Ant colonies > Internal structure of ants > Alimentary tract

Alimentary tract

The Alimentary Tract

This extends the entire length of the body from the mouth to the anus as a tube with but a slight tendency to convolution in the gaster. The walls of this tube are curiously modified in different portions of its length, so that we can recognize a number of regions known as the infrabuccal chamber, buccal tube, pharynx, oesoph­agus, crop, proventriculus, stomach, small intestine and rectum. The shape and extent of these regions are indicated in the accompanying diagram taken from Janet (Fig. 13).

Owing to the volume of the brain and cephalic glands, to the narrowness of the thorax and pedicel in the worker, and the great development of the wing muscles and glands in

FIG. 13. Sagittal section of worker Myrmica rubra. (Janet,) t, Tongue; lbr, labrum ; clp, clypeus ; sg, opening of salivary gland; bo, mouth opening; hp, inf ra­buccal chamber; ph, pharynx; phg, pharyngeal glands; oe, oesophagus; cr, crop; gz, gizzard; st, stomach; lin, large intestine; ynp, Malpighian vessels; rc, rectum; rcg, rectal gland; an, anus; f gl, frontal ganglion; rec, recurrent nerve; br, brain; mdg, mandibular ganglion; mxg, maxillary ganglion; lg, labial ganglion; soe, suboesophageal ganglion; cho, prothoracic chordotonal organ; thg1, thg2, thg3, pro-meso- and meta­thoracic ganglia; ag'-ag=, ag, 8-11, first to eleventh abdominal ganglia; sym, sympa­thetic connective, running along oesophagus to prestomachal ganglion (stg) ; st, sting; vg, vagina; ten, tentorium in section.

the thorax of the male and female, the alimentary canal is cramped for space and hence very tenuous, except in the gaster, where its most important parts are situated. The mouth openin, which, as we have seen, is bounded above by the labrum and ciypeus, on the sides by the maxillae, and below by the protrusible tongue, leads into a short, com­pressed buccal tube, dilated ventrally to form a spheroidal sac, the inf ra­buccal cavity or chamber (hp). This chamber is of great importance to the ant as a receptacle both for the fine particles of solid and semi­solid food rasped off or licked up by the tongue, and for the foreign matter scraped from the surfaces of the body by this organ and the strigils. Any juices that may be contained in the substance are sucked back through the pharynx into the crop and the useless solid residuum is eventually thrown out as a little body which preserves the form of the chamber in which it was moulded. Such bodies, called by Janet "corpuscles de nettoyage," are often seen scattered about the floors of artificial nests after the ants have been fed on starchy substances or after their bodies have been dusted with plaster of paris (Fig. 14). The short buccal cavity is continued back into the muscular pharynx which narrows still further to form long oesophagus traversing as a slender tube the head, thorax and pedicel (Fig. 13, oe ) .

FIG. 14. Pellets or castings from the in­ frabuccal chamber of Formica rufa, enlarged (Janet.)

The buccal tube, which, according to Janet, " has a protractor and a retractor muscle, is provided with soft lips that can be applied to the surface of the substances previously rasped off by means of the tongue for the purpose of obtaining any liquid they may contain. Transverse scale-like folds with their points turned outward line the walls of the buccal tube and serve to retain any solid particles not sufficiently minute."
" The pharynx is a flattened cavity the dorsal and ventral walls of which are moved by powerful dilator muscles. Behind it is furnished with two expansions arising laterally and united at their tips by a transverse constrictor muscle. During aspiration the pharynx, through the action of its dilators and a kind of posterior sphincter, opens in front and closes behind. In swallowing there is first produced a steel-­yard-like movement of the dorsal wall, whereupon the pharynx is opened behind, while the buccal tube is closed in front. Then, owing to the action of the transverse constrictor, the dorsal approaches the ventral wall from before backward. The two walls thus come in con­tact with each other and the liquid which was contained in the pharynx is pushed into the oesophagus." Immediately behind the pharynx two groups of finger-shaped post-pharyngeal glands open by a pair of orifices into the alimentary tract (Fig. 13, phg ) .

The thin chitinous lining of the oesophagus is covered with delicate hairs which point backwards. At the base of the gaster the oesophagus begins to dilate to form the ingluvies, or crop (Fig. 15, cr), a thin-walled, pyriform bag, whose walls, like those of the oesophagus, consist of a layer of longitudinal and one of transverse or ring-shaped muscle fibers and a delicate chitinous lining. In the oesophagus the chitinous lining is beset with fine hairs pointing backwards. There are no glands in. the crop and the chitinous walls completely resist the absorption of food, so that this organ serves merely as a reservoir for the liquid that has been imbibed or lapped up directly or sucked out of the more solid substances moulded in the infrabuccal chamber.

FIG. 15. Gaster of female Myrmica rubra in sagittal section. (Janet.) ppt, Post­petiole ; str, stridulatory organ ; gs'-gse, first to sixth gastric segments ; ht, heart ; v, cardiac valve; pc, pericardial cells; u, urate cell; f, adipocyte ; on, oenocyte ; ot, ovarian tubules; od, oviduct ; ut, uterus ; rs, receptaculum seminis ; bc, bursa copu­latrix ; vg, vagina; vv, vulva ; st, stylets of sting; gt, gorgeret ; pg, poison gland; ag, accessory gland. Remaining letters as in Fig. 13.

Forel aptly calls the crop "the social stomach," because the food it contains is at least in great part fed by regurgitation to the other ants of the colony or to the brood. The crop is remarkably distensible, especially in certain Camponotinae, like the honey-ants, so that its replete or deplete condition determines the volume, and in a measure also the shape of the gaster in the worker.
The crop is succeeded by a remarkable structure, the proventriculus, or pumping stomach, which has been carefully studied by Forel (1878b)
and Emery (1888c), who have found it to vary greatly and to afford valuable characters for the delimitation of genera and even of sub­families. The proventriculus of our common carpenter ants (Cam­ponotus) may be described as a paradigm (Fig. 16, A ) . It is a narrowed or constricted portion of the alimentary tract and consists of several successive sections. The most anterior of these is the calyx (c). As the name implies, this is a cup-shaped section with chitinous walls dif­ferentiated into eight bands, four greatly thickened, and very convex towards the lumen, alternating with four thinner chitinous bands which are more or less concave -towards the lumen. The thickened bands have been called the sepals. At the posterior narrow end of the calyx these can be applied so closely to one another as to shut off the lumen and thus assume the function of a valve at this point.

FIG. 16. The gizzard, or proventriculus, of various ants. (Emery.) A, Cam¢o­notus ligniperdus; B, Livmetopum microcephalum; C, Atta sexdens; D, Cry¢tocerus atratus; E, Technomyrmex strenuus, seen from the anterior end; F, sagittal section of same; a, (esophagus; b, crop; c, sepal; d, membrane between sepals; e, valve; f, bulb of calyx (pumping stomach proper) ; g, cavity of bulb; h, cylindrical portion; ti, knob-shaped valve; k, stomach, or ventriculus.

Posterior to this valve, the walls of the organ again dilate suddenly to form a globose section, the bulb (f ), which repeats the structure of the calyx with some modification. This is the pumping stomach proper. It is succeeded by a slender, thin-walled tube, the cylindrical section ( h ), opening behind into the much more voluminous stomach on the summit of a knob, which is also valvular in structure (i). At this point the chitinous lining of the alimentary tract stops abruptly. The walls of the proventriculus, especially of its bulb, are furnished with powerful transverse and feebler longitudinal muscles.

The function of the proventriculus as a pump has been explained by Emery. It is clear from the shape of the chitinous folds in the bulb and the arrangement of the musculature that the contraction of the latter must bring the folds close together and occlude the lumen, whereas the relaxation of the muscles permits the chitinous folds to flatten out through their own elasticity and thus enlarge the cavity and suck the liquid back out of the crop. Hence the organ functions like a rubber bulb with a tube and an appropriately constructed valve at each end. When the bulb is squeezed its liquid contents are forced into one tube, and when it is permitted to expand, it draws the liquid out of the other tube. The proventriculus has an important function, not only in passing the liquid food back from the crop to the true stomach, but also in filling the crop in the first place.

The proventriculus of Camponotus may be regarded as representing a structure from which we can pass on the one hand through greater simplification to the Myrmicine and Ponerine proventriculus, and on the other through greater complication to that of the other Campon.otinae (Plagiolepis, Pyenolepis, etc.) and Dolichoderinae. This complication consists, in great part, in a shortening of the calyx and a spreading and recurving of its lips till they form a bell-shaped structure more or less completely enclosing the remainder of the proventricultls. Extreme forms of this kind are seen in lyidomyrmex and Technoohyyniex (Fig. 16, E, F). In these ants it is possible to. see how the proventriculus may play an important role in regurgitation as well as in ingurgitation, for the contraction of the walls of the crop, especially of the ring­muscles at the posterior end, and the pressure of its liquid contents must tend to close the openings between the sepals, thus preventing the liquid from moving backward and determining its flow in the oppo­site direction. As the musculature of the crop is poorly developed, some authors, like Janet, regard the pharynx as the organ which by its peristaltic contractions probably initiates regurgitation and may even be of great importance in filling the crop during ingurgitation.

All of the above-described regions of the alimentary tract arise in the embryo as a tubular infolding of the outside skin, or ectoderm, the so-called Stomodaeum. This is indicated in the adult by the almost complete absence of glands and the presence of a chitinous lining which is continuous at the mouth with the chitinous investment of the body and appendages. The true or individual stomach ( ventriculus ) which succeeds the proventriculus, represents a sudden departure in structure and function (Figs. I 3 and 15, st ) . It is a small, elliptical sac, hardly capable of dilatation, with very glandular walls devoid of
a chitinous lining. This region alone arises from the inner germ-layer of the embryo, in which it is called the mesenteron. Its structure shows very clearly that it is adapted to digesting and absorbing the liquid food that may be permitted to pass the valve at the posterior end of the proventriculus. Though of relatively large size in the embryo and larva, the stomach in the adult ant forms but a small portion of the alimentary tract. The portion lying between the stomach and anus, and comprising the small intestine (Fig. 15, lin), Malpighian vessels (nip) and the rectum (rc), arises in the embryo like the stomodaeum from a tubular infolding of the ectoderm, the proctodaeum, and, like the stomodaeum, has a chitinotls lining, which in this case is continuous with the integument at the anus and ends abruptly at the junction with the posterior end of the stomach.

The small intestine is a narrow tube usually more or less wrinkled by the action of its transverse musculature. Its histological structure is similar to that of the cylindrical section of the proventriculus. Near its insertion into the stomach, where it forms a valve, it receives the Malpighian, or urinary, vessels, which are merely so many long, tubular evaginations of its walls. These vessels seem to vary considerably in number in different ants. Thus, according to Adlerz (1886) there are 6 in Leptotlzorax, Foryiiicoxenus and Harpago:renies, 8 in Anergates, 8-io in Lasius, 12 in Tapinoma, 14. in Polyergus and 20 in For»iica and Cancpoytotacs. According to Xleinert ( i86o) the number may vary in the different castes of the same species. Thus the female of Lasi-us flavus is said to have 7-i4, the male fr16 and the worker 7-8. Accord­ing to Janet there are 6 in all three phases of Myrniiica rubra.

The rectum consists of an ampulliform enlargement which narrows posteriorly to its termination in the antis. Its thin walls are furnished with a single dorsal and a pair of lateral lentiform glands. The f0eces and the urinary excretions from the Malpighian vessels accumulate in the rectal ampulla and are expelled by a contraction of the thin muscle­layer in its walls. The anus (Fig. 15, an) is provided with a sphincter muscle and is situated on a papilla, which, in a state of repose, is con­cealed within the small, telescoped terminal segments of the -aster. In the Campotlotinx the anal orifice is fringed with a regular row of deli­cate hairs, or cilia.