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 ht

FIG. 15. Gaster of female Myrmica rubra in sagittal section. (Janet.) ppt, Postpetiole; str, stridulatory organ; gs'-gs, first to sixth gastric segments; ht, heart; v, cardiac valve; pc, pericardial cells; u, urate cell; f, adipocyte; on, anocyte; 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.

substances moulded in the infrabuccal chamber. Forel aptly calls the

"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 Camponotinæ, 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

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

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. 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 Camponotinæ (Plagiolepis, Prenolepis, etc.) and Dolichoderinæ. 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 proventriculus. Extreme forms of this kind are seen in Iridomyrmex and Technomyrmex (Fig. 16, E, F). In these ants it is possible to see how the proventriculus may play an important rôle 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 stomodæum. 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. 13 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 (mp) and the rectum (rc), arises in the embryo like the stomodæum from a tubular infolding of the ectoderm, the proctodæum, and, like the stomodæum, has a chitinous 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 Leptothorax, Formicoxenus and Harpagoxenus, 8 in Anergates, 8-10 in Lasius, 12 in Tapinoma, 14 in Polyergus and 20 in Formica and Camponotus. According to Meinert (1860) the number may vary in the different castes of the same species. Thus the female of Lasius flavus is said to have 7-14, the male 6-16 and the worker 7-8. According to Janet there are 6 in all three phases of Myrmica rubra.

The rectum consists of an ampulli form enlargement which narrows posteriorly to its termination in the anus. Its thin walls are furnished with a single dorsal and a pair of lateral lentiform glands. The fæces 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 gaster. In the Camponotinæ the anal orifice is fringed with a regular row of delicate hairs, or cilia.

The Glandular System.-Glands are well-developed in ants, and, owing to their importance in the ethological relations of these insects, deserve particular notice. They have been studied by Meckel (1846), Leydig (1859), Meinert (1860), Forel (1874, 1878), Lubbock (1882), Nassonow (1889) and Janet (1894, 1898). The following groups may be distinguished:

1. Integumentary glands, arising in the embryo, larva or pupa as invaginations of the ectodermal cell-layer (hypodermis), and including the antennary, mandibular, maxillary, labial and metasternal glands, those of the sixth abdominal (third or fourth gastric) segment, and of the fore metatarsus. Here, too, may be included the unicellular glands connected with the olfactory and tactile organs, to be considered in the next chapter. All the integumentary glands are present in the male as well as in the worker and female ant.

2. Reproductive glands, including the penial glands of the male, and in the worker and female the homologous glands of the sting-sheath, belonging to the ninth abdominal (sixth or seventh gastric) segment; the poison, accessory and repugnatorial, or anal glands of the worker and female, and the glands of the seminal vesicle of the male.

3. Glands of the alimentary canal. These comprise the postpharyngeal, ventricular and rectal glands and the Malpighian vessels.

4. Glands of the circulatory system, including the cenocytes, pericardial cells and adipocytes, or fat body. These, unlike the three other categories of glands, are ductless.

FIG. 17. Frontal section of head of Myrmica levinodis worker. (Janet.) Cc, Central body of brain; cp, pedunculate bodies; ol, optic lobe; on, optic nerve; e, eye; lo, olfactory lobe with glomeruli; mg, mandibular gland; rs, reservoir; cr, cribellum; d, ducts from gland cells; tr, trachea; mr, maxillary gland; Ibr, labrum ; mc, buccal cavity.

The glands of the alimentary tract have been briefly described, and those of the circulatory and reproductive systems will be taken up later, so that here only the integumentary glands will be considered. The antennary glands consist of a few isolated cells with slender ducts opening on a small area in a depression at the base of each antenna. The mandibular glands (Fig. 17, mg) are well-de