THE GLANDULAR AND EXCRETORY APPENDAGES OF THE DIGESTIVE CANAL
Into each primary division of the digestive canal open important glands. The salivary and silk-glands are offshoots of the œsophagus (stomodæum); the cœcal appendages open into the stomach (mesenteron), while the urinary tubes grow out in embryonic life from the primitive intestine (proctodæum), and there are other small glands which are connected with the end of the hind-intestine.
a. The salivary glands
We will begin our account of these glands with those of the Orthoptera, where they are well developed. In the cockroach a large salivary gland and accompanying reservoir lie on each side of the œsophagus and crop. The gland is a thin, leaf-like, lobulated mass, divided into two principal lobes. These open into a common trunk, which after receiving a branch from a small accessory lobe, and from the salivary reservoir, unites with its fellow to form the unpaired salivary duct which opens into the under side of the lingua. Each salivary reservoir is a large oval sac with transparent walls. (Miall and Denny, also Figs. 299, sr, and 327.) The ducts and reservoirs have a chitinous lining, and the ducts are, like the tracheæ, surrounded by a so-called spiral thread, or by separate, incomplete, hooplike bands, which serve to keep the duct permanently distended. In the locust (Fig. 298) the lobules are more scattered, forming small separate groups of acinose glands. In the embryo of Forficula Heymons has observed a pair of salivary glands opening on the inner angle of the mandibles, a second pair opening in the second maxillæ, while a third pair of glands, whose function is doubtful, is situated in the hinder part of the head, opening to the right and left on the chitinous plate (postgula) behind the submentum. In Perla, there are two pairs segmentally arranged (Fig. 343).
Fig. 323.—Left side of the head of the silkworm: a, adductor muscle of the mandible, from which the muscular fibres have been removed; b, upper fibres of the same; c, lower fibres cut away to show the adductor muscle (e); d, fibres inserted on the accessory adductor lamella; f, œsophagus, much swollen; g, salivary gland; h, dorsal vessel; i, l, tracheæ of the mandibular muscles; k, trachea; n, optic nerve.—After Blanc.
Fig. 324.—Lower side of the head of the silkworm exposed, the spinning apparatus, the œsophageal ganglion, and the adductor of the left mandible removed: M, mandible; P, abductor of the mandible; R, adductor; N, salivary gland attached at O to the edge of the adductor muscle; o, o, transverse portion of the “hyoid”; 3, masticator nerve and its recurrent branch (7); L, tongue cut horizontally.—After Blanc.
Here we might refer to a pair of glands regarded by Blanc as the true salivary glands. They do not appear to be the homologues of the salivary glands of other insects, though probably functioning as such. The functional salivary glands of lepidopterous larvæ have been overlooked by most entomotomists, and the spinning glands have been, it seems to us, correctly supposed to be modified salivary glands. Lucas also regards those of case-worms (Trichoptera) as morphologically salivary glands. Those of the silkworm were figured by Réaumur (Tom. i, Pl. v, Fig. 1), but not described; while those of Cossus, which are voluminous, were regarded by Lyonet as “vaisseaux dissolvans.” Dr. Auzoux (1849), in his celebrated model of the silkworm, represented them accurately, while Cornalia briefly described them as opening into the mouth. The first satisfactory description is that of Blanc (1891), who states that in the silkworm “the two salivary glands” are small, flexuous, yellow tubes, which occupy a variable position on the sides of the œsophagus (Fig. 323). The glandular portion passes into the head, ending at the level of the adductor plate of the mandibles (Fig. 324, o), and entering the buccal cavity at the base of the mandible, as seen in Fig. 323. It is plain, when we recognize the direct homology of the silk-glands of the caterpillars with the salivary glands of other insects, and of the spinneret with the hypopharynx, that these so-called “salivary glands” in lepidopterous larvæ are different structures. They are probably modified coxal glands, belonging to the mandibular segment.
Fig. 325.—One of the two salivary glands of Cæcilius burmeisteri: d, excretory duct; cn, the lumen or canal; cg, gland-cells; ct, salivary fluid.—After Kolbe.
The polygonal epithelial cells of these glands contain branched nuclei, recalling those of the spinning-glands. In those caterpillars which feed on leaves, the salivary glands are slightly developed, but in such as bore into and eat wood, as the Cossidæ, the glands are, as figured by Lyonet, very large, forming two sausage-shaped bodies passing back to the beginning of the mid-intestine, each ending in a long convoluted filament. The salivary glands of the imago are very long and convoluted (Fig. 310, sd).
In the Panorpidæ these glands differ in the sexes, the males having three pairs of very long tortuous tubes, while, in the females, they are reduced to two indistinct vesicles. (Siebold.)
In the Diptera in general there are two pairs, one situated in the beak, the other in the thorax. In the larvæ there is a single pair (Fig. 341). Kraepelin describes a third pair in the Muscidæ at the point of transition from the fulcrum to the œsophagus, but Knüppel has apparently found only what may be fat cells at this point, so that the supposed presence of a third pair in Diptera needs confirmation. In the Psocidæ there are two salivary glands, of simple tubular shape (Fig. 325).
In the Nepidæ the salivary glands are four in number, and of conglomerate structure, two being long and extending back into the beginning of the abdomen, while the other two are about one-fourth as long. (Figs. 327, 328.) In Cicada, besides a pair of simple tortuous tubes, there is in the head another pair of glands, each composed of two tufts of short lobes, situated one behind the other. (Dufour.) In many Hemiptera (Pyrrhocoris, Capsus, etc.) there is but a single pair, each gland consisting of four lobes; in the Coccidæ each gland is divided into two lobes (Fig. 326); in the Aphidæ, according to Witlaczil, they consist of two lobes grown together. In the Psyllidæ they are said to be absent.
In Phylloxera vastatrix the saliva is forced through a salivary passage out of the duct and into the mouth by a pumping apparatus furnished with special muscles. (Krassilstschik.)
In the Odonata acinose glands are present in the imago, but not in the nymph until in its last stage, Poletaiew accounting for their absence in the earlier stages by the fact that the larva swallows more or less water while taking its food.
In the Coleoptera, as we have observed in Anopthalmus, there are three pairs of salivary glands (Fig. 74). In the Blapsidæ these glands consist of many ramifying tubes united on each side of the œsophagus into a single duct; in others they are but slightly developed, while in still others they are wanting.
The salivary glands are most highly differentiated in the Hymenoptera, and especially in the bees (Bombus and Apis), where Schiemenz found not less than five systems of glands (Fig. 329; also 87), of which four systems are paired. One pair of these glands lies in the tongue, three in the head, and one in the thorax.
Fig. 326.—Acinous salivary glands of Orthezia cataphracta. In some acini the nuclei and boundaries of the cells are shown.—After List, from Field’s Hertwig.
System I is situated in the head, and consists of unicellular glands; the duct from each cell leads into a common, strongly chitinized duct, opening into the gullet.
System II, composed of acinose glands, lies also in the head; its duct is united with that of System III, situated in the thorax. (Fig. 329, 2, 3.)
System IV is situated at the base of the upper surface of the mandibles, and forms a delicate sac lined within with glandular cells; its duct opens at the insertion of the mandibles.
System V lies in the beak, and is a single gland consisting of unicellular glands; it opens into the common opening of Systems II and III. This system is wanting in the honey-bee, but occurs in Bombus and other genera.
Fig. 327.—Appendages of digestive canal of Belostoma.—After Locy.
Fig. 328.—Salivary and other glands of Ranatra.—After Locy.
In all the five systems there constantly occur three cellular layers: the intima, epithelial, and propria. As regards their origin Schiemenz states that Systems I and IV are new structures, that System III arises in part, and Systems II and V wholly, from the silk-glands of the larva. As the glands differ much in the sexes, and in different species and genera, Schiemenz believes that their function is very manifold.
In addition to those previously discovered by Schiementz, Bordas has detected two additional pairs of salivary glands in the worker and male honey-bee, i.e. the internal mandibular and sublingual glands, so that in Apis there are in all six pairs, and apparently one unpaired.
The delicate chitinous external layer of the gland is perforated by many very fine pores through which the salivary fluid secreted by the epithelial cells passes into the salivary duct. The glands are externally bathed by the blood.
In many insects, including lepidopterous larvæ, the single median opening of the salivary duct is converted into a spraying apparatus.
In the adult Lepidoptera, according to Kirbach:—
Fig. 329.—Salivary glands of the honey-bee: systems No. 1–3, × 15: sv, salivary valve (of systems 2 and 3) at base of tongue; lp, labial palpus; mx, maxilla; so, salivary opening of system 1 in hypopharyngeal plate; no, openings in plate for termination of taste-nerve; œ, œsophagus; sd, salivary duct; b, junction of ducts of system No. 2; c, junction of ducts of system No. 3; sc, sc, salivary sacs; fl, front lobe; bl, back lobe; a, chitinous duct, with spiral thread. B, single acinus of system No. 1, × 70: n, nucleus; st, salivary tract; d, large duct. C, single pouch, or acinus, from system No. 2: a, propria or outer membrane; sc, secreting cells. D, termination of system No. 3:1,2,3,4, lines marking end of section; d, duct in section; sc, secreting cells in section; n, nucleus.—After Cheshire.
“Its lower half forms a thick chitinous gutter, with a concave cover above, in which the similarly shaped upper half lies encased, so that between the two only a small semicircular opening remains. Powerful muscles extend from the cover to the lower side and to the two ridges of the bottom plate; through their contraction the upper channel is elevated, and presses out of the hinder part of the ducts into the space thus formed a great quantity of the saliva, which by allowing the contraction of the cover-muscle through the crevice-like opening, which is situated in the lower edge of the mouth-opening, becomes squeezed out in order either to mix with the fluid where the 2d maxillæ fuse, passing up into the canal in the proboscis, or to penetrate into and thus dilute the semi-fluid or solid substances taken, into the proboscis.”
The morphology and general relations of the salivary glands have been sketched out by Hatschek, Patten, and by Lucas, from observations on those of the case-worms or larval Trichoptera.
Fig. 330.—Eight pairs of glands of Andrena: I, thoracic; II, postcerebral; III, supracerebral; IV, lateropharyngeal; V, mandibular; VI, internomandibular; VII, sublingual; VIII, lingual; Md, mandible; L, tongue; o, eye; œ, œsophagus; J, honey-sac.—After Bordas.
Patten states that the spinning-glands in Neophylax are formed by a pair of ectodermal invaginations on the ventral side of the embryo, between the base of the 2d maxillæ and the nervous cord. They increase rapidly in length, and “they also unite to form a common duct, which opens at the end of the upper lip.”
The salivary glands in the same insect are “formed by invagination of the ectoderm on the inner sides of the mandibles, in the same manner as are the spinning glands.”
Lucas has shown that in trichopterous larvæ (Anabolia) there are three pairs of salivary glands in the head, which are serially arranged. The first pair belong to the mandibular, the second pair to the 1st maxillary, and the third pair, or spinning glands, to the 2d maxillary segment. The first or mandibular glands open into the mouth at the base of the mandibles directly behind the dorsal condyle. The second pair open between the 1st and 2d maxillæ; at the base of the latter, near the ventral condyle of the mandibles. The third pair open into the hypopharynx, which is modified to form the spinneret. Lucas agrees with Korschelt in regarding them as modified coxal glands, Schiemenz having previously regarded the headglands of the imago of the bee as belonging to the segments bearing the three pairs of buccal appendages, so that each segment originally contained a pair of glands. It is thus proven that the silk-glands are modified salivary glands adapted to the needs of spinning larvæ, and indeed in the imago the sericteries revert to their primitive shape and use as salivary glands.
The serial arrangement of the salivary glands in the Hymenoptera, where the number varies from five to ten pairs, is clearly proved by Bordas. He has detected five more pairs than were previously known, and names the whole series as follows:—1, the thoracic salivary glands, which are larger than the others, and nine other pairs, which are all contained in the head as follows: 2, postcerebral; 3, supracerebral; 4, lateropharyngeal; 5, mandibular; 6, internomandibular, situated on the inner side of the base of mandible; 7, sublingual; 8, lingual (these and 1 to 7 common to all Hymenoptera); 9, paraglossal (in Vespidæ); 10, maxillary (very distinct in most wasps). These glands do not all occur in the same species, being more or less atrophied.
Bordas further shows the segmental arrangement of the cephalic glands by stating that the supracerebral glands correspond to the antennal segment, the sublingual glands to the labial, the mandibular glands (external and internal) to the mandibular segment, the maxillary glands to the 1st maxillary segment, the lingual glands to the 2d maxillary segment, while the thoracic and postcerebral salivary glands, he thinks, correspond to the ocular segment, a view with which we are indisposed to agree, although conceding that each of the six segments of the head has in it at least one pair of salivary glands.
Functions of the different salivary glands in Hymenoptera.—The secretion of the thoracic glands is feebly alkaline. The postcerebral salivary glands, considered by Ramdohr to be organs of smell, secrete, like the preceding, a distinctively alkaline fluid, which mingles with the products of the thoracic glands. The supracerebral glands, also equally well developed in all Hymenoptera, though much atrophied in the females and especially the males of Apis mellifica, also in the Vespinæ and Polistinæ, secrete an abundant, feebly acid liquid, which is actively concerned in digestion.
As to the mandibular glands, which Wolf supposed to be olfactory organs, their acid secretion, though smelling strongly, acts energetically on the food as soon as introduced into the mouth.
The sublingual glands, atrophied in most Apidæ, but relatively voluminous in Sphegidæ, Vespinæ, Polistinæ, Crabronidæ, etc., empty their secretion into a small prebuccal excavation, where accumulate vegetable and earthy matters collected by the tongue, and the saliva secreted by these glands, acts upon them before they pass into the pharynx. The lingual glands secrete a thick, sticky liquid, which causes foreign bodies to adhere to the tongue, and also agglutinates alimentary substances. The uses of the other glands, maxillary and paraglossal, are from their minuteness undetermined. (Bordas.)