _Libellulina_ the bile-vessels are
numerous, short, and free, as in the _Orthoptera_[526]. In _Hemerobius_
and _Myrmeleon_ there is a gizzard[527], and just above it a _cœcum_,
in the former very remarkable, is connected with the gullet[528].
The _Hymenoptera_ appear all to be distinguished by a long slender
gullet, terminating in a dilated crop forming the honey-bag; their
stomach is variable, their small intestine slender, and the rectum
dilated;--their bile-vessels, like those of the two preceding
Orders, are numerous, short, and free[529]. In the ants and
ichneumons there is an approach to a gizzard[530]. In the wasp and
humble-bee the stomach is very long, with muscular rings surrounding
it[531]. In this Order the larvæ at first have no lower intestines
and void no excrement[532], but as they approach to the pupa state
one begins to appear[533].
The next insects whose alimentary canal we are to consider, are those
which, taking their food by _suction_, have no occasion for masticating
organs: this may in part be predicated of the preceding Order, in
which most of the tribes in their perfect state _imbibe_ fluid food,
and use the ordinary organs of _mastication_ principally in operations
connected with their economy; and their crop, in which the honey in
many is stored up for regurgitation, may be regarded in some degree as
analogous to the food-bag of the _Diptera_ and other suctorious insects.
The two sections of the _Hemiptera_ Order differ widely in the canal
we are considering, and I shall therefore give a separate account of
each. In the _Heteropterous_ section, appended to the gullet by a
long convoluted capillary tube, besides the usual saliva-reservoirs
there is often a double vessel, which Ramdohr regards as discharging
the same function, but which in many respects seems rather analogous
to the food-reservoir of the _Diptera_[534]. As I have had no
opportunity of examining this vessel, I shall content myself with
stating this idea, and describe the vessel more fully hereafter. The
gullet, in these, usually terminates in an ample crop consisting
of many folds[535], followed by a long, slender, cylindrical tube,
dilated at its base into a spherical tumour; these two may be said to
form the first stomach: to this succeeds a second[536], which Ramdohr
denominates the _bug_-stomach (_Wanzen-magen_), which varies in its
figure, and in _Pentatoma_ consists of four demi-tubes, so as to form
a quadrangular canal[537]. In the _Homopterous_ section of this Order
Ramdohr seems to have examined but few; _Chermes_ however and _Aphis_
exhibit one remarkable feature; they have _no bile-vessels_, at least
he could discover no trace of these organs[538]. Their intestinal
canal is very simple, their stomach very long, widest above, and
somewhat convoluted, with a very slender gullet[539]. In _Cercopis
spumaria_ the structure is more complex, and extremely singular. It
has _two_ or rather _three_ stomachs; the two first of a _horny_
substance, and the last a slender somewhat convoluted _membranous_
tube, which becoming reversed, is attached by what should be deemed
its lower extremity to the first stomach, from the other side of
which emerge the lower intestines, terminating in a thick pear-shaped
rectum. At the same point of the first stomach the four bile-vessels
are attached, they grow gradually thicker for about a third of their
length, when they become twisted like a cord, and taper towards the
rectum, to which also they are attached[540]. From this structure
it should seem that the food has to pass twice through the first
stomach, before the process of digestion is complete, and it is
rejected at the anus.
The next suctorious Order is the _Lepidoptera_: in these the gullet
is long and slender, surrounded at the beginning with a loose
transparent skin, and at the base furnished with a pair of lateral
sacs, forming the honey-stomach, and probably analogous to the
food-reservoirs of the _Diptera_, which when blown up are of an oval
form; the stomach, as in the bugs, consists of _two_ portions, the
first being the longest[541]. There are three _free_ bile-vessels
on each side, proceeding from a single branch[542]. It will not be
uninteresting here to abstract from Herold the progressive changes
which take place in the intestinal canal in this Order, during the
transition of the animal from the larva to the imago state. In the
_larva_, the gullet, the small intestine, and the rectum, are short
and thick[543], there are a pair of silk reservoirs (_sericteria_),
as well as vessels for the secretion of saliva (_sialisteria_):
if you examine it two days after its first change, you will find
the gullet and the small intestine much lengthened and become very
slender; the stomach contracted both in length and size; the rectum
also changed, and the silk vessels contracted[544]. These in a
_pupa_ eight days old have wholly disappeared; the gullet is become
still longer, its base is dilated into a crop or food-reservoir;
the stomach is still more contracted, and instead of a cylinder
represents a spindle; the small intestine also is lengthened[545]:
at a still more advanced period, when it is near appearing under
its last form, the gullet and small intestine are still more drawn
out; and the honey-bag, though very minute, has become a lateral
appendage of the gullet[546]; and lastly, in the butterfly it appears
as a large vesicle[547]; the small intestine is grown very long[548];
and the rectum has changed its form and acquired a cœcum[549]. When
we consider the adaptation of all these changes of form, the loss
of old organs and the acquisition of new ones, to the new functions
and mode of life of the animal, we see evidently the all-powerful
hand of that ALMIGHTY BEING who created the universe, upholding by
his providence, and the law that he has given to every creature, the
system that he at first brought into existence.
We now come to the _Diptera_. These have a very slender gullet, to
which is attached on one side a long filiform tube, terminating in
the food-reservoir, which in some instances is simple[550], but
most generally consists of two or more vessels[551], collapsing
when empty, but varying in shape and size when inflated with food:
the mouth of the stomach in many cases is dilated into a kind of
ring[552]; sometimes there is on each side a blind appendage or
_cœcum_ opening into it, in _Bombylius_ covered with shags, which
though not connected with the mouth by a tube, Ramdohr regards as
saliva-reservoirs[553]; in _Musca vomitoria_ the beginning of this
organ below the mouth is covered with hemispherical prominences, and
in _Tipula_ it is dilated and marked with transverse folds. There are
usually _two_ pairs of bile-vessels; in the _Muscidæ_ pedunculate
and _free_[554]; in _Tipula_, _Bombylius_, and _Leptis_, sessile
and _united_[555]; and in _Tabanus_ sessile and _fixed_[556]. It is
remarkable that in some of this Order--the reverse of what usually
happens--the alimentary canal appears to be much longer in the larva
than it is in the imago; in _Musca vomitoria_, its length in the
_former_ is two inches and a quarter, while in the _latter_ it is
only one inch and one third[557]. A singular organ distinguishes the
imago of this species, the use of which appears not to be discovered.
It succeeds the _rectum_, and has on each side two short club-shaped
appendages, open at the end, which receive _tracheæ_, and terminate
in a short piece that opens into the anus[558].
In _Hippobosca_ and its affinities the canal in question differs
from that of other _Diptera_, in having no food-reservoir; in other
respects it resembles it[559].
From the above statement it appears that the principal character
which distinguishes those that take their food by _suction_,
from those that _masticate_ it, is the faculty with which they
are furnished by means of an ample crop, honey-stomach, or
food-reservoir, of _regurgitating_ the food they may have stored
up. Another distinction still more striking, which will appear more
evidently hereafter, is to be seen in the _saliva-secretors_ with
which the _suctorious_ tribes are furnished, to be found in very few
_masticators_, by which they are enabled to render the juices more
fluid and fit for suction.
The only insect amongst the _Aptera_ whose alimentary canal I
shall notice, is the common harvest-man (_Phalangium Opilio_): in
this, though the stomach and lower intestine are remarkably simple,
yet their cœcal appendages are numerous and singular: the former,
which has no distinct gullet, is pear-shaped[560]; and the latter,
tapering downwards, and truncated at the end[561]; connected with
it above are no less than twenty-three _cœca_ or blind appendages,
of various forms and dimensions; the last pair but one of which is
very remarkable, being bent like a bow, and furnished externally with
four short clavate processes[562]. It is probable that some of these
organs are analogous to the bile-vessels of other insects.
* * * * *
When the CREATOR in his wisdom fixed the limits of the various tribes
of animals, he united them all into one harmonious system by means
of certain intermediate forms, exhibiting characters taken some
from those that were to precede, and others from those that were
to follow them, and this not only in their _external_ structure,
but likewise in their _internal_ organization; so that we are
not to wonder if in the same individual we meet with organs that
belong to two distinct tribes, or if, remaining nearly the _same_
in their _prima facie_ appearance, they begin to exercise _new_
functions. An instance of this we have seen in the dorsal vessel of
insects, which in the _Arachnida_, though not materially different
in situation or general form, by the addition of a small apparatus
of arteries and veins becomes the centre and fountain of a regular
system of circulation[563]. From the circumstances here alluded to,
physiologists have been led to entertain very different sentiments
with regard to the structure of the alimentary organs of the Class we
are now to enter upon, the _Arachnida_: what some regard as a real
_liver_, others look upon as an _epiploon_ or caul; and what the
last denominate _bile_-vessels are by some of the former considered
as appropriated to the secretion of _chyle_[564]. Yet both these
opinions have some foundation in nature. When, in the _Arachnida_,
we discover a lobular substance consisting of granules filling the
whole cavity of the body and wrapped round the intestines, every one
will see in it no small analogy to the _epiploon_ which in insects
performs the same function: but when, upon a further examination,
we detect certain vessels communicating with this substance and the
intestinal canal[565], the idea that these may be _hepatic ducts_,
and this substance analogous to the _liver_, immediately strikes us
as not improbable. Again: when we discover pairs of other capillary
and tortuous vessels connecting with the intestinal canal either at
the _pylorus_[566] or below it[567], which in appearance strikingly
resemble the bile-vessels which we so constantly find in insects,
we seem warranted in concluding that they are of the same nature
and use: but when a nearer inspection enables us to detect the
hepatic ducts just mentioned in the scorpion, and we find that these
capillary vessels in the spider are in a very different situation
from those in insects which we suppose them to represent, it occurs
to us as not unlikely, that their _function_ may be different.
Let us now consider how the intestinal canal is circumstanced in
the two sections into which the Class _Arachnida_ is divided; the
_Scorpionidea_, and _Araneidea_. In the Scorpions, this organ proceeds
from the mouth to the anus without any flexure or convolution, so that
its length is scarcely equal to that of the body[568]; it is slender,
and its diameter, with the exception of an irregular dilatation here
and there, is nearly the same in its whole extent; the gullet is short;
the stomach long, and nearly cylindrical; the _duodenum_ shorter and
thicker than the stomach, from which, as well as from the _rectum_, it
is separated by a valve; the latter is cylindrical, and opens at the
anus above the insertion of the vesicle that secretes the poison[569].
With regard to the _biliary_ system and its organs: The _liver_ is of
a pulpy granular consistence and of a brownish colour, fills the whole
cavity of the trunk and abdomen, and serves as a bed for the other
intestines. It is divided longitudinally into two portions, by the
channel in which the heart reposes--its anterior part is formed into
many irregular lobes, by the sinuosities of the trunk; at the other
extremity, it terminates in two acute ends, which enter the first joint
of the tail; its surface presents a reticular appearance, the result of
the approximation of polygonous _lobuli_; its interior is a tissue of
infinitely minute glands: in _Scorpio occitanus_ there are about forty
pyramidal _lobuli_ detached from each other, the summits of which, by
their union, form bunches that have their excretory canals, varying in
number in different species, which convey the bile to the alimentary
tube; in the above insect there are six pairs, three in the trunk and
three in the abdomen, and in _S. Europæus_ a smaller number[570]; these
vessels run transversely from the liver, or aggregation of conglomerate
glands, to the intestinal canal[571]; the bunches consist of an
infinite number of spherical glands, generally filled with a brown
thick fluid[572]: besides the transverse vessels, from the base of the
stomach there issue two pairs of very slender tortuous ones, seemingly
analogous to the common bile-vessels; one pair of which runs upwards,
one on each side that organ towards the mouth, forming here and there
some ramifications which enter the liver; and the other runs nearly
transversely to it[573]. As the fluid contained in these vessels is
different from that contained in the glands of the liver, M. Marcel de
Serres supposes they may be chyliferous[574].
In the _Araneidea_ also the alimentary canal is nearly straight,
and scarcely exceeds the length of the body: the _gullet_ is rather
thick and cylindrical[575]; the _stomach_ is distinguished anteriorly
by two pairs of sacs, the upper pair being much the largest and
nearly triangular, the lower linear[576]; from these sacs a narrow
tube runs towards the _rectum_, but which is so entangled with the
liver, muscles, &c., as not to be easily made out[577]; the _rectum_
is rather tumid, and has a lateral _cœcum_[578]. The disposition of
the liver or conglomerate glands is stated to be similar to that of
the scorpion[579]; it is usually white, but in some species it is
yellowish, or reddish, and its lower surface has sometimes regular
excavations[580]; no transverse hepatic ducts connecting it with the
alimentary canal, as in the scorpion, appear to have been at present
discovered: two pairs of capillary _free_ vessels are attached to the
base of the _rectum_ on one side, which, except in their situation,
seem analogous to the bile-vessels of insects[581].
From the above detailed account of the alimentary canal of the
animals whose internal anatomy we are considering, it appears that
M. Cuvier's observation--that the length and complication of the
intestines indicate a less substantial kind of nutriment--does not
hold universally: thus, in _Necrophorus_ and _Silpha_, _carnivorous_
insects, the intestinal canal in its length and convolutions exceeds
those of most _herbivorous_ ones, and in _Cassida viridis_ and
some others of the _latter_ tribe are not longer than those of the
_predaceous_ beetles. In _herbivorous larvæ_ also, in general, the
length of the alimentary canal does not exceed that of the body, but
in those of some _flesh_-flies (_Musca vomitoria_) it very greatly
exceeds it[582]. So true is the observation--that there is no general
rule without exceptions.