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].

With regard to their _apex_, the bile-vessels are sometimes _fixed_ 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.

In this letter it may not be out of place to say a few words upon the _excrements_ of insects; which, strange as the observation may seem, but it is no less true than strange, are sometimes pleasing to the eye, from their symmetry, and to the taste, from their sweetness. In those that masticate their food they are solid, and in those that take it by suction, fluid or semi-fluid. In the caterpillars of _Lepidoptera_ they are of the former description, and every grain wears some resemblance to an insect's egg: as the passage in many of these consists of _six_ fleshy parts separated by channels, so the excrement represents six little prisms separated by six channels[583]. The _Aphides_ all secrete a fluid excrement as sweet as honey, of which the ants are so fond[584], which is ejected not only at the anal passage, but, in many, by two little siphonets also above it[585]. A semi-fluid excrement is produced by some species of _Chermes_, as that which inhabits the Box, which often comes from the animal in long convoluted strings resembling vermicelli. Reaumur says its taste is agreeable, much more so than that of manna[586]. Under this head should be included the abundant spume with which the larva of _Cercopis spumaria_ envelopes itself[587].

I am, &c.

FOOTNOTES:

[418] _Anat. Comp._ iv. 129.

[419] Cuv. _Anat. Comp._ iv. 129.

[420] PLATE XXI. FIG. 3. c, d, e, is the intestinal canal of the larva of the _Cossus_.

[421] Cuv. _Ibid._ 112.

[422] Ramdohr _Anat. der Ins._ 6.

[423] _Ibid._ 25.

[424] _Ibid._ 6.

[425] Cuv. _ubi supr._ 113.

[426] Comp. Ramdohr _Anat._ 7.

[427] PLATE XXI. FIG. 3. _c._

[428] _Tenebrio_ Ramdohr, _ubi supr._ 9. _t._ iv. _f._ 1.

[429] _Agrion._ _Ibid._ _t._ xv. _f._ 4. _a, b._

[430] _Ibid._

[431] Many other insects that live by suction have something similar, as the honey-bag of butterflies, PLATE XXX. FIG. 10, 11. _a._ Ramdohr _t._ xviii. _f._ 2. with _t._ xix. _f._ 1-3. and xxi. 1, 3, &c.

[432] Ramdohr _Anat._ 11--.

[433] PLATE XXI. FIG. 3. _d._

[434] Ramdohr _Ibid._ 28--.

[435] Herold (_Schmetterl._ 24) says that Ramdohr is mistaken here, and denies the existence of this juice in insects; but as Ramdohr's researches were so widely extended, he is most likely to be right.

[436] Ramdohr _Ibid._ 29.

[437] _Ibid._ 31.

[438] _Ibid._ 28.

[439] _Anat. Comp._ iv. 135. Comp. Dr. Kidd in _Philos. Trans._ 1825. 223. _t._ xv. _f._ 6, 7.

[440] Ramdohr _Anat._ 15.

[441] _Ibid._ 15.

[442] _Ibid._ 18.

[443] _Ibid._

[444] _Ibid._

[445] Swamm. _Bibl. Nat._ i. 94. b. Cuv. _Anat. Comp._ iv. 134.

[446] _Ubi supr._ 18.

[447] _Ibid._ _t._ i. _f._ 1. _e._ 5. _c._ 9. _g, h._

[448] _Ibid._ _t._ xxv. _f._ 4. _bb._

[449] Ramdohr _Anat._ _t._ viii. _f._ 3. _cc._

[450] _Ibid._ _t._ vii. _f._ 2.

[451] _Ibid._ 20.

[452] _Anat. Comp._ iv. 132.

[453] _Ibid._ and 136.

[454] _Ubi supr._ 30.

[455] _Ibid._ 31. _t._ iv. _f._ 2. _c._ _t._ v. _f._ 1. _d. f._ 4. D.

[456] _Ibid._ 32.

[457] _Ibid._ 34.

[458] Ramdohr _Anat._ 35.

[459] _Ibid._ _t._ xxiv. _f._ 1. _F._

[460] _Ibid._ 36. _t._ vii. _f._ 2. _kk._ _t._ viii. _f._ 3. _g, hh._

[461] _Ibid._ _t._ xii. _f._ 1. _t._ xvii. _f._ 1. _t._ vii. _f._ 5.

[462] _Ibid._ 37.

[463] _Ibid._ 38.

[464] _Ibid._

[465] Ramdohr _Anat._ 40.

[466] _De Bombyc._ 18--.

[467] _Anat. Comp._ iv. 153.

[468] _Ibid._

[469] _Ibid._

[470] Ramdohr 43. _Cicindela campestris_, _t._ iii. _f._ 1. K.

[471] _Phryganea grandis_, _Ibid._ _t._ xvi. _f._ 2.

[472] _Notonecta glauca_, _Ibid._ _t._ xxiii. _f._ 5.

[473] Of _Musca vomitoria_, _Ibid._ _t._ xix. _f._ 5.

[474] _Ibid._ _t._ viii. _f._ 1. H. and G. _f._ 2.

[475] _Ibid._ 50.

[476] _Ibid._

[477] _Ibid._

[478] _Ibid._ 44. _t._ i. _f._ 9.

[479] _Ibid._

[480] _Ibid._ _t._ vi. _f._ 5. H.

[481] Kidd in _Philos. Trans._ 1825. _t._ xv. _f._ 6.

[482] _Ibid._ _t._ xix. _f._ 1. _N, N, O,_ _f._ 2. _P, P, O._

[483] _Ibid._ _t._ 1. _f._ 1. _kkk._

[484] Ramdohr, _t._ xiii. _f._ 1-3.

[485] _Ibid._ 44.

[486] _Ibid._ 45.

[487] _Ibid._ 45. PLATE XXI. FIG. 3. _f. f._

[488] Rhamdohr, _Ibid._ _t._ iii. _f._ 6. E.

[489] _Ibid._ _t._ i. _f._ 1. 5. 9. _t._ xiv. _f._ 1-3.

[490] _Ibid._ 46. _t._ vi. _f._ 3.

[491] Ramdohr, _t._ vii. _f._ 2.

[492] _Ibid._ _t._ ii. iii. &c. _t._ xx. _f._ 1, 2. 6. _t._ xxii. _f._ 1-5. &c.

[493] _Ibid._ _t._ xviii. _f._ 1. 5. _t._ iv. _f._ 1. See also _t._ vi. _f._ 1. 3.

[494] _Ibid._ _Anat._ _t._ xvii. _f._ 1, 2. 6.

[495] _Ibid._ _t._ xiv. _f._ 3.

[496] _Ibid._ _t._ xiii. _f._ 4.

[497] _Ibid._ _t._ xv. _f._ 3, 4. _t._ 1. _f._ 1. 5. 9. _t._ xii. _f._ 4, 5, 6, &c.

[498] _Ibid._ _t._ xi. _f._ 4. _t._ xii. _f._ 4-6. _t._ xiii. _f._ 2-4, &c.

[499] _Ibid._ _t._ vii. _f._ 1. _t._ viii. _f._ 1, &c.

[500] Ramdohr _Anat._ _t._ ii. iii. xxv.

[501] _Ibid._ _t._ iii. _f._ 6. _t._ iv. _f._ 2. _t._ v. _f._ 1.

[502] _Ibid._ _f._ l. _e. f._ 3.

[503] _Ibid._ 122.

[504] _Ibid._ 123.

[505] _Ibid._ _t._ v. _f._ 4. B.

[506] _Ibid._ 94.

[507] _Ibid._ 96--.

[508] Ramdohr _t._ x. _f._ 1. 8.

[509] _Ibid._ _f._ 8. _b. c._

[510] _Ibid._ 98. _t._ x. _f._ 2-4. From Ramdohr's figure, compared with the size of the insect, it appears that the gizzard could scarcely have been of greater diameter.

[511] _Ibid._ _f._ 2.

[512] See W. Curtis in _Linn. Trans._ i. 88.

[513] Ramdohr _t._ x. _f._ 1. _d._

[514] _Ibid._ _l l._

[515] _Ibid._ _t._ ix. _f._ 1, 2. _t._ xi. _f._ 3. _t._ xxiv. _f._ 1, 2.

[516] Ramdohr 103.

[517] _Ibid._ 104. _t._ vi. _f._ 4. D.

[518] _Ibid._ _f._ 2. B.

[519] _Ibid._ _t._ vi. _f._ 3. E.

[520] _Ibid._ 101.

[521] _Ibid._ _t._ i. _f._ 1. 5. 9.

[522] _Ibid._ _f._ 2, 3, 4. 7, 8. 12.

[523] _Ibid._ _f._ 1. _e,_ _f._ 5. _c._ _f._ 9. _g h._

[524] _Ibid._ _f._ 1. 9. _k._

[525] _Ibid._ _t._ xv. _f._ 3, 4. _t._ xvii. _f._ 2. 6.

[526] _Ibid._ _t._ xv. _f._ 3, 4, _f._

[527] _Ibid._ _t._ xvii. _f._ 2. _c._ _f._ 6. _d._

[528] _Ibid._ _f._ 2. _b._ _f._ 6. _c._

[529] Ramdohr _t._ xii. _f._ 6. _H._ _t._ xiii. _f._ 1. _f._

[530] _Ibid._ _t._ xiv. _f._ 2, 3, _C._

[531] _Ibid._ _t._ xii. _f._ 6. _D._ _t._ xiii. _f._ 1. _b._

[532] _Ibid._ 133. _t._ xii. _f._ 1-3.

[533] _Ibid._ _f._ 4.

[534] Comp. Ramdohr _t._ xxii. _f._ 3. _M._ FIG. 4. 3. with _t._ xxi. _f._ 1. _I._

[535] Ramdohr _t._ xxii. _f._ 1. c. _f._ 3, 4. _B_--.

[536] _Ibid._ _f._ 1. _D E._ _f._ 3. _C D._

[537] _Ibid._ _t._ xxii. _f._ 1. _D, E._ _f._ 3. _C, D._ _f._ 4. _C._

[538] _Ibid._ 198.

[539] _Ibid._ _t._ xxvi. _f._ 2. 4.

[540] _Ibid._ _t._ xxxiii. _f._ 3.

[541] Ramdohr _t._ xviii. _f._ 1. _F, G._

[542] _Ibid._ _L, K._

[543] PLATE XXX. FIG. 7.

[544] Ibid. FIG. 8.

[545] Ibid. FIG. 9.

[546] PLATE XXX. FIG. 10.

[547] Ibid. FIG. 11. _a._

[548] Ibid. _e._

[549] Ibid. _d._

[550] Ramdohr, _Ibid._ _t._ xx. _f._ 1. _E._ _f._ 6. _C._

[551] _Ibid._ _t._ xix. _f._ 2. _C._ _f._ 3. _CCD._ _t._ xx. _f._ 2. _E._

[552] _Ibid._ _t._ xix. _f._ 2. _D._

[553] _Ibid._ _t._ xx. _f._ 2. _FF._ _f._ 6. _DD._ 184. 180.--

[554] _Ibid._ _t._ xix. _f._ 1. _ON._ _f._ 2. _OP._ _f._ 3. _F._ _t._ xxviii. _f._ 1, 2. _p. q._

[555] Ramdohr, _Ibid._ _t._ xx. _f._ 1. _G._ _f._ 2, 3. _L._

[556] _Ibid._ _t._ xxi. _f._ 1. _D._

[557] _Ibid._ 172.

[558] _Ibid._ _t._ xix. _f._ 2. _K L._ This organ seems analogous to that with four retractile fleshy horns, observed by Reaumur and De Geer in other species of _Muscidæ_. Reaum. iv. _t._ xxviii. _f._ 13. _a, s._ De Geer vi. _t._ iii. _f._ 18. _c, d._

[559] Ramdohr _t._ xxi. _f._ 6.

[560] Ramdohr _t._ xxix. _f._ 1 *. _A._

[561] _Ibid._ and _f._ 3. _B, D._

[562] _Ibid._ _f._ 2, 3. 5. &c.

[563] See above, p. 99--.

[564] Treviranus and Ramdohr are of the former opinion; and Meckel, Cuvier, Marcel de Serres, and Leon du Four, of the latter.

[565] Treviran _Arachnid._ _t._ 1. _f._ 6. _v._

[566] _Ibid._ _n._

[567] _Ibid._ _t._ ii. _f._ 24. β.

[568] Treviran _Arachnid._ _f._ 6. _B B._

[569] _N. Dict. d'Hist. Nat._ xxx. 423--. Comp. Treviranus, _Arachnid._ _t._ i. _f._ 6.

[570] Treviranus, _Ibid._ _v._

[571] _N. Dict. d'Hist. Nat._ xxx. 421--. Comp. Treviran. _Ibid._

[572] _N. Dict. d'Hist. Nat._ _Ibid._

[573] Treviran. _Ibid._ _t._ i. _f._ 6. _i i, c c._

[574] _N. Dict. d'Hist. Nat._ _Ibid._

[575] Treviran. _Ibid._ _t._ ii. _f._ 24. _a._

[576] _Ibid._ _v, b._

[577] _Ibid._ _c, d, f._

[578] _Ibid._ _g, n._

[579] _N. Dict. d'Hist. Nat._ _Ibid._

[580] Treviran. _Ibid._ 28.

[581] _Ibid._ _t._ ii. _f._ 24. β.

[582] Ramdohr, _t._ xix. _f._ 1.

[583] Reaum. i. 143. _t._ v. _f._ 9.

[584] VOL. II. p. 88--.

[585] De Geer iii. 26.

[586] Reaum. iii. 357. _t._ xxix. _f._ 6-10.

[587] VOL. II. p. 225.