A glandular cell of the chylific stomach, when at rest, is always furnished with a striated “platform,” or flat surface, or face, on the side facing the cavity of the stomach, and the free edge of the platform, or plateau, is provided with filaments projecting into the digestive cavity (Fig. 322, f). These glandular cells, when active, differ much in appearance. In a great number, the platform (plateau) has disappeared, and is replaced by a simple, regular membrane. During the process of secretion, a finely granular mass, in direct continuity with the protoplasm, swells, and raises the membrane over the entire breadth of the cell, causing it to project into the intestinal cavity (Fig. 322, A, B). These vesicles, or drops of the secretion, whether free or still attached by a web to the cells, are clear and transparent in the living insect, but granular in the portions of the digestive canal fixed for cutting into sections. Gehuchten then asks: “How does a cell gorged with the products of secretion empty itself?” Both Ranvier and also Heidenhain believe that one and the same glandular cell may secrete and excrete several times without undergoing destruction, but their researches made on salivary glands have not answered the question. Gehuchten explains the process thus: when the epithelial cell begins to secrete, the clear fluid elaborated in the protoplasm of the cell increases the intracellular tension, until, finally, the fluid breaks through certain weak places in the swollen basal membrane of the platform, and then easily passes through the closely crowded filaments, and projects out into the intestinal cavity as a pear-shaped vesicle of a liquid rich in albumens at first attached to the free face of the cell, but finally becoming free, as at Fig. 322, A, B.

When the elaboration of the substance to be secreted is more active, the mechanism of the secretion is modified. The basal membrane of the platform may then be raised at several places at once; instead of a single vesicle projecting into the intestinal cavity, each cell may present a great number more or less voluminous. If all remain small and rapidly detach themselves from the glandular cell, the filaments of the platform are simply separated from each other at different points of the free face, as in Fig. 322, C. On the other hand, when the different vesicles of a single cell become larger, the filaments of the platform are compressed and crowded against each other in the spaces between the vesicles remaining free, and the undisturbed portions of the platform appear homogeneous (Fig. 322, D). After the excretion of the secretory products by this process of strangulation, the cell then assumes the aspect of a glandular cell at rest, and may begin again to form a new secretion.

To sum up: The process of excretion may occur in two ways:

1. Where the membrane ruptures and the substances secreted are sent directly out into the digestive cavity. 2. Where the vesicles become free by strangulation, floating in the glandular or intestinal cavity, and ending by rupturing and coming into contact with the neighboring vesicles or with the food.

Absorbent cells.—Besides the glandular or secreting cells in Ptychoptera, there is between the two regions of the chyle-stomach lined with these cells a region about a centimetre long composed of absorbent cells. The absorbent cells are very large, polygonal, and contain a large nucleus, in which is a striated convoluted chromatic cord.

The food on entering the chyle-stomach is brought into contact with the products secreted in the proventriculus, in the first part of the chyle-stomach, and in the tubular glands. These products of secretion act on the food, extracting from them useful substances which they render soluble. These substances, after having been absorbed by the absorbent cells in the middle region of the stomach, undergo special modifications, and are transformed into solid products, which are situated at the bottom of these cells. Afterwards the alimentary substances freed from a portion of their useful substances are again placed in contact with the products of secretion in the distal part of the chylific ventricle, and reach the terminal part of the intestine.

“The products of secretion,” adds Gehuchten, “diverted into the intestinal canal do not come into immediate contact with the alimentary substances; they are separated from it by a continuous, structureless, quite thick membrane (the peritrophic membrane), which directly envelops the cylinder of food matters, extending from the orifice of the œsophageal valvule to the end of the intestine. Between this membrane and the free face of the epithelial lining there exists a circular space, into which are thrown and accumulate the excreted substances. The latter then cannot directly mingle with the aliments; but when they are liquid they undoubtedly pass through this membrane by osmose, and thus come into contact with the nutritive substances. It is the same with the products of absorption. The absorption of soluble products of the intestinal cavity is not then so simple a phenomenon as it was at first thought to be, since these products are nowhere brought into immediate contact with the absorbent cells” (pp. 90, 91).

The most recent authority, Cuénot, states that absorption of the products of digestion takes place entirely in the mid-intestine, and in its cæca when these are present. The mid-intestine exercises a selective action on the constituents of the food comparable to the action of the vertebrate liver.

LITERATURE ON THE PHYSIOLOGY OF DIGESTION

Davy, J. Note on the excrements of certain insects, and on the urinary excrement of insects. (Edinburgh New Phil. Journ., 1846, xl, pp. 231–234, 335–340; 1848, xlv, pp. 17–29.)

—— Some observations on the excrements of insects, in a letter addressed to W. Spence. (Trans. Ent. Soc. London, Ser. 2, iii, 1854, pp. 18–32.)

Bouchardat, A. De la digestion chez le ver à soie. (Revue et Mag. de Zool., Sér. 2, 1851, iii, pp. 34–40.)

Lacaze-Duthiers, H., et A. Riche. Mémoire sur l’alimentation de quelques insects gallicoles et sur la production de la graisse. (Ann. Scienc. natur., 1854, Sér. 4, ii, pp. 81–105.)

Basch, S. Untersuchungen über das chylopoetische und uropoetische System von Blatta orientalis. (Sitzungsber. d. math.-naturwiss. Classe d. Akad. d. Wissensch. Wien., 1858, xxxiii, pp. 234–260, 5 Taf.)

Lambrecht, A. Der Verdauungsprozess der stickstoffreichen Nährmittel, welche unsere Bienen geniessen, in den dazu geschaffenen Organen derselben. (Bienenwirtschaftl. Centralbl., viii Jahrg., 1872, pp. 73–78, 83–89.)

Plateau, F. Recherches sur les phénomènes de la digestion chez les insectes, (Mém. Acad. roy. de Belgique, Sér. 2, xli, 1 Part, 1873, pp. 124, 3 Pls.)

—— Note additionelle au mémoire sur les phénomènes de la digestion chez les insectes. (Bull. Acad. roy. de Belgique, Sér. 2, xliv, 1877, pp. 710–733.)

Tursini, G. Fr. Un primo passo nella ricerca dell’ assorbimento intestinale degli artropodi. (Rend. d. R. Accadem. di Sc. fis. e matemat. di Napoli, 1877, xvi, pp. 95–99, 1 Pl.)

Jousset de Bellesme, Physiologie comparée. Recherches expérimentales sur la digestion des insectes et en particulier de la blatte. Paris, 1876, vii, and 96 pp., 3 Pls.

—— Recherches sur les fonctions des glandes de l’appareil digestif des Insectes. (Compt. rend., lxxxii, Paris, 1876, pp. 97–99.)

—— Travaux originaux de physiologie comparée. (i, Insectes, Digestion, Métamorphoses.) Paris, 1878, 5 Pls.

Simroth, H. Einige Bemerkungen über die Verdauung der Kerfe. (Zeitschr. f. d. gesammten Naturwiss., xli, 1878, pp. 826–831.)

Krukenberg, C. Fr. W. Versuche zur vergleichenden Physiologie der Verdauung und vergleichende physiologische Beiträge zur Kenntnis der Verdauungsvorgange. (Untersuch, a. d. physiolog. Institut d. Universität Heidelberg, 1880, i, 4, pp 327, Figs.; ii, 1, p. 1, Figs.)

Metschnikoff, E. Untersuchungen über die intrazelluläre Verdauung bei wirbellosen Tieren. (Arb. d. zool. Instit. Wien., 1883, v, pp. 141–168, 2 Taf.)

Locy, William A. Anatomy and physiology of the family Nepidæ. (Amer. Naturalist, xviii, 1884, pp. 250–255, 353–367, 4 Pls.)

Vangel, E. Beiträge zur Anatomie, Histiologie, und Physiologie des Verdauungsapparates des Wasserkäfers, Hydrophilus piceus. (Termész. Füzet., x, 1886, pp. 111–126 (in Hungarian); pp. 190–208 (in German), 1 Pl.)

Schönfeld. Die physiologische Bedeutung des Magenmundes der Hönigbiene. (Archiv f. Anat. u. Physiol., Physiol. Abt., 1886, pp. 451–458.)

Faussek, V. Beiträge zur Histiologie des Darmkanals der Insekten. (Zeitschr. f. wiss. Zool., 1887, xl, pp. 694–712, 1 Taf.; Abstract in Zool. Anz., Jahrg. x, pp. 322, 323, 1 Taf.)

Frenzel, J. Ueber Bau und Thätigkeit des Verdauungskanals der Larve des Tenebrio molitor, mit Berücksichtigung anderer Arthropoden (Berlin. Ent. Zeitschr., 1882, pp. 267–316, 1 Taf.); Inaug.-Diss. Göttingen, 1882.

—— Einiges über den Mitteldarm der Insekten, sowie über Epithel-regeneration. (Archiv f. Mikrosk. Anat., 1885, xxvi, pp. 229–306, 3 Taf.)

—— Zum feineren Bau des Wimperapparates. (Ibid., 1886, xxviii, pp. 53–80, 1 Taf.)

—— Die Verdauung lebenden Gewebes und die Darmparasiten. (Archiv f. Anat., 1891.)

Gehuchten, A. van. Recherches histologiques sur l’appareil digestif de la Ptychoptera contaminata, I Part. Étude du revêtment épithélial et recherches sur la sécrétion. (La Cellule, 1890, vi, pp. 183–291, 6 Pls.)

Cuénot, L. Études physiologiques sur les Orthoptères. (Arch. Biol., xiv, 1895, pp. 293–341, 2 Pls.)

Needham, James G. The digestive epithelium of dragon-fly nymphs. (Zool. Bull., i, 1897, Chicago, pp. 104–113, 10 Figs.)

With the writings of Mingazzini (see p. 323), Kowalevsky, Ranvier, Haidenhain, Beauregard (p. 323), Sadones.

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).

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.

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.

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.

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:—

“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.

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.)

LITERATURE ON THE SALIVARY GLANDS

Leydig, F. Zur Anatomie der Insekten. (Archiv Anat. und Phys. 1859.)

—— Untersuchungen zur Anatomie und Histiologie der Tiere. Bonn, 1883, pp. 174, 8 Taf.

—— Intra- und interzellulare Gänge. (Biolog. Centralblatt, x, 1890, pp. 392–396.)

Dohrn, A. Zur Anatomie der Hemipteren. (Stettin. Entom. Zeit., 1866, salivary glands, pp. 328–332.)

Kupffer, C. Die Speicheldrüsen von Periplaneta orientalis und ihr Nervenapparat. (Beiträge zur Anatomie und Physiol., 1875.)

Schiemenz, P. Ueber das Herkommen des Futtersaftes und die Speicheldrüsen der Biene. (Zeitschr. f. wissens. Zool., xxxviii, 1883, pp. 71–135, 3 Taf.)

Korschelt, E. Ueber die eigentümlichen Bildungen in den Zellkernen der Speicheldrüsen von Chironomus plumosus. (Zool. Anzeiger, 1884, pp. 189–194, 221–225, 241–246.)

Hofer, B. Untersuchungen über den Bau der Speicheldrüsen und des dazu gehörenden Nervenapparates von Blatta. (Nova Acta d. Kais. Leopold.-Carol. Deutsch. Akad. d. Naturforscher, li, 1887, pp. 345–395, 3 Taf.)

Knüppel, A. Ueber Speicheldrüsen von Insekten. (Archiv für Naturg., 1887, Jahrg. 52, pp. 269–303, 2 Taf.)

Blanc, Louis. La tête du Bombyx mori à l’état larvaire, anatomie et physiologie. (Extrait des Travaux du Laboratoire d’Études de la Soie, 1889–1890; Lyon, 1891, p. 180, many figs.)

Bordas, L. Anatomie des glandes salivaires des Hyménoptères de la famille des Ichneumonidæ. (Zool. Anzeiger, 1894, pp. 131–133.)

—— Glandes salivaires des Apides, Apis mellifica. ♂ and ♀. (Comptes rendus Acad. Sc., Paris, cxix, pp. 363, 483, 693–695, 1894; also two articles in Bull. Soc. Philomath. Paris, 1894, pp. 5, 12, 66.)

—— Appareil glandulaire des Hyménoptères. (Ann. Sc. Nat. Zool., xix, Paris, 1894, pp. 1–362, 11 Pls.) (See also p. 366.)

Berlese, Antonio. Le cocciniglie Italiane viventi sugli agrumi. Firenze, 1896, 12 Pls. and 200 Figs.

With the writings of Mark, Minot, Locy, List, Krassilstschik, Nagel (1896).

b. The silk or spinning glands, and the spinning apparatus

The larvæ of certain insects, chiefly those of the Lepidoptera, possess a pair of silk or spinning glands (sericteries) which unite to form a single duct opening in the upper lip at the end of the lingua, which is modified to form the spinneret. (See pp. 71, 75.) All caterpillars possess them, and they are best developed in the silkworms, which spin the most complete cocoon. Silk-glands also occur in the larvæ of the Tenthredinidæ, in the case-worms or larval Trichoptera, also in certain chrysomelid beetles (Donacia, Hæmonia), and in a weevil (Hypera). In a common caddis-worm (Limnophilus) the glands are of a beautiful pale violet-blue tint, and two and a half times as long as the larva itself; viz. the body is 20 mm. and the glands 55 mm. in length.

In caterpillars the glands are of tubular shape, shining white, and much like the ordinary simple tubular salivary glands of the imago. When only slightly longer than the body they are twice folded, the folds parallel and situated partly beneath and partly on the side of the digestive canal; not usually, when folded in their natural position, extending much behind the end of the stomach; but in the silkworms they are so long and folded as to envelop the hinder part of the canal. In geometrid caterpillars the glands when stretched out only reach slightly beyond the end of the body; in Datana they are half again as long as the body. Helm thus gives their relative length in certain Eurasian caterpillars, and we add that of Telea polyphemus:—

Thus in Telea the silk-glands are about 18.50 inches in length, being about seven times as long as the body.

For the most complete accounts of the spinning glands of Lepidoptera and their mechanism we are indebted to Helm and to Blanc, and for that of the Trichoptera to Gilson.

The unpaired portion, or spinning apparatus (filière of Lyonet), is divided into two portions; the hinder half being the “thread-press,” the anterior division the “directing tubes.” The silk material, stored up in the thickest portion of the glands, passes into the thread-press (Fig. 334, A), which is provided with muscles which force the two double ribbon-like threads through the directing tube, as wire is made by molten iron being driven through an iron plate perforated with fine holes. The entire spinning apparatus, or filator, as we may call it, is situated in the tubular spinneret. The opening of the spinneret is directed anteriorly, and the anterior end of the directing tube passes directly into this opening so that the directing tube may be regarded as an invagination of the lingua.

The silk thread which issues from the mouth of the spinneret is, as Leeuwenhoek discovered, a double ribbon-like band, as may be seen in examining the silk of any cocoon.

The process of spinning.—Since the appearance of Helm’s account, Gilson, and also Blanc, have added to our knowledge of the way in which the silk is spun and of the mechanism of the process. Gilson has arrived, in regard to the function of the press or filator, at the following conclusions: 1, the press regulates the thread, it compresses it, gives it its flattened shape; 2, it regulates the layer of gum^[52] (grès) which surrounds the thread; 3, it may render the thread immovable by compressing it as if held by pincers.

The process of spinning in the silkworm, says Blanc, comprises all the phenomena by which the mass of silk contained in the reservoir is transformed into the silk fluid of which the cocoon is spun. The excretory canals each contain a cylindrical thread of silk having a mean diameter of 0.2 mm. and surrounded by a layer of gum (grès) which in the fresh living organ exactly fills the annular space situated between the fibroin cylinder and the wall. Arrived within the common duct, the two threads receive the secretion of Filippi’s gland, where the silken fluid is formed, but has not yet assumed its definite external characters. The two threads press through the common canal and arrive at the infundibulum (Fig. 334, c) of the press, at the bottom of which is situated the orifice of the spinning canal, almost completely divided into two by the sharp edge of the rachis (Figs. 334, a, 335, l). The threads each pass into one of the two grooves, and the layer of gum (grès) fills the rest of the canal of the press or filator.