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].