II. The circulation of theArachnidais 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.—Inspiders, the heart in general is a long dorsal vessel as in insects, but supposed to be confined to theabdomen, growing slenderer towards each extremity, particularly the anal. In some also, as inAranea domestica, like that of insects, it has lateral muscular appendages; but in others, as inClubiona 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 theepiploon, supposed to be theirliver[415]. Whether these last are to be regarded merely asveins, 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 theabdomen, it may probably extend into thetrunk.
The heart of thescorpionhas 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 toveins. 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 thearteries. Before it enters the tail, the heart throws out two vascular branches which do not go to the gills, but distributing the bloodto 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.
"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 theScarabæiand 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 theother 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 theirfood, 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 essentialtunicksas 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—theflocky 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]. Theexteriortunick is thicker and stronger than theinterior, 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, inthe 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 itsparts, it may be considered as consisting oftwolarger 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. Thegullet(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 thehead[428], it is usually continued through thetrunk, and sometimes even extends into the middle of theabdomen[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 theHymenoptera,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 thefood-bag(Speisesack), as he thinks, peculiar toDiptera[431]. From the mouth in these proceeds a narrow tube into the abdomen, whereit 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 areservoirin which to store up their food; the product therefore of a single meal will require several days to digest it.
2. Thestomach(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 herbivorousinsectsit contains no acid, but, like the gastric juice of herbivorousquadrupeds, 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 theinnerskin, is first collected in the intermediate cellular part, and ultimately forced through theouterskin[436]. At its posterior end it terminates in thepylorus, 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 theOrthoptera, PredaceousColeoptera, and several other insects, an organ of thiskind precedes the ordinary stomach, which from its structure Cuvier denominates a second stomach orgizzard[439]; Posselt improperly calls itCardia[440]; and by Ramdohr it is named theplaited-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. Theinnerskin 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 otherappendagesthat require notice. In mostOrthoptera, a pair or more of blind intestines orcœcamay be found at the point of union of the gizzard with the stomach[447], which have been regarded as forming athirdstomach: they also begin the stomach in the louse[448]; they form a coronet roundthe 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 callsshags(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. Thesmall intestines(Intestina parva) are the portion of intestines next the stomach, and consist often ofthreedistinct canals;—the first is supposed to be analogous to theduodenum; it is found only in the Coleopterous generaSilphaL. andLampyrisL., and is distinguished from the succeeding intestine by being perfectly smooth[455]. Next follows thethin intestine(Dünndarm), which in the above insects is wrinkled; it most commonly immediately follows the stomach. Sometimes it is wholly wanting, as inAgrion, theHemiptera[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 perhapsmay in some cases be regarded as analogous to thejejunumin vertebrate animals. The third pair of the small intestines, which perhaps represents theileum, Ramdohr distinguishes by the name ofclub-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 sometimesseparatedfrom the thin intestine, as inCerambyx moschatus[459].
4. Thelarge intestines(Intestina magna) consist sometimes of two portions. Thethick intestine(Dicken-darm), which may be regarded as a kind ofcœcum, 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 therectum(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 ofTelephorus, at least inT. 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, therectumis usually so likewise; but when it follows a club-shaped or thick intestine, it is narrow[463]. It generally may be termedshort[464]. Whenwide, 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 orcœcum(Blind-darm), being a continuation of the same skin; but perhaps this enlargement is really analogous to what Ramdohr calls thethick intestine, though in these cases he regards it as an appendage of the rectum[465].
I must now call your attention to thebile-vesselsof insects. These, by Malpighi[466]and the earlier physiologists, who regarded them as a kind oflacteals, were denominatedvaricosevessels: but Cuvier—and his opinion after some hesitation has been adopted by Ramdohr—considers them as vessels for the secretion ofbile, and as analogous to theliverof 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 theDytisciof 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]. Insome, 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 thepylorus, 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 otherMuscidæ, 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 theirapex, the bile-vessels are sometimesfixedsingly 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 theColeoptera[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 vesselsuniteso as to form a double one: this may be seen in those ofPhilonthus politus[488], and probably other rove-beetles: and lastly, in others the bile-vessels arefree, hanging down by the intestinal canal, without being attached to it or to each other. This structure is constantly found in theOrthopteraandHymenopteraOrders, &c.[489].
With regard to theirnumber, 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 inGalleruca Vitellinæ, where on each side are two long ones and one shorter[490]; the most usual numbers are,four—six—ormany, that is, more thantwenty—
The bile-vessels vary considerably inlength: in many cases where they arefreethey areshort[498]; they are often very long, and perhaps those that arefixedmay 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.
TheColeopteraalone, 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, asmall intestine and rectum, and not more thanthreepairs offixedorunitedbile-vessels. In the Predaceous beetles, thegulletmostly widens at the base into a considerablecrop, followed by agizzard, a shaggystomach, and two pairs ofunitedbile-vessels. The whole alimentary canal in these, is never less thandouble, and sometimestreblethe length of the body[500]. In thecarnivorousbeetles, at least theStaphylinidæandSilphidæ, there is little or nocrop, and thegizzardis hidden: in the former, the whole length of the intestinal canal is nottwice, while in the latter it is more thanfourtimes that of the body[501]. In these also the intermediate portion of the large intestine is singularly annulated[502]. In thePetalocerathestomachis usually longer than all the rest of the intestines together, and often convoluted: in the cockchafer the whole intestinal canal is nearlyfivetimes the length of the body,fourparts of which is occupied by the stomach[503]. In the grub the canal scarcely exceeds the length of the animal[504]. InLampyristhe stomach exhibits a remarkable appearance, having on each side a series of sphericalfoldsorvesicles[505]. Have these any thing to do with the secretion of its phosphoric matter?Tenebriohas agizzardarmed internally with calluses, and a shaggy stomach, andBlapsdoes not differ materially; their entire canal is more than twice the length of the body[506]. In thevesicatorybeetles (Cantharis,Meloe, &c.) there is nogizzard, 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 arostrum(Rhyncophora).In the only two that appear to have been examined,Rhynchites BetuletiandCryptorhynchus 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 smallcœcaor shags, and there arethreepairs of bile-vessels[509]; while in the latter the gullet is dilated into a crop which includes a gizzard in which the skill of aDivineartist 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 theCapricornbeetles, the part we are considering varies much: in general we may observe that it is more thandoublethe 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 arethreepairs of bile-vessels[515]. In the Herbivorous beetles (Chrysomela,Cassida, &c.) the canalis more than double the length of the body, and in some much longer[516], the stomach is long, and commonly naked; but inChrysomela violaceait is covered with hemispherical prominences[517], and inChrysomela Populiit is shaggy[518]; in the insect last named andGalleruca Vitellinæthe rectum consists oftwopieces[519]. In this tribe the intestines of the larva resemble those of the perfect insect[520].
In theOrthopterathe 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 ofcœcaat its upper extremity[523]; the lower intestines are not distinct, and the bile-vessels numerous, short and free[524].
In theNeuroptera, many of the genera are distinguished by the remarkable length of the gullet, and by the lower intestines forming one short piece[525]. In theLibellulinathe bile-vessels are numerous, short, and free, as in theOrthoptera[526]. InHemerobiusandMyrmeleonthere is a gizzard[527], and just above it acœcum, in the former very remarkable, is connected with the gullet[528].
TheHymenopteraappear 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 bysuction, 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 stateimbibefluid food, and use the ordinary organs ofmasticationprincipally 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 theDipteraand other suctorious insects.
The two sections of theHemipteraOrder differ widely in the canal we are considering, and I shall therefore give a separate account of each. In theHeteropteroussection, 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 theDiptera[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, inthese, 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 thebug-stomach (Wanzen-magen), which varies in its figure, and inPentatomaconsists of four demi-tubes, so as to form a quadrangular canal[537]. In theHomopteroussection of this Order Ramdohr seems to have examined but few;Chermeshowever andAphisexhibit one remarkable feature; they haveno 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]. InCercopis spumariathe structure is more complex, and extremely singular. It hastwoor ratherthreestomachs; the two first of ahornysubstance, and the last a slender somewhat convolutedmembranoustube, 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 theLepidoptera: 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 theDiptera, which when blown up are of an oval form; the stomach, as in the bugs, consists oftwoportions, the first being the longest[541]. There are threefreebile-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 thelarva, 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 apupaeight 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 lateralappendage 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 thatAlmighty Beingwho 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 theDiptera. 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 orcœcumopening into it, inBombyliuscovered with shags, which though not connected with the mouth by a tube, Ramdohr regards as saliva-reservoirs[553]; inMusca vomitoriathe beginning of this organ below the mouth is covered with hemispherical prominences, and inTipulait is dilated and marked with transverse folds. There are usuallytwopairs of bile-vessels; in theMuscidæpedunculate andfree[554]; inTipula,Bombylius, andLeptis,sessile andunited[555]; and inTabanussessile andfixed[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; inMusca vomitoria, its length in theformeris two inches and a quarter, while in thelatterit 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 therectum, and has on each side two short club-shaped appendages, open at the end, which receivetracheæ, and terminate in a short piece that opens into the anus[558].
InHippoboscaand its affinities the canal in question differs from that of otherDiptera, 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 bysuction, from those thatmasticateit, is the faculty with which they are furnished by means of an ample crop, honey-stomach, or food-reservoir, ofregurgitatingthe food they may have stored up. Another distinction still more striking, which will appear more evidently hereafter, is to be seen in thesaliva-secretorswith which thesuctorioustribes are furnished, to be found in very fewmasticators, by which they are enabled to render the juices more fluid and fit for suction.
The only insect amongst theApterawhose alimentarycanal 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-threecœcaor 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 theCreatorin 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 theirexternalstructure, but likewise in theirinternalorganization; 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 thesamein theirprima facieappearance, they begin to exercisenewfunctions. An instance of this we have seen in the dorsal vessel of insects, which in theArachnida, 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 entertainvery different sentiments with regard to the structure of the alimentary organs of the Class we are now to enter upon, theArachnida: what some regard as a realliver, others look upon as anepiploonor caul; and what the last denominatebile-vessels are by some of the former considered as appropriated to the secretion ofchyle[564]. Yet both these opinions have some foundation in nature. When, in theArachnida, 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 theepiploonwhich 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 behepatic ducts, and this substance analogous to theliver, immediately strikes us as not improbable. Again: when we discover pairs of other capillary and tortuous vessels connecting with the intestinal canal either at thepylorus[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 theirfunctionmay be different.
Let us now consider how the intestinal canal is circumstanced in the two sections into which the ClassArachnidais divided; theScorpionidea, andAraneidea. 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; theduodenumshorter and thicker than the stomach, from which, as well as from therectum, 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 thebiliarysystem and its organs: Theliveris 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 polygonouslobuli; its interior is a tissue of infinitely minute glands: inScorpio occitanusthere are about forty pyramidallobulidetached 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 alimentarytube; in the above insect there are six pairs, three in the trunk and three in the abdomen, and inS. Europæusa 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 theAraneideaalso the alimentary canal is nearly straight, and scarcely exceeds the length of the body: thegulletis rather thick and cylindrical[575]; thestomachis 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 therectum, but which is so entangled with the liver, muscles, &c., as not to be easily made out[577]; therectumis rather tumid, and has a lateralcœcum[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 capillaryfreevessels are attached to the base of therectumon 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, inNecrophorusandSilpha,carnivorousinsects, the intestinal canal in its length and convolutions exceeds those of mostherbivorousones, and inCassida viridisand some others of thelattertribe are not longer than those of thepredaceousbeetles. Inherbivorous larvæalso, in general, the length of the alimentary canal does not exceed that of the body, but in those of someflesh-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 theexcrementsof 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 thecaterpillars ofLepidopterathey are of the former description, and every grain wears some resemblance to an insect's egg: as the passage in many of these consists ofsixfleshy parts separated by channels, so the excrement represents six little prisms separated by six channels[583]. TheAphidesall 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 ofChermes, 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 ofCercopis spumariaenvelopes itself[587].
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