Chapter XVIII: Part I: Morphology and Physiology (14)

=Müller, H.= Ueber die angebliche Afterlösigkeit der Bienenlarven.
(Zool. Anzeiger, 1881, pp. 530, 531.)

=Schiemenz, Paulus.= Ueber das Herkommen des Futtersaftes und die
Speicheldrüsen der Bienen, nebst einem Anhänge über das Riechorgan.
(Zeitschr. f. wissens. Zool., xxxviii, 1883, pp. 71–135, 3 Taf.)

=Rovelli, G.= Alcune ricerche sul tubo digerente degli Atteri,
Ortotteri e Pseudo-Neurotteri. (Como, 1884, p. 15.)

=Beauregard, H.= Structure de l’appareil digestif des Insectes de la
tribu des Vésicants. (Compt. rend. Acad. Paris, 1884, xcix, pp.
1083–1086.)

—— Recherches sur les Insectes vésicants., 1 Part, Anatomie. (Journ.
Anat. Phys. Paris, 1885, xxi Année, pp. 483–524, 4 Pls.; 1886, xxii
Année, pp. 85–108, 242–284, 5 Pls.)

—— Les Insectes vésicants, Paris, 1890, Chap. III, Appareil digestif,
pp. 63–99; (Phénomènes digestifs, pp. 161–170; Pls. 6–9.)

=Wertheimer, L.= Sur la structure du tube digestif de l’_Oryctes
nasicornis_. (Compt. rend. Soc. Biol. Paris, 1887, Sér. 8, iv, pp.
531, 532.)

=Kowalevsky, A.= Beitrage zur Kenntniss der nachembryonal Entwicklung
der Musciden. (Zeitschr. f. wissens. Zool., xlv, 1887, pp. 542–594,
5 Taf.)

=Schneider, A.= Ueber den Darm der Arthropoden, besonders der
Insekten. (Zool. Anzeiger, 1887, x Jahrg., pp. 139, 140.)

—— Ueber den Darmkanal der Arthropoden. (Zool. Beiträge von A.
Schneider, ii, 1887, pp. 82–96, 3 Taf.)

=Fritze, A.= Ueber den Darmkanal der Ephemeriden. (Berichte der
Naturforsch.-Gesellsch. zu Freiburg i. Br., 1888, iv, pp. 59–82, 2
Taf.)

=Emery, C.= Ueber den sogenannten Kaumagen einiger Ameisen. (Zeitschr.
f. wissens. Zool., 1888, xlvi, pp. 378–412, 3 Taf.)

=Meinert, F.= Contribution à l’anatomie des Fourmilions. (Overs.
Danske Vidensk. Selsk. Forhandl. Kjöbenhavn, 1889, pp. 43–66, 2
Pls.)

=Mingazzini, P.= Richerche sul canale digerente dei Lamellicorni
fitofage (Larve e Insetti perfetti). (Mitteil. Zool. Station zu
Neapel, ix, 1889–1891, pp. 1–112, 266–304, 7 Pls.)

=Fernald, Henry T.= Rectal glands in Coleoptera. (Amer. Naturalist,
xxiv, pp. 100, 101, Jan., 1890.)

=Visart, O.= Digestive canal of Orthoptera. (Atti Soc. Toscana Scient.
Natur., vii, 1891, pp. 277–285.)

=Eberli, J.= Untersuchungen an Verdauungstrakten von _Gryllotalpa
vulgaris_. (Vierteljahresschr. d. Naturforsch. Gesells. Zurich,
1892, Sep., p. 46, Fig.)

=Holmgren, Emil.= Histologiska studier öfver några lepidopterlarvers
digestionskanal och en del af deras Körtelartade bildningar. (Ent.
Tidskr. Årg. xiii, pp. 129–170, 1892, 6 Pls.)

=Ris, F.= Untersuchung über die Gestalt des Kaumagens bei den Libellen
und ihren Larven. (Zool. Jahrb. Abth. Syst., ix, 1896, pp. 596–624,
13 Figs.)

See also the works of Straus-Dürckheim, Newport, Mark, Witlaczil,
Vayssière, Landois, Jordan, Oudemans, Berlese, List, Grassi, Verson,
Miall and Denny, Leidy, Cheshire, Kowalevsky, Gehuchten, Locy, etc.

_b_. Digestion in insects

For the most complete and reliable investigation of the process of digestion, we are indebted to Plateau, whose results we give, besides the conclusions of later authors:

In mandibulate or biting insects, the food is conducted through the œsophagus by means of the muscular coating of this part of the digestive canal. Suctorial insects draw in their liquid food by the contractions followed by the dilatations of the mid-intestine (chylific stomach). Dragon-flies, Orthoptera, and Lepidoptera swallow some air with their food.

Where the salivary glands are present, the neutral alkaline fluid secreted by them has the same property as the salivary fluid of vertebrates of rapidly transforming starchy foods into soluble and assimilable glucose. In such forms as have no salivary glands, their place is almost always supplied by an epithelial lining of the œsophagus, or, as in the Hydrophilidæ, a fluid is secreted which has the same function as the true salivary fluid.

Nagel states that the saliva of the larva of Dyticus is powerfully digestive, and has a marked poisonous action, killing other insects, and even tadpoles of twice the size of the attacking larva, very rapidly. The larvæ not only suck the blood of their victims, but absorb the proteid substances. Drops of salivary juice seem to paralyze the victim, and to ferment the proteids. The secretion is neutral, the digestion tryptic. Similar extra-oral digestion seems to occur in larvæ of ant-lions, etc. (Biol. Centralbl., xvi, 1896, pp. 51–57, 103–112; Journ. Roy. Micr. Soc., 1896, p. 184.)

In carnivorous insects and in Orthoptera, the œsophagus dilates into a crop (ingluvies) ended by a narrow, valvular apparatus (or gizzard of authors). The food, more or less divided by the jaws, accumulates in the crop, which is very distensible; and, when the food is penetrated by the neutral or alkaline liquid, there undergoes an evident digestive action resulting, in carnivorous insects, in the transformation of albuminoid substances into soluble and assimilable matter analogous to peptones, and, in herbivorous insects, an abundant production of sugar from starch. This digestion in the crop, a food-reservoir, is very slow, and, until it is ended, the rest of the digestive canal remains empty.

“Any decided acidity found in the crop is due to the injection of acid food; but a very faint acidity may occur, which results from the presence in the crop of a fluid secreted by the cæcal diverticula of the mesenteron.” (Miall and Denny.)

When digestion in the crop is accomplished, the matters are subjected to an energetic pressure of the walls through peristaltic contractions, and then, guided by the furrows and chitinous teeth, pass along or gradually filter through the valvular apparatus or proventriculus, whose function is that of a strainer.

At the beginning of the “chyle-stomach” (mesenteron) of Orthoptera are glandular cæca which secrete a feebly acid fluid. This fluid emulsifies fats, and converts albuminoids into peptones. It passes forwards into the crop, and there acts upon the food.

In the mesenteron (mid-intestine) the food is acted upon by an alkaline or neutral fluid, never acid, either secreted, as in Orthoptera, by local special glands, or by a multitude of minute glandular cæca, as in many Coleoptera, or by a simple epithelial layer. It has no analogy with the gastric juices of vertebrates; its function differs in insects of different groups; in carnivorous Coleoptera it actively emulsionizes greasy matters; in the Hydrophilidæ it continues the process of transformation of starch into glucose, begun in the œsophagus. In the Scarabæidæ, it also produces glucose, but this action is local, not occurring elsewhere; in caterpillars, it causes a production of glucose, and transforms the albuminoids into soluble and assimilable bodies analogous to peptones, and also emulsionizes greasy matters. Finally, in the herbivorous Orthoptera there does not seem to be any formation of sugar in the stomach itself, the production of glucose being confined to the crop (jabot).

When digestion in the crop is finished, the proventriculus relaxes, and the contents of the crop, now in a semi-fluid condition, guided by the furrows and teeth, passes into the mesenteron, which is without a chitinous lining, and is thus fitted for absorption.

The contents of the mid-intestine (chylific stomach) then slowly and gradually pass into the intestine, the first anterior portion of which, usually long and slender, is the seat of an active absorption. The epithelial lining observed in certain insects seems, however, to indicate that secondary digestion takes place in this section. The reaction of the contents is neutral or alkaline.

The second and larger division of the intestine only acts as a stercoral reservoir. (The voluminous cæcum occurring in Dyticidæ, Nepa, and Ranatra, whether full or empty, never contains gas, and it is not, as some have supposed, a swimming-bladder.) The liquid product secreted by the Malpighian tubes accumulates in this division, and, under certain circumstances, very large calculi are often formed. In his subsequent paper on the digestion of the cockroach, Plateau states that in the intestine are united the residue of the work of digestion and the secretion of the urinary or Malpighian tubes, this secretion being purely urinary.

These organs are exclusively depuratory and urinary, freeing the body from waste products of the organic elements. The liquid they secrete contains urea (?), uric acid and abundant urates, hippuric acid (?), chloride of sodium, phosphates, carbonate of lime, oxalate of lime in quantity, leucine, and coloring-matters.

The products of the rectal or anal glands vary much in different groups, but they take no part in digestion, nor are they depuratory in their nature.

Insects have nothing resembling chylific substances.[51] The products of digestion, dissolved salts, peptones, sugar in solution, emulsionized greasy matters, pass through the relatively delicate walls of the digestive canal by osmose, and mingle outside of the canal with the blood.

Whatever substances remain undigested are expelled with the excrements; such are the chitin of the integuments of insects, vegetable cellulose, and chlorophyll, which is detected by the microspectroscope all along the digestive canal of phytophagous insects.

In his experiments in feeding the larvæ of Musca with lacmus, Kowalevsky found that the œsophagus, food-reservoir, and proventriculus, with its cæcal appendages, always remained blue, and had an alkaline reaction; the mid-intestine, also, in its anterior portion, remained blue, but a portion of its posterior half became deep red, and also exhibited a strong reaction. The hind-intestine, however, always remained blue, and also had an alkaline reaction. (Biol. Centralbl., ix, 1889, p. 46.)

=The mechanism of secretion.=—Gehuchten describes the process of secretion in insects, the following extract being taken from his researches on the digestive apparatus of the larva of Ptychoptera. The products of secretion poured into the alimentary canal are more or less fluid; for this reason, it is impossible to say when an epithelial cell at rest contains these products. For the secreting nature of these cells is only apparent at the moment when they are ready for excretion; then the cellular membrane swells out, and a part of the protoplasmic body projects into the intestinal cavity.

Before going farther, the terms _secretion_ and _excretion_ should, he says, be defined. With Ranvier, he believes that the elaboration in the protoplasm of a definite fluid substance is, _par excellence_, the secretory act, while the removal of this substance is the act of excretion.

FIG. 322.—Different phases of the mechanism of secretion and of
excretion.—After Gehuchten.
]

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.

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.)

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_:—

_Vanessa io_ length of body 32 mm.; of the silk glands 26 mm. _Smerinthus tiliæ_ length of body 63 mm.; of the silk glands 205 mm. _Bombyx mori_ length of body 56 mm.; of the silk glands 262 mm. _Antheræa yamamaya_ length of body 100 mm.; of the silk glands 625 mm. _Telea polyphemus_ length of body 60 mm.; of the silk glands 450 mm.

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.

FIG. 331.—Longitudinal section of the spinneret: _a_, horizontal
portion of the tongue; _b_, vertical portion; _c_, _f_, circle of
the tongue; _d_, tongue-pad; _e_, orifice of the spinneret; _g_,
body of the lyre; _h_, prebasilar membrane forming a fold; _i_,
internal canal of the spinneret; _k_, filator.—After Blanc.
]

FIG. 332.—The lower lip (labium) of _Bombyx mori_, isolated, seen from
the left side: _A_, lyre; _B_, spinneret; _C_, labial palpus; _D_,
vertical part of the labium; _E_, horizontal part of the same; _H_,
_L_, silk-canal; _K_, right gland of Filippi; _L_, canal of the left
gland; _N_, labial nerve; _a_, oblique fibre of the elevator of the
labium; _b_, right fibre of the same; _c_, depressor of the labium;
_d_, superior spinning muscles.
]

FIG. 333.—The labium in a horizontal position, seen from the side:
_f_, the filator or press situated under the external part of the
spinneret (_d_), between the branches (_b_), of the lyre (_a_); _e_,
labial palp; _c_, tongue.
]

FIG. 334.—Longitudinal section of the spinneret and press (_filator_):
_A_, filator or press; _B_, spinneret; _C_–_D_, body of the lyre;
_F_, lower part of the labium; _E_, common canal; _eh_, its
epithelium; _G_, superior muscle of the press; _a_, rachis; _b_, its
posterior enlargement; _c_, infundibulum; _d_, cuticle; _o_, orifice
of the spinning canal; _op_, central canal of the lyre and of the
spinneret; _fi_, hypodermis of the lyre; _f_, _f_, hypodermic pad of
the lyre.
]

The silken substance is then pressed by the more or less powerful contractions of the muscles of the filator, so that the passage of the threads is facilitated. If the muscles totally contract, the spinning canal is opened wide, the threads pass easily upwards and assume the form of a triangular prism (Fig. 336).

FIG. 335.—Spinning apparatus, seen from above: _A_, opening of the
spinneret; _B_, central canal of the spinneret (_C_); _D_, common
canal; _E_, canal of Filippi; _F_, excretory canal of a silk-gland;
_i_, orifice of the canal of Filippi’s gland; _l_, rachis; _k_, ring
of the infundibulum; _b_, _c_, _d_, _e_, _f_, cavity of the
different canals; _h_, spur which separates the two excretory
canals.—This and Figs. 331–334 after Blanc.
]

If this contraction diminishes, the chitinous wall of the spinneret comes together, owing to its elasticity; the ceiling of the canal approaches the floor; the cavity tends to take the form of a semicircular slit, and the threads are compressed, flattened. As each mass or thread of silk is much more voluminous than the canal, except when the latter is extremely dilated, it follows that the two threads are always compressed, or squeezed together, and that each of them is compelled to mould itself in the groove it occupies and to take its shape. Hence the variations in the appearance of the two masses or divided portions of silk, which as stated present all grades between the form of an isosceles-triangular prism and that of a nearly flat ribbon; but this last case is quite rare. The use of the spinneret, then, is to compress the thread and to change its form more or less considerably, at the same time as it diminishes its diameter.

FIG. 336.—Diagram of the press and its muscles: _a_, lower; _b_,
lateral; _c_, upper muscles of the press.—After Blanc.
]

Moreover, this constant compression of the thread as it passes through the press keeps it in a certain state of tension so as to allow the caterpillar while spinning to firmly hold its thread.

Finally, when the worm suspends the contraction of its spinning muscles, the press flattens, vigorously compresses the thread, and arrests its motion, in such a way that if there was a strain on the silken fluid (_bave_), it would break rather than oblige the caterpillar to let go any more of it.

The press does not act directly on the silken thread, but through the gummy layer (_grès_) which transmits over the whole surface of the silken fluid (_brin_) the pressure exerted on it. After having overcome this difficult passage, the silk thread has acquired its definite form; it rapidly passes out of the spinneret.

=How the thread is drawn out.=—Having seen, says Blanc, how the two masses of silk (_brins_), in passing through the spinning apparatus (or press), join each other, constituting the frothy silken fluid, thus becoming modified in form, it remains to examine the way in which the thread is drawn out of the spinneret. If we examine a caterpillar while spinning, it will be seen that in moving its head it draws on the frothy mass of silk fixed to the web of the cocoon. This traction certainly aids very much the exit of the thread, but it is not the only cause.

The silk, Blanc affirms, is pushed out by a force _a tergo_, developed by different agents, such as the pressure of the distended cuticle or the silky mass contained in the reservoir, as seen in the section of a worm which has spun its cocoon. But if we consider a caterpillar before it has begun to spin, it is difficult to explain the mechanism of spinning. As Blanc has often observed, in making sections of the heads of silkworms, two cases arise. Sometimes the worm has already spun a little, and a certain length of the frothy silk (_bave_) issues from the orifice of the spinneret, where it forms a small twisted bundle. At other times the worm has not spun since its last moult or the frothy mass of silk has broken within the head, and we find the end in the common tube. In the first of these two cases, the worm, dilating its press, is able by a general contraction to discharge a little of the gritty material (_grès_) which lines the ball of silk hanging at the end of the spinneret. It can also reject a certain quantity of the secretion of Filippi’s glands and thus soften the gritty substance. The little plug of silk can then adhere to the body with which it comes in contact.

In the same case it is necessary that the two bits or portions of silk traverse the press, and this normally has a calibre less than their diameter. The worm should then distend the spinning tube as much as is practicable, so as to make the openings as large as possible. It has been stated that the press is, in this condition, at least as large as the mass of frothy silk. This Blanc believes (although Gilson thinks otherwise) is pushed by a force _a tergo_, and reaches the funnel of the spinning canal; its two bits of silk (_brins_) unite there, penetrate into the canal itself, and, owing to successive impulses produced by the general contractions of the worm, press through and pass out of the spinneret.

While the silkworm is engaged in spinning its cocoon, the spinneret and press execute very varied movements, determined by the elevator, depressor, retractor, and protractor muscles of the labium, as well as those of the press. These movements, originally very numerous, may combine among themselves, so that the spinneret is susceptible of assuming during the process of spinning still more diverse positions.

FIG. 337.—Portion of the silk-gland of _Bombyx mori_: _p_, tunica
propria; _i_, tunica intima; _s_, secretion-cell with branched
nuclei; _a_, separate secretion-cell from the anterior part of the
silk-gland of _Amphidasis betularia_; _b_, the same of _Vanessa
urticæ_; _c_, the same in _Smerinthus tiliæ_.—After Helm.
]

FIG. 338.—_A_, section of gland of lepidopter: _B_, section of
silk-gland of a saw-fly larva; _n_, nucleus; _i.d_, canals; _d.s_,
cavity.—After Gilson.
]

Histologically the silk-glands are composed of three layers,—the outer, or _tunica propria_ (Fig 337); the inner, the _tunica intima_; the middle layer being composed of extraordinarily large epithelial cells which can be seen with the naked eye, and are also remarkable for the branched shape of the nuclei (_a_, _b_, _c_, 337), the branches being more or less lobed, and the larger the cells the more numerous are the branches of the nucleus. Gilson[53] finds that those of Trichoptera, Lepidoptera, Diptera, and Hymenoptera ordinarily consist of a small number of cells; and it is quite common, he says, to find only two cells in a transverse section (Fig. 338, _A_). In the Tenthredinidæ, however, “the organ still consists of a tube, the wall of which is composed of flat cells, but in addition to that, two series of spheroidal cells are attached to the sides. Each of these cells contains a system of tiny canals running through their cytoplasm (_B_, _i. d_). These cells are the secreting elements; they continually cast the silk substance into the tube.” A peculiarity of the _tunica intima_ is its distinct transverse striation.

FIG. 339.—Branching nucleus of spinning gland of Pieris larva.—After
Korschelt, from Wilson.
]

FIG. 340.—Filippi’s glands (_G_) isolated and seen from above: _e_,
_e_, its lobules; _d_, its excretory canal; _E_, silk-duct; _C_,
common canal; _c_, upper spinning muscle; _b_, lower muscle; _a_,
lateral muscle; _T_, spinneret.—After Blanc.
]

The lining of the glands and of their common duct is moulted when the caterpillar casts its skin, and this, as well as the mode of development, shows that the glands are invaginations of the ectoderm. Gilson finds that the silk-glands and silk-apparatus of Trichoptera are very similar to those of caterpillars, and that the silk is formed in the same way.

=Appendages of the silk-gland (Filippi’s glands).=—In most larvæ there is either a single or a pair of secondary glands which open into the spinning glands near their anterior end. They are outgrowths of the gland provided with peculiarly modified excretory cells or evaginations of the entire glandular epithelium. Those of _Bombyx mori_ (Fig. 340) are very well developed, and, according to Blanc, form two whitish, lobulated masses in the labium on each side of the common duct of the spinning gland. Externally they appear to be acinose; but their structure, as described by Blanc and by Gilson, is very peculiar. Helm thinks, with Cornalia, that the function of these glands is to secrete the adhesive fluid which unites the silk threads, and also to make the silk more adhesive in the process of spinning, but Blanc states that this is done before the thread passes into the common excretory tubes, and he is inclined to think that the secretion serves to lubricate the spinneret, and thus to facilitate the passage of the thread. On the other hand, in certain caterpillars these glands are situated quite far from the spinning apparatus.

The silk-glands in the pupa state undergo a process of degeneration,
and finally completely disappear. They are specific larval organs
evolved in adaptation to the necessity of the insect’s being
protected during its pupal life by a cocoon. (Helm.)

Morphologically the silk-glands are by Lang regarded as modified
coxal glands, and homologues of the setiparous parapodial glands of
chætopod worms, the coxal glands of Peripatus, and the spinning
glands of spiders.

In Scolopendrella, spinning glands are situated in the two last
segments of the body, opening out at the end of the cercopods (Fig.
15, _s.gl_), and the larvæ of the true Neuroptera (Chrysopa,
Myrmeleon, etc.) which spin cocoons, have spinning glands opening
into the rectum. The silk forming the cocoon of the ant-lion, as
Siebold and the older observers have stated, is secreted by the
walls of the rectal or anal sac. Siebold (Anatomy of the
Invertebrates, p. 445) states that in the larva of Myrmeleon, the
silk-apparatus is very remarkable, “for the rectum itself is changed
into a large sac and secretes this substance which escapes through
an articulated spinneret projecting from the opening of the
anus”[54] (Fig. 307, _e_). The larvæ of the Mycetophilidæ have
spinning glands at the hinder end of the body, as also the imago of
the female of the tineid moth Euplocamus. (Kennel.) The larvæ of
ichneumons, wasps, bees, of Cecidomyia, and other Diptera, spin
silken cocoons, but their glands have not yet been examined.

It should also be observed that during the process of pupation the
larvæ of butterflies, of certain flies (Syrphus), and beetles
(Coccinellidæ and some Chrysomelidæ) attach themselves by silk spun
from the anus, so that the pupa is suspended by its tail; such
glands are probably homogenetic with the coxal glands.

The silk in its fluid or soft state is mucilaginous, and according
to Mulder, in the silkworm consists of the following substances,
varying somewhat in their relative proportions by weight:

Silk-fibre material 53.67
Glue (Leim) 20.66
Protoplasm 24.43
Wax 1.39
Coloring matter 0.05
Fat and resin 0.10