THE DIGESTIVE CANAL AND ITS APPENDAGES

Fig. 297.—Transverse section through an abdominal segment of larva of Megalopyge crispata, showing the relations of the digestive canal to the other organs: int, hind-intestine, with its mucous or epithelial layer (ep), and ml its outer or muscular layer; ng, ventral ganglion; ht, heart; mp, urinary tubes; f, fat-body; sc, thickened portion of the hypodermis (hy) containing the setigenous cells; m, muscles; m′, a pair of retractor muscles inserted near the base of the lateral glandular process (lgp); cut, cuticula; l, legs. Also compare Figs. 142–144 and 234.

Fig. 298.
The alimentary or digestive canal of insects is a more or less
straight tube, which connects the mouth and anus, the latter invariably
situated in the last segment of the body, under the last
tergite or suranal plate. It lies directly over the ventral nervous
cord and under the dorsal vessel, passing through the middle of the
body (Fig. 297). It is loosely held in place by delicate retractor
muscles (retractores ventriculi, found by Lyonet in the larvæ of
Lepidoptera, and occurring in those of Diptera), but is principally
supported by exceedingly numerous branches of the main tracheæ.
Fig. 298.—Internal anatomy of Melanoplus femur-rubrum: at, antenna and nerve leading
to it from the “brain” or supraœsophageal ganglion (sp); oc, ocelli, anterior and vertical ones,
with ocellar nerves leading to them from the brain; œ, œsophagus; m, mouth; lb, labium or under
lip; if, infraœsophageal ganglion, sending three pairs of nerves to the mandibles, maxillæ, and
labium respectively (not clearly shown in the engraving); sm, sympathetic or vagus nerve, starting
from a ganglion resting above the œsophagus, and connecting with another ganglion (sg) near the
hinder end of the crop; sal, salivary glands (the termination of the salivary duct not clearly
shown by the engraver); nv, nervous cord and ganglia; ov, ovary; ur, origin of urinary tubes;
ovt, oviduct; sb, sebaceous gland; bc, bursa copulatrix; ovt, site of opening of the oviduct (the
left oviduct cut away); 1–10, abdominal segments. The other organs labelled in full.—Drawn from
his original dissections by Mr. Edward Burgess.

Fig. 299.—Digestive canal of Anabrus: m, mouth: œ, œsophagus; sm, the sympathetic nerve passing along the crop; t, tongue; fg, frontal ganglion; br, brain, the nervous cord passing backward from it; sr, salivary reservoir; sg, salivary gland; pv, proventriculus; ur, origin of urinary tubes; sb, sebaceous gland; 1–10, the ten abdominal segments.—Burgess del.

It is in the higher adult insects differentiated into the mouth and pharynx, the œsophagus or gullet, supplementary to which is the crop (ingluvies) or “sucking stomach” of Lepidoptera, Diptera, and Hymenoptera; the proventriculus or gizzard; the ventriculus, “chyle-stomach,” or, more properly, mid-intestine, and the hind-intestine, which is divided into the ileum, or short intestine, the long intestine, often slender and coiled, with the colon and the rectum. Morphologically, however, the digestive or enteric canal is divided into three primary divisions, which are indicated in the embryo insect; i.e., the fore-intestine (stomodæum of the embryo), mid-intestine or “chyle-stomach,” and hind-intestine or proctodæum (Fig. 300). The three primary regions, with their differentiations, may be tabulated thus:—

Fore-intestine (Stomodæum).Mouth and pharynx.
Pumping apparatus of Hemiptera, Lepidoptera, and Diptera.
Œsophagus.
Crop or ingluvies, food reservoir, or “sucking stomach.”
Proventriculus.
Mid-intestine (Mesenteron).Mid-intestine, “chylific stomach,” or ventriculus (with cœcal glands).
Hind-intestine (Proctodæum).Ileum, or short intestine (with the urinary tubes).
Long intestine.
Colon.
Rectum (with rectal glands).
Anus (with anal glands).

Fig. 300.—The three primary divisions of the alimentary canal of an embryonic orthopterous insect: br, brain; sbg, subœsophageal ganglion; ng, nervous cord; st, stomodæum; pr, proctodæum; mv, malphigian tubes; mesen, mid-intestine; ht, heart; md, mandibles; mx, mx′, 1st and 2d maxillæ.—After Ayers, with some changes.

The appendages of the alimentary canal are: (1) the salivary and poison glands, which arise from the stomodæum in embryonic life; (2) while to the chylific stomach a single pair of cœcal appendages (Orthoptera and larval Diptera, e.g. Sciara), or many cœca may be appended; (3) the urinary tubes, also the rectal glands and the paired anal glands. In a Hemipter (Pyrrhocoris apterus) appendages arise from the intestine in front of the origin of the urinary tubes. In certain insects a single cœcal appendage (Nepa, Dyticus, Silpha, Necrophorus, and the Lepidoptera) arises from the proctodæum.

Fig. 301—Larva of honey-bee: g, brain; bm, ventral nervous cord; œ, œsophagus; sd, spinning-gland; cd, mid-intestine or chyle-stomach; ed, hind-intestine, not yet connected with the mid-intestine; vm, urinary tube; an, anus; st, stigmata.—After Leuckart, from Lang.

In certain larval insects, as those of the Proctotrypidæ (first larval stage), the higher Hymenoptera (ichneumons, ants, wasps, and bees, Fig. 301), in the Campodea-like larvæ of the Meloidæ and Stylopidæ, the larva of the ant-lion (Myrmecoleo), and those of Diptera pupipara (Melophagus), the embryonic condition of the separation of the proctodæum and mid-gut (mesenteron) persists, the stomach ending in a blind sac; in such cases the intestine, together with the urinary tubes, is entirely secretory.

The anus is wanting in the larva of the ant-lion, as also in the wasps (in which there is a rudimentary colon) and in freshly hatched bees, though it becomes perfectly formed in the fully grown larvæ (Newport, art. Insecta, p. 967, and H. Müller).

In the larvæ of lamellicorn Coleoptera (Melolontha vulgaris) the digestive tube is nearly as simple as in bees, though there is a large colon, which at its beginning forms an immense cœcum, and has also one anal aperture (Newport).

The length and shape of the digestive canal is dependent on the nature of the food and also on the mode of life, especially the ease or difficulty with which the food is digested.

Newport, while stating that the length of the alimentary canal in larvæ is not in general indicatory of the habits of the species, makes this qualification after describing the digestive canal of Calandra as compared with that of Calosoma: “The length and complication of the intestines, therefore, appear to have some reference to the quality of the food to be digested, since it is well known that the food of these latter insects (weevils) is of difficult assimilation, being as it is chiefly the hard ligneous fibres of vegetable matter; but they cannot be received as always indicatory of a carnivorous [or] vegetable feeder, since, as above remarked, the length of the canal is considerable in one entirely carnivorous larva, while it is much shorter in some herbivorous, and particularly in pollenivorous larvæ, as in the Melolontha and the apodal Hymenoptera.”

Fig. 302.—Digestive canal of a carabid beetle: b, œsophagus; c, crop; d, proventriculus; f, mid-intestine, or “chyle-stomach,” with its cœeca; g, posterior division of the stomach; i, the two pairs of urinary tubes; h, intestine; k, rectum; l, anal glands.—After Dufour, from Judeich and Nitsche.

Newport also contends that the length of the alimentary canal is not more indicative in the perfect insect of the carnivorous or phytophagous habits of the species than in the larva. It is nearly as long (being from two to three times the length of the whole body), and is more complicated, in the rapacious Carabidæ (Fig. 302) than in the honey-sipping Lepidoptera, whose food is entirely liquid. Referring to the digestive canal of Cicindelidæ, which is scarcely longer than the body, he claims that “we cannot admit that the length of the digestive organs, and the existence of a gizzard and gastric vessels, are indicatory of predacity of habits in the insect, because a similar conformation of parts exists often in strictly vegetable feeders. The existence and length of these parts seem rather to refer to the comparative digestibility of the food than to its animal or vegetable nature.” Newport then refers to the digestive canal of Forficulidæ (in which the gizzard is present, the canal, however, passing in an almost direct line through the body, making but one slight convolution), “a farther proof that the length of the canal must not be taken as a criterion whereby to judge of the habits of a species.” He adds this will apply equally well to the omnivorous Gryllidæ, in which there exists a short alimentary canal, but a gizzard of more complicated structure than that of the Dytiscidæ.

In larval insects and others (Synaptera, Orthoptera, etc.), in which the digestive canal is simplest, it is scarcely longer than the body, and passes through it as a straight tube.

In the caterpillar, which is a voracious and constant feeder, the digestive canal is a large straight tube, not clearly differentiated into fore-stomach, stomach, and intestine; but in the imago, which only takes a little liquid food, it is slender, delicate, and highly differentiated. In the larva the mid-gut forms the largest part of the canal; in the imago, the intestine becomes very long and coiled into numerous turns; at the same time the food-reservoir (the “sucking stomach”) develops, and the excretory tubes are longer.

a. The digestive canal

Fig. 303.—Interior view of the bottom of the head of Danais archippus, the top having been cut away, showing, in the middle, the pharyngeal sac with its five muscles: the frontal (f.m), dorsal pair (d.m), and the lateral pair (l.m); cl, clypeus; cor, cornea; œ, œsophagus; p.m, one of the large muscles which move the labial palp.—After Burgess.

It will greatly simplify our conception of the anatomy of the digestive canal if we take into account its mode of origin in the embryo, bearing in mind the fact that during the gastrula condition the ectoderm is invaginated at each pole to form the primitive mouth and fore-gut (stomodæum) and hind-gut (proctodæum). The cells of the ectoderm secrete a chitinous lining (intima), which forms the continuation of the outer chitinous crust, and thus the lining of each end of the digestive canal is cast whenever the insect molts; while the mid-intestine (mesenteron), arising independently of the rest of the canal much later in embryonic life from the mesoderm, is not the result of any invagination, being directly derived from the mesoderm, and is not lined with chitin.

The mouth, or oral cavity, and pharynx.—This is the beginning of the alimentary bounded above by the clypeus, and labrum, with the epipharynx, and below by the hypopharynx, or tongue, as well as the labium. Into it pour the secretion of the salivary glands, which passes out through an opening at the base of the tongue or hypopharynx. On each side of the mouth are the mandibles and first maxillæ.

The sucking or pharyngeal pump.—This organ has been observed by Graber in flies and Hemiptera, but the fullest account is that by Burgess, who was the first to discover it in Lepidoptera. In the milk-weed butterfly (Danais archippus) the canal traversing the proboscis opens into a pharynx enclosed in a muscular sac (Figs. 303, 304, and 310).

The pharyngeal sac, says Burgess, serves as a pumping organ to suck the liquid food through the proboscis and to force it backwards into the digestive canal.

Fig. 304.—Longitudinal section through the head of Danais, showing the interior of the left half: mx, left maxilla, whose canal leads into the pharynx; hph, floor of the latter, showing some of the taste-papillæ; oe, œsophagus; ep, epipharyngeal valve; sd, salivary duct; d.m, f.m, and cl, as in Fig. 302.—After Burgess.

Meinert (“Trophi Dipterorum”) has made elaborate dissections of the mouth and its armature, including the pharynx of several types of Diptera, with its musculature. He describes the pharynx as the principal, and in most Diptera, as the only part of the pump (antlia), and says: “By the muscles of the pump (musculis antliæ) the superior lamina of the pharynx is varied that the space between the two laminæ may be increased, and the liquid is thus led through the siphon formed by the mouth-parts into the mouth” (Fig. 81).

The œsophagus.—This is a simple tube, largest in those insects feeding on solid, usually vegetable, food, and smallest in those living on liquid food. It usually curves upwards and backwards, passing directly under the brain, and merges into the crop or proventriculus either at the back part of the head or in the thorax, its length being very variable. Its inner walls longitudinally are folded and lined with chitin.

According to Newport, in the œsophagus of the Gryllidæ, of the two layers of the mucous lining the second is distinctly glandular and secretory, and in it there are many thousands of very minute granular glandular bodies, which probably secrete the “molasses” or repellent fluid often ejected by these and other insects when captured.

The crop or ingluvies.—This, when present, is an enlargement of the end of the œsophagus, and lined internally with a muscular coat. It is very large in locusts (Fig. 298), Anabrus (Fig. 299), and other Orthoptera (the Phasmidæ excepted), in the Dermaptera, and most adult Coleoptera. A crop-like dilatation in front of a spherical gizzard is also present in the Synaptera (Poduridæ and Lepismidæ), as well as in the Mallophaga (Nirmidæ).

Fig. 305.—Digestive canal of Calandra: H, pear-shaped œsophagus; I, crop; K, gastric cœca L, ilium; MN, colon; P, urinary tubes.—After Newport.

Fig. 306.—Section of the crop (H), gizzard (I), and stomach (K) of Athalia.-After Newport.

Fig. 307.—Upper side of head and digestive canal of Myrmeleon larva: a, crop; b, “stomach”; c, free ends of two urinary tubes; c′, common origin of other six tubes; d, cœcum; e, spinneret; ff, muscles for protruding its sheath; gg, maxillary glands.—After Meinert, from Sharp.

In the larvæ of weevils (Calandra sommeri) there is a crop (Fig. 305), but not in the larva of Calosoma; also, according to Beauregard, in the pollen-eating beetles Zonitis, Sitaris, and Malabris it is wanting, while in Meloe it is highly developed (Kolbe).

The crop forms a lateral dilatation of the end of the œsophagus in the larvæ of weevils and of saw-flies (Athalia centifoliæ, Fig. 306).

The “sucking stomach” or food-reservoir.—This is a thin muscular pouch connected by a slender neck with the end of the œsophagus or the crop, when the latter is present. There is no such organ in Orthoptera, except in Gryllotalpa. It is wanting in the Odonata and in the Plectoptera (Ephemeridæ); in Platyptera (Perlidæ and Termitidæ), in Trichoptera, and in Mecoptera (Panorpidæ). In most adult Neuroptera (Myrmeleonidæ, Hemerobiidæ, and Sialidæ), but not in Rhaphidiidæ, the long œsophagus is dilated posteriorly into a kind of pouch or crop, and besides there is often a long “food-reservoir” arising on one of its sides, that of Myrmeleon (Fig. 307) and Hemerobius being on the right side.

Fig. 308.—Digestive canal of Sarcophaga carnaria: a, salivary gland; b, œsophagus; c, food reservoir; f-g, stomach; h, intestine; i, urinary tubes; k, rectum.—From Judeich and Nitsche.

A true food-reservoir is present in most Diptera (Fig. 308) as well as in the larvæ of the Muscidæ, but according to Dufour it is wanting in some Asilidæ and in Diptera pupipara, and according to Brauer in the Œstridæ. The food-reservoir in Diptera is always situated on the left side of the digestive canal; there is usually a long neck or canal, while the reservoir is either oval or more usually bilobed, and often each lobe is itself curiously lobed.

In Lepidoptera (Figs. 309, 310) the so-called “sucking stomach” is, as Graber has proved, simply a reservoir for the temporary reception of food; though generally found to contain nothing but air, Newport has observed that in flies it is filled with food after feeding. He has found this to be the case in the flesh fly, and in Eristalis he has found it “partially filled with yellow pollen from the flowers of the ragwort upon which the insect was captured,” the pollen grains also occurring in the canal leading to the bag, in the gullet, and in the stomach itself. Graber has further proved by feeding flies with a colored sweet fluid that this sac is only a food-receptacle. As he says: “It can be seen filling itself fuller and fuller with the colored fluid, the sac gradually distending until it occupies half the hind-body.”

The food-reservoir of the Hymenoptera is a lateral pouch at the end of the long, slender œsophagus, and has been seen in the bee to be filled with honey.

Fig. 309.—Digestive canal of Sphinx ligustri: h, œsophagus; i, rudiment of the gizzard; k, “stomach”; q, its pyloric end; t, food reservoir; p, urinary tubes; l, ilium; m, cœcum of colon; n, rectum; v, vent.—After Newport.

In the mole-cricket the hinder part of the crop is armed within with hook-like bristles directed backwards so as not to prevent the energetic pressure of the food backwards into the proventriculus, and to obviate the possibility of a regurgitation. (Eberli.)

The fore-stomach or proventriculus.—This is especially well developed in the Dermaptera, in the Orthopterous families Locustidæ, Gryllidæ, and Mantidæ, while in the Thysanura (Lepisma) there is a spherical gizzard provided with six teeth. It also occurs in many wood-boring insects, and in most carnivorous insects, notably the Carabidæ, Dyticidæ, Scolytidæ, in the Mecoptera (scorpion-flies), in the fleas, and in many kinds of ants, as well as Cynips, Leucospis, and Xyphidria. It is very muscular, lined within with chitin, which is usually provided with numerous teeth arising from the folds. These folds begin in the œsophagus or crop, and suddenly end where the mesenteron (“chylific stomach”) begins. It has been compared with the gizzard of birds, and is usually called by German authors the chewing or masticating stomach. (Kaumagen.)

The proventriculus is best developed in the Gryllidæ (Acrida viridissima), where the six folds at the end of the crop close together to form a valve between the crop and proventriculus. “They are each armed with five very minute hooked teeth; and, continued into the gizzard, develop many more in their course through that organ. These first teeth are arranged around the entrance to the gizzard, and seem designed to retain the insufficiently comminuted food and to pass it on to that organ.

Fig. 310.—Anatomy of Danais archippus after removal of right half of the body. Lettering of the head: a, antenna; ph, pharynx; pl, labial palpi; r, proboscis; g, brain; usg, subœsophageal ganglion. Lettering of the thorax: I. II. III. thoracic segments; b1, b2, b3, the coxal joints of the three pairs of legs; bm, muscles of the wings; ac cephalic aorta with its swelling; œ, œsophagus; bg, thoracic ganglia of the ventral cord; sd, salivary glands of one side, those of the other side cut off near their entrance into the common salivary duct. Lettering of the abdomen: 1–9. abdominal segments; h, heart; sm, so-called sucking-stomach (food-reservoir); cm, chyle-stomach; ag, abdominal ganglia: ed, hind intestine with colon (c) and rectum (r); rm, urinary vessels; ov, ovarial tubes, those of the right side cut off; ove, terminal filaments of the ovaries; bc, bursa copulatrix; obc, its outer aperture; od, oviduct; vag, vagina; wo, its outer aperture; ad, glandular appendages of the vagina partly cut away; vk, connective canal between the vagina and bursa copulatrix with swelling (receptaculum seminis); an, anus.—After Burgess, from Lang.

Fig. 311.—Transverse section of the proventriculus of Gryllus cinereus: muc, muscular walls; r, horny ridge between the large teeth (sp).—After Minot.

Fig. 312.—Transverse section of the proventriculus of the cockroach.—After Miall and Denny.

Fig. 313.—Digestive canal of the honey-bee: A, horizontal section of the body; lp, labial palpus; mx, maxilla; e, eye; pro. t, prothorax; mesa. t, mesothorax; meta. t, metathorax; dv, dorsal vessel; v, v, ventricles of the same; No. 1, No. 2, No. 3, salivary gland systems; œ œsophagus; g, g, ganglia of chief nerve-chain; n, nerves; hs, honey-sac; p, petaloid stopper or calyx of honey-sac or stomach-mouth; c. s, chyle stomach; bt, urinary tubes; si, small intestine (ilium); l, “lamellæ or gland-plates of colon,” rectal glands; li, rectum. B, cellular layer of stomach; gc, gastric cells, × 200. C, urinary tube; bc, cells; t, trachea. D, inner layer, with gastric teeth (gt).—After Cheshire.

“Next to these in succession on each of the longitudinal ridges are four flat, broad, somewhat quadrate teeth, each of which is very finely denticulated along its free margin. These extend about half-way through the gizzard. They appear to be alternately elevated and depressed during the action of the gizzard, and to serve to carry on the food to the twelve cutting teeth, with which each ridge is also armed, and which occupy the posterior part of the organ. These teeth are triangular, sharp-pointed, and directed posteriorly, and gradually decrease in size in succession from before backward. Each tooth is very strong, sharp-pointed, and of the color and consistence of tortoise shell, and is armed on each side by a smaller pointed tooth. These form the six longitudinal ridges of the gizzard, between each two of which there are two other rows of very minute teeth of a triangular form, somewhat resembling the larger one in structure, occupying the channels between the ridges. The muscular portion of the gizzard is equally interesting. It is not merely formed of transverse and longitudinal fibres, but sends from its inner surface into the cavity of each of the large teeth other minute but powerful muscles, a pair of which are inserted into each tooth. The number of teeth in the gizzard amounts to 270, which is the same number in these Gryllidæ as found formerly by Dr. Kidd in the mole-cricket. Of the different kinds of teeth there are as follows: 72 large treble teeth, 24 flat quadrate teeth, 30 small single-hooked teeth, and 12 rows of small triangular teeth, each row being formed of 12 teeth. This is the complicated gizzard of the higher Orthoptera.” (Newport.)

In the more generalized cockroach, there are six principal folds, the so-called teeth, which project so far inwards as to nearly meet (Fig. 312). The entire apparatus of muscles and teeth is, as Miall and Denny state, “an elaborate machine for squeezing and straining the food, and recalls the gastric mill and pyloric strainer of the crayfish. The powerful annular muscles approximate the teeth and folds, closing the passage, while small longitudinal muscles, which can be traced from the chitinous teeth to the cushions, appear to retract these last, and open a passage for the food.”

As in the fore-stomach or proventriculus of the lobster, the solid, rounded teeth do not appear to triturate the solid fragments found in the organ, but act rather as a pyloric strainer to keep such bodies out of the chylific stomach. We accept the view of Plateau that this section of the digestive canal in insects, which he compares to the psalterium of a ruminant, is a strainer rather than a masticatory stomach, and both Forel and Emery, as well as Cheshire, take this view.

The proventriculus of the honey-bee (Fig. 313, hs) is called by apiarians the “honey-sac” or “honey-stomach.” Cheshire states that if it be carefully removed from a freshly killed bee, its calyx-like “stomach-mouth” may be seen to gape open and shut with a rapid snapping movement. The entrance to the stomach is guarded by four valves, each of which is strongly chitinous within, and fringed along its edge with downward-pointing fine stiff bristles. By the contraction of the longitudinal muscles (lm), the valves open to allow the passage of food from the honey-sac to the “chyle-stomach.” It is closed at will by circular muscles (tm). Then the bee can carry food for a week’s necessities, either using it rapidly in the production of wax, or eking it out if the weather is unfavorable for the gathering of a new store.

Fig. 314.—“Honey-sac stopper,” “stomach-mouth,” or calyx-bell of honey-bee, × 50. A, front view of one of the lobes of the calyx-bell; l, lip-like point, covered by down-turned bristles (b); sm, side membrane. B, longitudinal section of the stomach-mouth, with continuations into entrance of chyle-stomach; l, l, lip-like ends of leaflets; s, setæ; lm, longitudinal muscles; tm, transverse muscles in cross-section; cl, cell-layer of honey-sac; LM, TM, longitudinal and transverse muscles of same; nc, nucleated cells of tubular extension of stomach-mouth into chyle-stomach; lm′, tm′, longitudinal and transverse muscles of chyle stomach; c, c, cells covered within by an intima. C, cross-section of stomach-mouth; m, cross-section of muscles seen at lm in B; tm, transverse muscles surrounding stomach-mouth. D, cross-section through small intestine; a and m, longitudinal and surrounding muscles.—After Cheshire.

Cheshire also shows that when bees suck up from composite and other flowers nectar together with much pollen, the outside wrinkled membrane (sm, A, Fig. 314) “is seen to continually run up in folds, and gather itself over the top of the stomach-mouth, bringing with it, by the aid of its setæ, the large pollen-grains the nectar contains.” The lips (l, l, B, Fig. 314), now opening, take in this pollen, which is driven forwards into the cavity made between the separating lips by an inflow of the fluid surrounding the granules. The lips in turn close, but the down-pointing bristles are thrown outwards from the face of the leaflet, in this way revealing their special function, as the pollen is prevented from receding while the nectar passes back into the honey-sac, strained through between the bristles aforesaid, the last parts escaping by the loop-like openings seen in the corners of C, Fig. 314. The whole process is immediately and very rapidly repeated, so that the pollen collects and the honey is cleared. “Three purposes, in addition to those previously enumerated, are thus subserved by this wondrous mechanism. First, the bee can either eat or drink from the mixed diet she carries, gulping down the pollen in pellets, or swallowing the nectar as her necessities demand. Second, when the collected pollen is driven forwards into the chyle-stomach, the tube extension, whose necessity now becomes apparent, prevents the pellets forming into plug-like masses just below p, Fig. 313, for, by the action of the tube, these pellets are delivered into the midst of the fluids of the stomach, to be at once broken up and subjected to the digestive process. And third, while the little gatherer is flying from flower to flower, her stomach-mouth is busy in separating pollen from nectar, so that the latter may be less liable to fermentation and better suited to winter consumption. She, in fact, carries with her, and at once puts into operation, the most ancient, and yet the most perfect and beautiful, of all ‘honey-strainers.’”

Forel’s experiments on the proventriculus of ants prove that through its valvular contrivance it closes the passage from the crop to the mid-intestine (“chylific stomach”), and allows the contents of the former to pass slowly and very gradually into the latter. Emery confirms this view, and concludes that the organ in the Camponotidæ and in the Dolichoderidæ provided with a calyx-bell, usually regarded as a triturating stomach (Kaumagen), but more correctly as a pumping stomach, consists of parts which perform two different functions. Under the operation of the muscles of the crop the entrance to the pumping stomach becomes closed, in order by such spasmodic contraction to prevent the flow of the contents of the crop into the proventriculus. By the pressure of the transverse muscles of the proventriculus its contents are emptied into the mid-intestine, while simultaneously a regurgitation into the crop is prevented. In the Dolichoderidæ and Plagiolepidinæ the closure in both cases is effected by the valves. In the true Camponotidæ there are two separate contrivances for closing; the calyx belonging to the crop-musculature, while the valves essentially belong to the proventricular pumping apparatus.

Opinions vary as to the use of this portion of the digestive canal. Graber compares it to the gizzard of birds, and likens the action of the rosette of teeth to the finer radiating teeth of the sea-urchin, and styles it a chopping machine, which works automatically, and allows no solid bits of food to pass in to injure the delicate walls of the stomach (mid-gut).

He also states that the food when taken from the proventriculus is very finely divided, while that found in the œsophagus contains large bits.

Kolbe says that this view has recently been completely abandoned, and that the teeth are used to pass the food backwards into the chylific stomach. “But Goldfuss had denied the triturating action of the proventriculus of the Orthoptera (Symbolæ ad Orthopterorum quorundam Œconomiam, 1843), stating that the contents of the same are already fluid in the gullet, so that the fore-stomach (Kaumagen) does not need to comminute the food” (Kolbe). In the Gryllidæ and Locustidæ, just before the posterior opening of the proventriculus into the stomach the chitinous lining swells into a ring and projects straight back as the inner wall of the cylindrical chylific stomach. The muscular layer forms two sac-like outgrowths or folds, which separate on the circular fold from the chitinous membrane. This apparatus only allows very finely comminuted food to pass into the stomach.

In the Acrydiidæ (Eremobia muricata) at the end of the proventriculus, where it passes into the stomach, is a small circular fold which hangs down like a curtain in the stomach.

The œsophageal valve.—Weismann[[50]] states that the origin of the proventriculus in the embryo of flies (Muscidæ) shows that it should be regarded as an intussusception of the œsophagus. While in the embryo the invaginated portion of the œsophagus is short, after the hatching of the larva it projects backwards into the mid-intestine. Kowalevsky also observed in a young muscid larva, 2.2 mm. in length, that the œsophagus, shaped like a tube, extends back into the expanded portion (proventriculus) and opens into the stomach (Fig. 315, A). In a larva 10 mm. long the funnel is shorter, the end being situated in the proventriculus (Fig. 315, B, pr). In the cavity between the outer (o) and inner wall (i) no food enters, and the use of this whole apparatus seems to be to prevent the larger bits of food from passing into the chylific stomach (Kowalevsky).

Fig. 315.—Œsophageal valve of young muscid larva: m, its opening: t, thickening of the cells; mes, mesoderm.—After Kowalevsky.

Beauregard has found a similar structure in the Meloidæ, and calls it the “cardiac valvule” (Fig. 318, Kl). It was observed by Mingazzini in the larvæ of phytophagic lamellicorn beetles, and Balbiani described it in a myriopod (Cryptops) under the name of the “œsophageal valvule.”

Gehuchten describes a homologous but more complicated structure in a tipulid larva (Ptychoptera contaminata), but differing in containing blood-cavities, as a tubular prolongation of the posterior end of the œsophagus which passes through the proventriculus and opens at various positions in the anterior part of the chylific stomach (Fig. 316).

The three layers composing this funnel are distant from each other and separated by blood-cavities, the whole forming “an immense blood-cavity extended between the epithelial proventricular lining and the muscular coat.”

According to Schneider the longitudinal muscular fibres of the fore and hind gut in insects pass into the stomach (mid-gut). The anterior part of the fore-gut has generally only circular fibres. When, however, the longitudinal fibres arise behind the middle, then they separate from the digestive canal and are inserted a little behind the beginning of the chylific stomach. Hence there is formed an invagination of the proventriculus, which projects into the cavity of the stomach.

Schneider describes this process, which he calls the “beak,” as an invagination of the fore-stomach which projects into the cavity of the stomach. The two layers of the invagination in growing together form a beak varying in shape, being either simple or lobed and armed with bristles or teeth. This beak is tolerably large in Lepisma, Dermaptera (Forficula), Orthoptera, and in the larvæ and adults of Diptera, but smaller in the Neuroptera and Coleoptera, while in other insects it is wanting.

Proventricular valvule.—Gehuchten also describes in Ptychoptera what he calls “the proventricular valvule,” stating that it is “a circular fold of the intestinal wall” (Fig. 310, vpr). He claims that it has not before been found, the “proventricular beak” of Schneider being regarded by him as the œsophageal valvule.

Fig. 316.—Digestive canal of Ptychoptera contaminata: gs, salivary glands; ra, œsophagus; pr, proventriculus; gt, crown of eight small tubular glands; im, mid-intestine; ga, two accessory white glands; vm, urinary vessels; ig, small intestine; gi, large intestine; r, rectum; A, the proventriculus in which the hinder end of the œsophagus extends as far as the chyle-stomach. B, longitudinal section of the proventricular region; sph, muscular ring or œsophageal sphincter; ppr, wall of the proventriculus; e, circular constriction dividing the cavity of the proventriculus in two; vpr, circular fold of the wall of the mid-intestine forming the proventricular valve; vœ, œsophageal valve.—After Gehuchten.

The peritrophic membrane.—This membrane appears first to have been noticed by Ramdohr in 1811 in Hemerobius perla. It has been found by Schneider, who calls it the “funnel.” On the hinder end of the fore-stomach, he says, the cuticula forms a fold enclosing the outlet of the fore-stomach, and extending back like a tube to the anus. This “funnel,” he adds, occurs in a great number of insects. It has been found in Thysanura, but is wanting in Hemiptera. In the Coleoptera it is absent in Carabidæ and Dyticidæ. It is generally present in Diptera and in the larvæ of Lepidoptera, but not in the adults. In Hymenoptera it has been found in ants and wasps, but is absent in Cynipidæ, Ichneumonidæ, and Tenthredinidæ. All those insects (including their larvæ) possessing this funnel eat solid, indigestible food, while those which do not possess it take fluid nourishment. It is elastic, and firmly encloses the contents of the digestive tract. Until Schneider’s discovery of its general occurrence, it had only been known to exist in the viviparous Cecidomyia larvæ (Miastor). Wagner, its discoverer, noticed in the stomach of this insect a second tube which contained food. Pagenstecher was inclined to regard the tube as a secretion of the salivary glands. Metschnikoff, however, more correctly stated that the tube consisted of chitin, but he regarded it as adapted for the removal of the secretions. (Schneider.) Plateau, however, as well as Balbiani, the latter calling it the “peritropic membrane,” considers this membrane as a secretion of the chylific stomach, and that it is formed at the surface of the epithelial cells. It surrounds the food along the entire digestive tract, forming an envelope around the fæcal masses. On the other hand, Gehuchten states that in the larva of Ptychoptera its mode of origin differs from that described by Plateau and by Schneider, and that it is a product of secretion of special cells in the proventriculus.

The mid-intestine.—This section of the digestive canal, often, though erroneously, called the “chylific stomach” or ventriculus, differs not only in its embryonic history, but also in its structure and physiology from the fore and hind intestine of arthropods, and also presents no analogy to the stomach of the vertebrate animals. In insects it is a simple tube, not usually lined with chitin, since it is not formed by the invagination of the ectoderm, as are the fore and hind intestine, the absence of the chitinous intima promoting the absorption of soluble food. Into the anterior end either open two or more large cœcal tubes (Fig. 299), or its whole outer surface is beset with very numerous fine glandular filaments like villi (Fig. 317 and Fig. 329).

The mid-intestine varies much in size and shape; it is very long in the lamellicorn beetles (Melolontha and Geotrupes), and while in Meloë it is very large, occupying the greatest part of the body-cavity, in the longicorn beetles and in Lepidoptera it is very small. The pyloric end consists of an internal circular fold projecting into the cavity. In the Psocidæ (Cæcilius) the pyloric end is prolonged into a slender tube nearly as long as the larger anterior portion.

The limits between the mid and hind intestine are in some insects difficult to define, the urinary tubes sometimes appearing to open into the end of the mid-intestine (“stomach”). The latter also is sometimes lined with an intima. The limits are also determined by a circular projection, directly behind which is an enlargement of the intestine in the shape of a trench (rigole), or circular cul-de-sac (the “pyloric valvule” of some authors, including Beauregard), while the walls of the small intestine contract so as to produce a considerable constriction of the cavity of the canal. This constriction exactly coincides with the beginning of the double layer of circular muscles in the wall of the small intestine. An internal layer, which is the continuation of the circular muscles of the chylific stomach, and an external layer much more developed probably belong to this part of the alimentary canal. Since the homologue of the circular fold occurs in the locust as well as in Diptera, it is probably common to insects in general.

Fig. 317.—Digestive canal of Carabus monilis: h, œsophagus; i, gizzard or proventriculus; k, “stomach,” with its cœca (r); p, urinary tubes; q, their point of insertion; m, n, colon, with cœcal glands; s, anal glands; a, b, c, a gastric cœcum; a, b, portion of lining of gizzard.—After Newport.

Fig. 318.—Digestive canal of Meloe: sch, œsophagus; Kl, œsophageal valve; mD, mid-intestine; eD, hind-intestine; Ei, eggs; g, sexual opening.—After Graber.

Gehuchten adds that the limit set by the circular projection does not exactly coincide with the opening into the intestine of the urinary tubes and the two annexed glands. He shows by a section (his Fig. 133) that the tubular glands open into the alimentary canal in front of the circular fold. It is the same with the Malpighian tubes. They are not, therefore, he claims, dependences of the terminal intestine, but of the mid-intestine. Beauregard has observed the same thing in the vesicating insects (Meloidæ). The Malpighian tubes, he says, open into the “chylific stomach” before the valvular crown. This arrangement does not seem to be general, because, according to Balbiani, the Malpighian vessels open into the beginning of the intestine in Cryptops. Compare also Minot’s account of the valve in locusts separating the stomach from the intestine, and in front of which the urinary or Malpighian tubes open.

Histology of the mid-intestine.—The walls of the stomach are composed of an internal epithelium, a layer of connective tissue, with two muscular layers, the inner of which is formed of unstriated circular muscular fibres, and the outer of striated longitudinal muscular fibres.

In the cockroach short processes are given off from the free ends of the epithelial cells, as in the intestine of many mammals and other animals. “Between the cells a reticulum is often to be seen, especially where the cells have burst; it extends between and among all the elements of the mucous lining, and probably serves, like the very similar structure met with in mammalian intestines, to absorb and conduct some of the products of digestion.” (Miall and Denny.)

Gehuchten shows that the epithelial lining of the mesenteron (chylific stomach) of the dipterous larva Ptychoptera is composed of two kinds of cells, i.e. secreting or glandular cells and absorbent cells, the former situated at each end of the stomach, and the absorbent cells occupying the middle region. The part played by these cells in digestion will be treated of beyond in the section on digestion. (See p. 327.)

The hind-intestine.—In many insects this is divided into the ileum, or short intestine, and the long intestine. The limit between the intestine and stomach is externally determined by the origin of the urinary tubes, which are outgrowths of the anterior end of the proctodæum. Like the fore-intestine the hind-intestine is lined with a thick muscular layer, and, as Gehuchten states, the passage from the epithelial lining of the stomach (mid-intestine) to the muscular lining of the intestine is abrupt.

Large intestine.—In Ptychoptera, as described by Gehuchten, there are no precise limits between the small and large intestine; the epithelium of the large intestine has a special character, and its constituents present a close resemblance to the absorbed cells of the chylific stomach, being like them large and polygonal. The muscular layer is not continuous, and is formed of longitudinal and circular fibres, the latter being the larger.

The ileum—Though in most insects slender, and therefore called the small intestine, the ileum is in locusts (Fig. 298) and grasshoppers (Anabrus, Fig. 299) as thick as the stomach. In many carnivorous beetles (Dyticus, Fig. 320, il, and Necrophorus) it is very long, but rather slender and short in the Carabidæ and Cicindelidæ, as well as those insects whose food is liquid, such as Diptera. In the Lepidoptera it varies in length, being in Sphinx quite long and bent into seven folds (Fig. 309), while it is very short in the Psocidæ, Chrysomelidæ, and Tenthredinidæ.

In the locust the ileum is traversed by six longitudinal folds with intervening furrows; outside of each furrow is a longitudinal muscular band. Seen from the inner surface the epithelium has an unusual character, the cells in the middle of each of the flat folds being quite large, polygonal in outline, while towards the furrows the cells become very much smaller. The walls are double when seen in transverse section, the inner layer consisting of epithelial cells resting on connective tissue, the outer layer formed of circular muscles. The cuticula is thin, but probably chitinous; it resembles that on the gastro-ileal folds, except that there are no spinules, but unlike the cuticula of the stomach it extends equally over the folds and the furrows. (Minot.) In the cockroach the junction of the small intestine with the colon is abrupt, a well-developed annular fold assuming the nature of a circular valve. (Miall and Denny.)

The gastro-ileal folds.—In the locust the intestine is separated from the chylific stomach by what Minot calls “the gastro-ileal folds,” which form a peculiar valve. The urinary vessels open just underneath and in front of this valve. In Melanoplus, and probably in the entire family of Acrydiidæ, they are indicated as “dark spots, round in front and lying at the anterior end of the ileum so as to form a ring around the interior of the intestine.” They are 12 in number, and all alike. They are pigmented and round in front where they are broadest and stand up highest; they narrow down backwards, the pigment disappears, and they gradually fade out into the ileal folds. Directly beneath them, and just at the posterior end of the stomach, there is a strong band of circular striated muscular fibres. The epithelium of these folds is covered with minute conical spines, which are generally, but not always, wanting between the folds. (Minot.)

The colon.—This section of the intestine (Fig. 319) is sometimes regarded as a part of the rectum. In the locust the six longitudinal folds of the ileum are continued into the colon, but their surface, instead of being smooth as in the ileum, is thrown up into numerous irregular curved and zigzag secondary folds. The cells of the epithelium are of uniform size, and the layer is covered by a highly refringent cuticula without spines; and, like that in the ileum, it rests on a layer of connective tissue, beyond which follows (1) an internal coat of longitudinal, and (2) an external coat of circular striated muscular fibres. (Minot.)

Fig. 319.—Digestive canal of Lucanus cervus: G, anterior muscles of the pharynx; H, œsophagus; I, gizzard; K, chyle-stomach; L, ilium; M, colon (cœcal part of); N, colon; O, rectum; a, frontal ganglion of the vagus; b, vagus nerve; c, anterior lateral ganglion connected with the vagus.—After Newport.

In butterflies (Pontia brassicæ), in Sphinx ligustri, and probably in most Lepidoptera the colon is distinct from the rectum, and is anteriorly developed into a very large more or less pyriform or bladder-like cæcum (Figs. 309, 310), which in certain Coleoptera (Dyticus, Fig. 320, d; Silpha, Necrophorus, etc.) is of remarkable length and shape; it also occurs in Nepidæ (Fig. 327). In the cockroach a lateral cæcum “is occasionally, but not constantly, present towards its rectal end,” and a constriction divides the colon from the rectum. (Miall and Denny.)

The rectum.—The terminal section of the hind-gut varies in length and size, but is usually larger than the colon, and with thick, muscular walls. In Lepidoptera it is narrow and short.

The rectum is remarkable for containing structures called rectal glands (Fig. 298). Chun describes those of Locusta viridissima as six flat folds, formed by a high columnar epithelium and a distinct cuticula; there is a coat of circular bands corresponding to the furrows between the glands. Minot states that this description is applicable to the locusts (Acrydiidæ) he has investigated, the only difference being in the structural details of the single layers. He claims that the rectal folds “do not offer the least appearance of glandular structure,” neither is their function an absorbent one, as Chun supposed. From their structure and position Fernald regards the rectal glands of Passalus as acting like a valve, serving to retain the food in the absorptive portions of the digestive tract till all nutriment is extracted.

Fig. 320.—Dyticus marginalis, ♂ opened from the back: a, crop; b, proventriculus; c, mid-intestine beset with fine cœcal glands; d, long cœcal appendage of the colon; apodemes; B1-B3, apodemes; vhm, coxal extensor muscle, moving the hind leg; ho, testis; dr, accessory gland; r, penis; e, reservoir of the secretion of the anal gland.—After Graber.

The epithelial folds of the larvæ of dragon-flies serve as organs of respiration, the water being admitted into this cavity, and when forcibly expelled serving to propel the creature forward. Paired and single anal glands (repugnatorial) enter the rectum of certain Coleoptera (Figs. 302, l; 317, s; 320, e).

The vent (anus).—The external opening of the rectum is situated in the end of the body, in the vestigial 10th or 11th abdominal segment, and is more or less eversible. It is protected above in caterpillars, and other insects with 10 free abdominal segments, by the suranal plate. It is bounded on the sides by the paranal lobes, while beneath is the infra-anal lobe.

The anus is wanting in certain insects, and where this is the case the hind-gut, owing to a retention of the embryonic condition, is usually separated from the mid-intestine. (See p. 300.)

Fig. 321.—Enteric canal of Psyllopsis fraxinicola: œ, œsophagus; md, mid-intestine; ed, hind-intestine; vm, urinary vessels; s, the coil formed by the hind-intestine and the most anterior part of the mid-intestine.—After Witlaczil, from Lang.

Some remarkable features of the digestive canal in hemipterous insects are noteworthy. In the Coccidæ, according to Mark, the anterior end of the long mid-intestine forms, with the hinder end of the œsophagus, a small loop, whose posterior end is firmly grown to the wall of the rectum, and forms a cup-like invagination of the latter. Then the rest of the tube-like stomach turns sidewise and forms a large loop, which turns back on itself and occupies a large part of the body-cavity. This loop receives on the anterior end, near the œsophagus, the two urinary vessels, and forms just below the opening into the rectum a short cæcum.

In other homopterous genera (Psyllidæ and some Cicadidæ) Witlaczil describes nearly the same peculiarity, the mid-gut and part of the intestine forming a loop growing together for a certain distance and winding round each other (Fig. 321).

Histology of the digestive canal.—In all the divisions of the digestive canal of insects the succession of the cellular layers composing it is the same: 1st, a cuticula; 2d, an epithelium; 3d, connective tissue; 4th, muscular tissue. In the locust, the first division of the canal (fore-gut), there are two muscular coats, an internal longitudinal and an external circular coat; the fibres are all striated. The lining epithelium is not much developed, but forms a thick, hard, and refringent cuticula, which is thrown up into spiny ridges. In the second division (mid-gut, “stomach”) the epithelium is composed of very high columnar cells, which make up the greater part of the thickness of the walls, while the cuticula is very delicate, slightly refringent, with no ridges, and is probably not chitinous; the fibres of the muscular coats are not striated, while this division is also distinguished by the presence of glandular follicles and folds. The stomach and the cæcal appendages have all these peculiarities in common, while no other part of the canal is thus characterized.

The third division (intestine and rectum) is composed of an epithelium, the cells of which are intermediate in size between those of the fore and mid gut. The cells are often pigmented, and they are covered by a much thicker cuticula than that of the stomach, but which is not so thick and hard as that of the œsophagus and proventriculus. The very refringent cuticula is not thrown up into ridges, though in some parts it is covered with delicate conical spines, which are very short. “The epithelium and underlying connective tissue (tunica propria) are thrown up into six folds, which run longitudinally, being regular in the ileum and rectum (as the rectal glands), but very irregular in the colon. Outside the depression between each two neighboring folds there is a longitudinal muscular band, these making six bands. This peculiar disposition of the longitudinal muscles does not occur in any other part of the canal; it is, therefore, especially characteristic of the third division.” (Minot.)