Bordas has found both the alkaline gland (gland of Dufour) and the acid gland to occur in a hundred species of Hymenoptera, including not only Aculeata, but also Ichneumonidæ (Fig. 348), Tenthredinidæ, and they may be safely said to be of general occurrence. The acid gland consists of three parts, the glandular portion, the reservoir for the poison, and the secretory canal. The alkaline gland is an irregular tube, with a striated surface and without a reservoir. In most Hymenoptera there is still a third gland, which is unpaired, granular, rectangular or lanceolate, with a short filamentous duct which opens beside the orifice of the alkaline glands.

The poison in ants, wasps, and bees consists of two substances, i.e. formic acid and a whitish, fatty, bitter residue in the secretion of the glands; the corroding active formic acid is the essential part of the poison. (Will.)

In Melipona the sting and poison-glands are aborted; in certain ants (Formica, Lasius, etc.) the sting is wanting, but the poison-sac is extraordinarily large.

Bordas finds in various species of Ichneumon three kinds of glands opening into the base of the sting. The first two correspond to the acid (Fig. 348, G.A) and alkaline (G.A) glands of bees and wasps (Vespidæ, etc.), and the third (G.ac) is situated between the two lateral muscular bundles which attach the base of the sting to the last abdominal segment. The poison-reservoir (Fig. 348, V) is recognized by its yellow color and diaphanous and striated appearance. It is situated on the left of the hind-intestine, a little in front of the rectum. The tubular gland (Ga) or alkaline gland of aculeate Hymenoptera is remarkably large; it is situated on the left side of the body. The accessory gland (G.A) is elongated, triangular, flat, its duct opening at the base of the alkaline gland; it is formed of small spherical cells. Bordas has met with well-developed poison-glands in forty species belonging to the Terebrantia, including that of Tenthredo, Emphytus, as well as various genera of Ichneumonidæ, but in all these species the accessory gland was wanting.

Under the name of cephalic glands (Fig. 349), Locy describes a pair of glands in the head of Nepidæ. The epithelial or secreting cells are 8–sided (Fig. 350). “When these insects are irritated,” he says, “a secretion is freely thrown out around the base of the beak, which produces death very quickly when introduced on a needle point into the body of an insect.” He infers that the cephalic glands may be the source of this poisonous secretion. The poisonous salivary fluid of the larva of Dyticus is referred to on p. 324.

That the mosquito injects poison into the wound it makes has been proved by Macloskie, who discovering fine droplets of a yellow oily-looking fluid escaping from the end of the hypopharynx, afterwards detected the poison-glands. It appears that the two salivary glands are subdivided, each into three lobes, the middle of which (Fig. 351, pg) differs from the others in having evenly granulated contents and staining more deeply than the others. Having examined the preparations, we agree with the discoverer that these lobes secrete the poison. The poison is diluted by the secretion of the salivary lobes, and the two efferent ducts, one from each set of glands, “carry forward and commingle the venomo-salivary products in the main duct; and the stream is then carried by the main duct to the reservoir at the base of the hypopharynx.”

f. Adhesive or cement-glands

Dewitz has discovered in ants and bees, in close connection with the poison-glands, and like them discharging their secretion through the sting, cement-glands. They arise by budding at the base of the poison-glands.

The two glands in these Hymenoptera correspond to the tubular glands of the Orthoptera, which open at the base of the inner sheath of the ovipositor (Fig. 299, sb), so that the secretion flows out through it as the poison of bees, etc., out of the sting. The use of the secretion of these glands is either to glue the eggs together, or to afford material for the egg-case of cockroaches and Mantidæ and the gummy egg-case of the locusts, etc. The contents of the cement-glands serves for the fixture of the eggs after deposition. In the stinging Hymenoptera one of the cement-glands is an accessory gland; the other becomes the poison-sac. The cement-glands are in the Hemiptera only short blind sacs, in the Lepidoptera and Diptera long convoluted tubes, tubular and branched in the Coleoptera, or richly branched in the Ichneumonidæ and Tenthredinidæ. In the cockroach there are two cement-glands, but the right one is probably of no functional importance. The left one is filled with a milky substance, containing many crystals and a coagulable fluid, out of which the egg-capsule (oötheca) is formed. (Miall and Denny.) In the locusts the sebific or cement-gland (Fig. 298, sb) secretes a copious supply of a sticky fluid, which is poured out as the eggs pass out of the oviduct and agglutinates the eggs into a mass, forming a thin coating around each egg, which from the mutual pressure of the eggs causes the tough coating to be pitted hexagonally. In other insects also (Trichoptera, Chrysopidæ, Lepidoptera, etc.) there are similar secretions for the protection or fastening of the eggs when laid.^[57] The Trichoptera lay their eggs either in or on the surface of the water in bunches or in strings or in annular gelatinous masses on stones or on plants. This jelly-like substance is secreted by two highly developed paired anal glands. (Weltner, in Kolbe, p. 621.) Also in certain dragon-flies (Libellula, Diplax, and Epitheca) the eggs are laid in jelly-like masses.

With a similar secretion, spun from the end of the abdomen, the Psocidæ cover their little bunches of eggs laid on the under side of leaves; and the silk thread forming the egg-sac of the great water-beetle (Hydrophilus) is secreted from such anal glands.

g. The wax-glands

Besides the honey-bee, which secretes wax in little scales on the under side of the abdomen, the bodies of many other insects, such as the plant and bark lice, as well as the Psyllidæ, Cicadidæ (especially Flata and Lystra), are covered with a waxy powder, or as in Chermes, Schizoneura, Flata, etc., with wool-like filaments of wax.

The wax is secreted by minute unicellular dermal glands, which in the lower insects (Hemiptera) are distributed nearly all over the body, but in the bees are restricted either to the under (Apis, Fig. 352) or upper side (Trigona) of the end of the abdomen.

The wax-glands of Pemphigus, Chermes, etc., lie under the little warts, seen in Lachnus strobi, the white-pine aphis, to be distributed in transverse lines across the back and sides of the abdominal segments (Fig. 353). These warts are surrounded by a chitinous ring, and divided into delicately marked areas. Through the delicate numerous pits in the chitinous membrane of these areas the little waxen threads project, since under each area ends a duct leading from a large glandular cell, which is a specially modified hypodermis cell (Claus). The wax threads are hollow, and all those arising from a single glued cell form a bundle, whose threads separate from each other and form a white woolly down or bloom covering the body. Witlaczil also shows that gall-forming Aphids secrete a wax-like substance, which, during the movements of the insects in the gall, is rubbed off, becoming a watery layer mixed with the fluid excrement, which forms a spherical impervious layer lining the gall, and thus rendering possible the mode of life of the gall-lice.

In the Psyllidæ Witlaczil has discovered wax-glands which also secrete slender waxen threads. They are situated in groups of two or three at the end of the abdomen near the anus, and arise from hypodermis cells. The wax threads surround the liquid excrement as it passes out of the vent, covering it with a continuous layer of wax. The excrement accordingly is discharged very slowly and gradually, in sausage-shaped masses slightly strung together and rolled into close spirals. The body becomes unavoidably smeared with the sticky excrement, since it is not entirely covered by the waxy layer. Moreover, in the larvæ of many Psyllidæ waxen threads are formed on the upper side of the abdomen; they are for the most part tightly curled or frizzly, like wool, and form, though partly torn, a waxen coat, chiefly on the side and back of the thorax and abdomen. The insects appear therefore as if covered with dust. The mature animals of many species are also covered with a waxen down. The wax threads rapidly dissolve and disappear in alcohol. From a wax-like substance more or less easily dissolved in alcohol arise peculiar hair-like structures which, in the larvæ of Psyllidæ, are situated on the side and end of the body and also on the rudiments of the wings. They are readily distinguished from ordinary hairs, as they arise from glandular cells, and are of very different lengths, more or less like bristles, but hollow, and very brittle. They are leaf-like in the first nymphal stages of Trioza rhamni, but in following stages become narrow and form a row around the entire periphery of the body.

The waxen dorsal shield which protects the body of bark-lice (Coccidæ) is a similar product.

Witlaczil has described the way it is formed in Aspidiotus and Leucaspis. The freshly hatched nymph shows no signs of a waxy secretion. But eventually waxen threads arise first on the hinder and anterior end of the body, and then over the whole surface. These threads interlace into a sort of felting and thus form the shield, which is usually much larger than the body and lies closely upon it. The shield is formed after the first moult. It is noteworthy that these threads are matted together to form as thick a tissue as that of the shield itself. The shield is whitish or gray and rather thin. On the thinnest part of the edge the single threads may be drawn out. The growth of the shield advances with the increase in size of the nymph around the entire edge, but is greatest behind. The first two larval skins are retained on the back under the shield. Also a very thin waxen pellicle remains on the resting place of the insect when it is raised. The wax-glands open in the pitted fields, and appear as clear brownish cells which are distinguished from the ordinary hypodermis cells by their greater size. (Witlaczil. Compare also Fig. 354.)

The wax-glands in the honey-bee are scale-shaped organs situated on the under side of the four last abdominal segments (Fig. 355). These secrete the wax, which appears as whitish scales, and secretion is only possible when the bees have sufficient honey and pollen. The wax is secreted by the hypodermal cells rather than by glands within the abdominal cavity; the wax traverses the cuticular layer, and accumulates on its outer surface (Carlet). According to Fritz Müller, in the stingless bees (Trigona) which he observed, the wax-glands are situated on the back of the abdomen, but Ihering states that in many species of Trigona and Melipona there are also slightly developed wax-organs on the ventral side.

It has been found that certain caterpillars secrete wax. Thus the cells of the Tortrix of the fir (Retinia resinella) formed of resin are lined with wax, as on dissolving away the resin with alcohol, Dr. Knaggs found a slight film of wax; also a secretion of wax has been detected in the larva of a butterfly (Parnassius apollo). The bodies of certain saw-fly larvæ are covered with a white powdery secretion, while the remarkable larva of a Selandria is clothed with snow-white, long, flocculent, waxy masses, nearly concealing the body (Fig. 356).

h. “Honey-dew” or wax-glands of Aphids

The so-called “honey-dew” of Aphids which oozes from two wart-like tubercles or tubes situated near the end of the body, is secreted by hypodermal unicellular glands which open into a modification of a pore-canal, the tube itself being an outgrowth of the cuticula.

Witlaczil states that both in the “honey” tubes and in the body beneath, the sugary matter exists in cells of the connective tissue in the form of granules. “These large ‘sugar-cells’ in contact with the air undergo destruction, while the sugar crystallizes into needles, and thus each cell is transformed into a radiated crystalline mass.”

“A muscle extends from a horseshoe-shaped place (a valve?) in the middle of the flat terminal plate of the honey tube, through this and down through the abdomen to the ventral surface. By this muscle the honey tube is at times erected, and we then find, as also when we lightly press the body of the insect, lumps of crystallized sugar which have been expressed through the tips of the honey tubes.” (Zool. Anzeiger, 1882, p. 241.)

Busgen, after careful research, denies that this is a sugar, but claims as the result of chemical analysis, that it is more like wax. He observed that on reaching the air the drops issuing from the “nectary” or “honey” tube stiffened almost instantly into a wax-like mass, which was easily crushed between the teeth, and had no taste at all. No sugar-like substance or urea could be detected. He therefore concludes that the secretion in question should be regarded as a wax-like mass, which agrees well with Witlaczil’s anatomical observations, and confirms the statements of previous observers. Thus, as early as 1815, Kyber stated that the Aphides expelled an excrementitious substance through the “sap tubes.” Burmeister states that the tubes give out a fluid which “dries gumlike, but, so far as I have observed, has no peculiar taste.” Réaumur, and also Kaltenbach, state that the “honey” does not issue from the tubes, but from the anus. Lastly, Forel emphatically states that “the two dorsal tubes of Aphides do not secrete a sweet fluid, but a gluey wax, which is not sought by the ants. Moreover the shield-lice and many leaf-lice have no such tubes, but yet are often sought by ants. The drops of sugar which the ants lick up are rather the excrement of the insects in question.” Hence the opinion first stated by Linné, that a sweet fluid is secreted by Aphides, must be abandoned.

On the other hand, Busgen, after careful observations, finds that the use of the sticky, waxen secretion is in reality a protective one, as he observed that when a larval Chrysopa rudely attacks the Aphides, they smear its face with the sticky wax, causing at least a momentary interruption in its attacks. He also observed that Aphides when invaded by coccinellid larvæ set their tubes in motion and besmear their heads and front part of the body. He thus seems to establish the fact that these tubes secrete a protective, sticky fluid.

i. Dermal glands in general

We have seen that certain of the hypodermal cells may be modified or specialized to form secretory unicellular glands. Such are those (trichogens) which secrete chitinous setæ, hairs, and spines, certain setæ in some insects being hollow and containing a poison (p. 187); others secrete wax, certain ones in Aphids “honey-dew”; in some cases dermal glands may excrete protective, sticky, or otherwise offensive matters, or may be depuratory, or facilitate the process of moulting.

There are other minute, unicellular, or compound dermal glands whose function is unknown.

Dermal glands may be segmentally or serially arranged. Thus Verson has detected a series of one or two pairs of unicellular glands near the stigmata in each thoracic, and the first eight abdominal segments of the silkworm (B. mori). In the earliest stages of growth of the caterpillar they give out oxalate of lime, and in later stages uric acid. They thus appear to act interchangeably with the urinary tubes, as excretory organs. They do not, however, carry their products directly outwards, but leave them between the hypodermis and cuticula, in order to facilitate the sloughing off of the latter in the process of moulting.

LITERATURE ON THE SECRETORY GLANDS

a. General

Sirodot, S. Recherches sur les sécrétions chez les Insectes. (Ann. sc. nat., 4 Sér., Zool., 1858, x, pp. 141–189, 251–328, 12 Pls.)

Gazagnaire, G. Des glandes chez les Insectes. (Compt. rend. Acad. Sc., Paris, 1886, cii, pp. 1501–1503; Annal. Soc. Ent. France, 1886, Bull., pp. 104–106.)

Leydig, F. Beitrage zur Anatomie und Histiologie der Insekten. 1887.

Hanow, Karl. Ueber Kerfabsonderungen und ihre Benutzung im eigenen Haushalte. (Programm des Realprogymnasiums zu Delitzsch für das Schuljahr 1889, xc; Delitzsch, 1890, pp. 3–22.)

Verson, E. Di una serie di nuovi organi escretori scoperti nel filugello. (Publ. R. Stazione Bacologica di Padova, v, 1890, pp. 30. 4 Pls.)

—— Altre cellule glandulari di origine postlarvale. (Ibid., vii, 1892, pp. 16, 1 Pl.)

—— ed E. Bisson. Cellule glandulari ipostigmatiche nel Bombyx mori. (Ibid., vi, 1891.)

Borgert, H. Die Hautdrüsen der Tracheaten. Jena, 1891, pp. 80.

Batelli, Andrea. Di una particolarità nell’ integumento dell’ Aphrophora spumaria. (Monitore Zool. Ital., 1891, Anno ii., pp. 30–32, dermal gland in last segment.)

Koschewnikow, G. A. On a new compound dermal gland found in the sting of the bee. (Journal of the Zoological Section of the Society of the Friends of Natural Science. Moscow, ii, Nos. 1, 2, 1892, p. 36. Preliminary notice. In Russian.)

Willem, V., et H. Salbe. Le tube ventral et les glandes céphaliques des Sminthurus. (Ann. Soc. Ent. Belg., 1897, xli, pp. 130–132.)

Henseval, Maurice. Les glandes à essence der Cossus ligniperda. (La Cellule, 1897, xii, pp. 19–26, 27, 29.)

—— Recherches sur l’essence der Cossus ligniperda. (La Cellule, 1897, xii, pp. 169–181, 183.)

b. Poison-glands

Macloskie, George. The poison-apparatus of the mosquito. (Amer. Nat., xxii, 1888, pp. 884–888. 1 Fig. Also in Science, 1887, p. 106.)

Beyer, Otto W. Der Giftapparat von Formica rufa, ein reduziertes Organ. (Jena. Zeitschr. f. Wissens, xxv, 1891, pp. 26–112, 2 Taf.)

Bordas, L. Sur l’appareil venimeux des Hyménoptères. (Comtes rend., cxviii, 1894, pp. 296–299 and 873–874; also Zool. Anzeiger, xvii Jahrg., 1894, pp. 385–387, Figs.)

—— Appareil glandulaire des Hyménoptères. (Glandes salivaires; Tubes de Malpighi et glandes venimeuses.) Paris, 1894, pp. 362, 11 Pls.

—— Description anatomique et étude histologique des glandes à venin des insectes Hyménoptères. Paris, 1897, pp. 53, 2 Pls.

See Forel (p. 186); also the standard authors, Kolbe, etc.; also Locy (Amer. Nat., xviii, 1884, p. 355), Nagel (p. 324), Fenger.

c. Wax-glands

Brandt und Ratzeburg. Medicinische Zoologie, ii, 1830, p. 179. Taf. xxv, Fig. 18.

Treviranus, George R. Ueber die Bereitung des Wachses durch die Bienen. (Zeitschrift für Physiologie, etc., iii, 1832, pp. 62, 225.)

Dufour, L. Note anatomique sur la question de la production de la cire des abeilles. (Comptes rend. Acad. Sc., Paris, 1843, xvii, pp. 809–813, 1248–1253; Revue Zool. 1843, l’Institut, 1843, xi.)

Dujardin, F. Mémoire sur l’étude microscopique de la cire, etc. (Ann. Sc. Nat., xii, 1849, pp. 250–259.)

Tarzione-Tozzetti, H. Studii sulle cocciniglie. Milano, 1867, 7 Pls.

—— Sur la cire qu’on peut obtenir de la cochenille du figuier (Coccus caricæ). (Comptes rend. Acad. Sc., Paris, lxv, 1867, pp. 246–247.)

Claus, C. Ueber die wachsbereitenden Hautdrüsen der Insekten. (Sitzungsber. Gesells. z. Beförd. d. Gesammt. Naturw. zu Marburg, June, 1867, No. 8, pp. 65–72.)

Witlaczil, E. Die Anatomie der Psylliden. (Zeitschr. wissens. Zool., xlii, 1885, pp. 582–586.)

—— Zur Morphologie und Anatomie der Cocciden. (Zeitschr. wissens. Zool., xliii, 1886, pp. 149–174, 1 Taf.)

Ihering, H. von. Der Stachel der Meliponen. (Ent. Nachrichten, xii Jahrg., 1886, p. 185.)

Carlet, G. Sur les organes sécréteurs et la sécrétion de la cire chez l’Abeille. (Comptes rendus, cx, pp. 361–363, 1890.)

—— La cire et ses organes sécréteurs. (Le Naturaliste, 1890, pp. 149–151, 2 Figs.)

Also the works of Siebold, Cheshire, Kolbe, Howard and Marlatt (Bull., 3, N. S., Div. Ent. U. S. Dept. Agr., 1896, p. 40), Knaggs, Berlese.

d. Wax-like glands of Aphides

Huber et Forel, A. Études myrmécologiques, 1875.

Witlaczil, E. Zur Anatomie der Aphiden. (Zool. Anzeiger, v Jahrg., 1882, pp. 239–241; Arbeiten a. d. Zool. Institut der Univ. Wien., iv, 1882, pp. 397–441, 3 Taf.)

Busgen, M. J. Der Honigtau. Biol. Studien an Pflanzen u. Pflanzenlause. (Jena. Zeitschrift, xxv, 1891, pp. 339–428.)

While these eversible glands are not found in marine or aquatic arthropods such as Crustacea or Merostomata (Limulus), they are often present in the air-breathing forms, especially insects. In the winged insects they are of frequent occurrence, existing under great variety of form, varying greatly in position, and appearing usually to be in immediate relation with their active volant habits. Their presence is in direct adaptation to the needs and habits of their possessors, and being repellent, warning, or defensive structures, the odors they secrete being often exceedingly nauseous, they appear to have been called into existence in direct response to their biological environment. The fact that these singular organs do not exist in marine or aquatic Crustacea suggests that the air-breathing, aërial, or volant insects by these eversible glands, usually in the form of simple evaginable hypodermic pouches, are enabled to protect themselves by emitting an infinitesimal amount of an offensively odorous fluid or ether-like spray which charges the air throughout an extent of territory which may be practically illimitable to the senses of their enemies. The principle is the same as in the mephitic sulphuretted oil ejected by the skunks, the slight quantity these creatures give out readily mixing with and charging the atmosphere within a radius of many miles of what we may call the centre of distribution.

As is now well known, the very delicate, attenuated highly volatile odors exhaled are perceived by insects with extreme ease and rapidity, the degree of sensitiveness to such scents being enormously greater than in vertebrates, their organs of sense being developed in a corresponding degree. Professors Fischer and Penzoldt, of Erlangen, have recently established the fact that the sense of smell is by far the most delicate of the senses. They find that the olfactory nerve is able to detect the presence of 1
2,760,000,000 of a grain of mercaptan.^[58] The smallest particle of matter that can be detected by the eye is sodium, when observed by the spectroscope, and this particle is 250 times coarser than the particle of mercaptan which can be detected by the human nose.

In those Arachnida which are provided with poison-glands, these scent-glands are absent, but in certain Acarina and Linguatulidæ, which have no poison-glands, there are various oil-glands, stigmatic glands, as well as scent-glands, and in seizing a Thelyphonus with the forceps we have observed it to send out from each side of the body a jet of offensive spray.

We not infrequently find in myriopods (Polydesmidæ, Julidæ, and Glomeris) repugnatorial or the so-called cyanogenic glands, which are either paired, opening on the sides of the body, or form a single row along the median line of the under side of the body. Leidy describes and figures the spherical glands of Julus marginatus, of which there are 50 pairs. These glands have been regarded as modified nephridia, but are more probably coxal glands, and the homologues of the parapodial glands of annelid worms.

Eversible coxal glands.—True coxal glands occur in Scolopendrella immaculata on the 2d to 11th segment, on the inner side of the base of the legs (Fig. 15, c.g.). Homologous glands also occur in the same position in Campodea staphylinus (also in C. cookei and C. mexicana) on the 1st to 8th abdominal segments, and Oudemans has described a pair of eversible sacs on each side of segments one to seven of Machilis. These eversible sacs in the synapterous insects are evidently modified coxal glands, and are probably repugnatorial as well as respiratory in function.

The apparatus consists of an eversible gland, composed of hypodermic cells, usually retracted by a slender muscle and with an efferent passage, but the glands vary greatly in shape and structure in different insects. In some cases these fœtid glands appear not to be the homologues of the coxal glands, but simply dermal glands.

These repugnatorial glands are of not infrequent occurrence in the lower or more generalized winged insects, and in situation and appearance are evidently the homologues of the coxal glands of the Symphyla and Synaptera.

Fœtid glands of Orthoptera.—In the ear-wigs (Forficula and Chelidura) Meinert has detected a pair of what he calls fœtid glands at the posterior margin of the dorsal plates of the 2d and 3d abdominal segments.

Vosseler also describes the same glands as consisting of a retort-shaped sac, in whose walls are numerous small hypodermal cells and large single glandular cells provided with an efferent passage, the fluid being forced out by the pressure of the dermal muscles, one acting specially to retract the gland. The creature can squirt to a distance of 5 and even 10 cm. (4 inches) a yellowish-brown liquid or emulsion with the odor of a mixture of carbolic acid and creosote.

The large eversible dorsal glands of the Blattidæ, since they contain numerous hairs, which, when everted, are fan-like or like tufts, serve, as in the spraying or scent apparatus, to disseminate the odor, and might be classified with the alluring unicellular scent-glands or duftapparat of other insects, as they are by some authors; but as the glands are large and compound they may prove to be the homologues of the coxal glands rather than of the dermal glands.

Evaginable organs in the Blattids were first observed by Gerstæcker in both sexes of Corydia; they are yellowish white, covered with hairs, and are thrust out from between the dorsal and ventral plates of the 1st and 2d abdominal segments.

In the cockroach (P. orientalis) Minchin detected two pouch-like invaginations of the cuticle, lying close on each side of the middle line of the body between the 5th and 6th tergites of the abdomen. They are lined by a continuation of the cuticle, which forms, within the pouches, numerous stiff, branched, finely pointed bristles, beneath which are a number of glandular epithelial cells. In the male nymph of P. decorata he also found beside these glandular pouches “an additional gland, opening by a tubular duct under the intersegmental membrane between the 5th and 6th terga above the glandular pouch of each side, and extending forward into the body cavity. The gland and its duct are proliferations of the hypodermis, and there is no invagination of the cuticle.” These eversible glands are most complicated in Phyllodromia germanica. While it is absent in the female, in the male it is relatively of enormous size, extending over the 6th and 7th somites, as well as projecting far into the body cavity (Minchin). Haase states that these glands become everted by blood-pressure and give out the well-known disagreeable smell of these insects. He states that in the male of P. germanica the dorsal glands in the 6th and 7th abdominal segments are without hairs and produce an oily secretion; they function as odoriferous organs in sexual union.

In the male of another Blattid (Aphlebia bivittata) of the Canary Islands, Krauss has detected two yellowish dorsal sacs 1.5 mm. in length, opening out on the 7th abdominal segment, and filled full of long yellowish hairs, the ends directed towards the opening, where they form a thick tuft. These eversible glands lined with hairs appear to be closely similar to the long slender eversible hairy appendages or scent organs of certain Arctian and Syntomid moths. (Fig. 359.)

We have found the external median wart with lateral lids or flaps in between the 5th and 6th tergites of Platyzosteria ingens Scudder, a large wingless Blattid living under the leaf scars of the cocoanut tree in Southern Florida (Fig. 360), but were unable to detect them in Polyzosteria or in the common Blabera of Cuba, or in another genus from Cordova, Mexico.

In another group of Orthoptera, the Phasmidæ, occur a pair of dorsal prothoracic glands, each opening by a pore and present in both sexes. In the walking-stick, Anisomorpha buprestoides, ♂ and ♀, these openings are situated on each side of the prothorax at its upper anterior extremity, situated at the bottom of a large deep pit. When seized it discharged a “milky white fluid from the pores of the thorax, diffusing a strong odor, in a great measure like that of the common Gnaphalium or life everlasting” (Peale in Say’s American Entomology, i, p. 84). Boll states that the females when captured “spurt from the prothorax, somewhat after the manner of bombardier beetles, a strong vapor, which slightly burnt the skin; when the females were seized by the males a thick fluid oozed from the same spot.” Scudder describes these glands in another Phasmid (Autolyca pallidicornis) as two straight, flattened, ribbon-like bodies, with thick walls, broadly rounded at the end, lying side by side and extending to the hinder end of the mesothorax. In Anisomorpha buprestoides the glands are of the same size and shape (Scudder). In Diapheromera femorata the repugnatorial foramina are very minute, and the apparatus within consists of a pair of small obovate or subfusiform sacs, one on each side of the prothorax, about 1 mm. in length, with a short and very slender duct opening externally at the bottom of the pit (Scudder).

In the Mantidæ these seem to be genuine coxal glands, as there is a pair situated between the coxæ of the first pair of legs. An evaginable organ like a wart, with a glandular appearance, occurs on the hind femora of the Acrydiidæ in a furrow on the under side, into which the tibia fits, about one-fourth from the base (Psyche, iii, p. 32).

In the male cricket, the anal odoriferous glands are small lobes opening into a reservoir on each side of the rectum (Dufour). Homologous glands also occur in the Coleoptera (Fig. 302, l and 317, s).

Most Hemiptera or bugs send out a fœtid or nauseous odor due to a fluid secreted by a single or double yellow or red pear-shaped gland, situated in the middle of the mesothoracic segment, and opening between the hinder or third pair of coxæ. In Belostoma Leidy describes these glands as consisting of two rather long cœcal tubes situated in the metathorax, beneath the other viscera, extending backwards into the abdomen, and opening between the coxæ of the third pair of legs. Locy states that the smell arising from these glands is pleasant, resembling that of well ripened pears or bananas. Other bugs, moreover, emit an agreeable odor, that of Syromastes resembling that of a fine bergamot pear. (Siebold.) The fluid given out by the European fire-bug (Pyrrhocoris apterus) has a sweetish smell, like ether. In the nymph there are three pairs of dorsal glands, on abdominal segments 2–5, which are atrophied in the mature insect. In the bed-bug, the nymph has three odoriferous glands each with paired openings in the three basal abdominal segments respectively, and situated on the median dorsal line, being arranged transversely at the edge of the tergites; but after the last moult these are aborted, and replaced by the sternal metathoracic glands (Künckel). Gissler has detected a pair of glands in Lachnus strobi (Fig. 361).

Anal glands of beetles.—Certain beetles are endowed with eversible repugnatorial glands. Eleodes gigantea and E. dentipes of both sexes are said by Gissler to possess these glands. When teased “they stand on their anterior and middle legs, holding the abdomen high up and spurting the contents of the glands right and left.” The glands (Fig. 366, 1) are two reddish brown, somewhat bilobed sacs, and extend from the base of the last up to the middle of the 2d abdominal segment, with an average length of 6.5 mm. The liquid stains the human skin, has an acid reaction, with a peculiar, “intensely penetrant odor, causing the eye to lachrymate. It is soluble in water, alcohol, and ether. Boiled with concentrated sulphuric acid and alcohol an ethereal aromatic vapor is produced, indicating the presence of one or more organic acids, though neither formic or acetic acid could be detected.” Williston has observed the same habits in seven other species of Eleodes, all ejecting a pungent vile-smelling liquid, one species (E. longicollis) ejecting a stream of fluid from the anal gland, backwards sometimes to the distance of 10 cm. or more, and he regards these beetles as “the veritable skunks of their order.” Leidy briefly describes the odoriferous glands of Upis pennsylvanica.

The anal glands consist, according to Meckel and also Dufour, of two long, simple, flexuous cœca with reservoirs having two short excretory ducts situated near the anus (Siebold).

Glands like those of Eleodes found in Blaps mortisaga are described in detail by Gilson (Fig. 366, 2). They form two pouches or cuticular invaginations situated in the end of the abdomen on the sides of the end of the intestine and unite on the median line underneath the genital organs, forming a very short tube with a chitinous wall, continuous with the cuticle of the last abdominal segment. Into each pouch open a large number of fine slender lobules varying in shape, giving a villous aspect to the surface. These lobules are composed of as many as fifty unicellular glands, each of which is composed of four parts: (1) A radiated vesicle, (2) a central sac, giving rise (3) to a fine excretory tube, and (4) a sheath near the origin of the excretory tube. These are all modifications of the cytoplasm of the cell with its reticulum; the nucleus with its chromosomes is also present, but situated on one side of the central sac. The fine excretory tubules form a bundle passing down into the mouth of each lobule.

Similar glands, though usually smaller, which have not been carefully examined, occur in Carabus (Fig. 300, 3) and Cychrus, which eject from the vent a disagreeable fluid containing butyric acid (Pelouse). The bombardier beetle Brachinus, with its anal glands, ejects a jet of bluish vapor accompanied with a considerable explosion, which colors the human skin rust red; it is caustic, smells like nitrous acid, and turns blue paper red. Westwood states that individuals of a large South American Brachinus on being seized “immediately began to play off their artillery, burning and staining the flesh to such a degree that only a few specimens could be captured with the naked hand, leaving a mark which remained for a considerable time.” The fluid ejected by another species, in Tripoli, blackened the fingers of the collector. “It is neither alkaline nor acid, and it is soluble in water and in alcohol.” (Kirby and Spence, iv, p. 149.)

Species of other genera (Agonum, Pheropsophus, Galerita, Helluo, Paussus, Ozæna) are also bombardiers, though less decidedly so than Brachinus. A Paussid beetle (Cerapterus) ejects explosively a fluid containing free iodine (Loman), while Staphylinus, Stenus, Ocypus olens, Lacon, etc., have similar anal fœtid glands, the liquid being more or less corrosive. The secretion of Mormolyce phyllodes is so corrosive that it is said to paralyze the fingers for 24 hours after. (Cuénot.)

The two pairs of remarkably large, soft, eversible, forked, orange-yellow glands of the European genus Malachius, are thrust out from the side of the 1st and the 3d thoracic segments. They are everted by blood-pressure, and retracted by muscles. The larva of Hydrophilus piceus ejects by the anus a black, fœtid fluid.

Claus has shown that the larva of Lina populi and other Chrysomelidæ possess numerous minute, eversible glands in each of the warts on the upper surface of the body, each gland containing a whitish, repellent fluid smelling like the oil of bitter almonds and containing salicylic acid derived from its food-plant, which issues as pearl-like drops. Candèze thinks the fluid may contain prussic acid. The fluid is secreted by a variable number of glandular cells, each provided with an efferent duct. The larvæ of saw-flies, notably Cimbex americana, also eject droplets of a clear fluid from non-evaginable glands situated near each stigma (Chlolodkovsky).

The blood as a repellent fluid.—In this connection it may be mentioned that though there are no special glands present, many beetles emit drops of blood from the femoro-tibial joints of their legs as a means of defence. Such are the oil-beetles (Meloë), Cantharis, Lytta. The cantharadine secreted by these beetles, according to Beauregard, is an efficient means of defence, as birds, reptiles, and carnivorous insects will not usually attack them. This substance is formed in the blood and also in the genital organs, and is so extremely caustic that scavenger insects which feed upon their dead bodies will leave untouched the parts containing cantharadine, and if May-beetles or mole-crickets are washed with the blood of Meloë or with cantharidate of potassa, it will for several days render them safe from the attacks of the carabids which usually prey upon them. The eggs even after deposition are strongly vesicant, and are thus free from the attacks of egg-eating insects (Cuénot). The Coccinellidæ are also protected by a yellow, mucilaginous, disagreeable fluid oozing out of the ends of the femora; in our common, two-spotted lady-bird (C. bipunctata) the yellow fluid is disagreeable, smelling like opium. Lutz has found that the blood in Coccinellidæ passes out through a minute opening situated at the end of each femur (Fig. 362). The blood is very repellent to insectivorous animals.

The Dyticidæ eject from the anus a colorless, disagreeable fluid, while these beetles, and especially the Gyrinidæ, when captured send out a milky fluid which appears to issue from the joints of the body. The Silphidæ throw out both from the mouth and vent a fœtid liquid with an ammoniacal odor. They possess but a single anal gland, the reservoir opening on one side of the rectum (Dufour).

Other malodorous insects have not yet been investigated; such are the very persistent odors of lace-winged flies (Chrysopa).

More agreeable secretions, but probably formed by similar glands, is the odor of rose or hyacinth given out by Cicindelæ, or the rose fragrance exhaled by the European Aromia moschata.

Eversible glands of caddis-worms and caterpillars.—Gilson, while investigating the segmentally disposed thoracic glands of larval Trichoptera, has found in the larva of Limnophilus flavicornis that the sternal prothoracic tubercle gives exit to an underlying tubular gland. In Phryganea grandis each thoracic sternum affords an exit to an eversible gland. Many caterpillars, as our subjoined list will show, are very well protected by eversible repugnatorial glands situated either in the under or upper side of the body. Since the time of De Geer (1750) the fork-tailed larva of Cerura has been known to throw out a secretion, which was described by Bonnet in 1755 as a true acid, sharp, sour, and biting. This spraying apparatus in Cerura (Harpyia) vinula has been well described by Klemensiewicz (Fig. 366, 4), though Rengger in 1817 noticed the general form of the secretory sac, and that it opens out in two muscular eversible tubes, out of which the secretion is ejected.

The fork-tailed larva of Macrurocampa marthesia, which is much like that of Cerura, when teased sends out a jet of spray to the distance of nearly an inch from each side of the neck. While examining the very gayly-colored and heavily-spined caterpillars of Schizura concinna we observed that when a fully-grown one was roughly seized with the forceps or fingers it sent out a shower of spray from each side of the prothoracic segment, exactly like that of Cerura and Macrurocampa.

In the European Cerura vinula the apparatus consists of a single sac, which opens by a narrow transverse slit on the under side of the neck, out of which is rapidly everted four lateral tubes, two on each side (Fig. 366, 4, t), which are withdrawn within the opening by the contraction of several fine muscles. The apparatus in the American C. multiscripta is as in the European C. vinula. In a living specimen the large secretory sac was seen to be of the same size and shape as in Macrurocampa, and of the color of raw silk. The sac when distended extends back to a little behind the middle pair of legs, and is nearly two-thirds as wide as the body. The caterpillar sent out the fluid when handled, but we could not make it spray.

In the larva of Macrurocampa marthesia the cervical or secretory gland (Fig. 366, 5) is situated in the 1st and 2d thoracic segments, extending to the hinder edge of the latter and lying between the nervous cord and the œsophagus and proventriculus, and when empty the bulk of it lies a little to one side of the median line of the body. It is partly held in place by small tracheæ, one quite large branch being sent to it from near the prothoracic spiracle. The short, large duct, leading from it to the transverse opening in the membrane between the head and prothoracic segment, is a little narrower than this opening, and is kept distended by tænidia, or a series of short, spiral threads which are pale, not honey-yellowish, in color. This duct lies on one side of the prothoracic ganglion, resting just under the commissures passing up to the brain; it is also situated between the two silk ducts.

The very distensible sac (Fig. 366, 5) is rendered elastic by a curious arrangement of the cuticle, the tænidia of the duct itself being represented by very thickly-scattered, irregular, separate, sinuous, chitinous ridges, which stand up from the cuticular lining of the wall of the sac (Fig. 366, 6). The secretory cells of the walls of this sac in Cerura vinula are said by Klemensiewicz to be large hexagonal cells, resembling those of silk-glands, having like them large branched nuclei.

The fluid thrown out is said by Poulton to be formic acid; it causes violent effervescence when allowed to fall upon sodium-bicarbonate, and colors blue litmus paper red. It also appears from the researches of Latter that these creatures in the imago state secrete free potassium hydroxid, a substance for the first time known to exist in the animal kingdom.

In the caterpillar of Astyanax archippus (Limenitis disippus) a dark, bladder-like sac is everted, but the lateral tubes appear to be wanting, and no spray is sent out; it occurs in the larvæ of many Nymphalidæ and other butterflies and moths.

These glands are functionally active in Perophora, but obsolete (at least the external openings) in Lacosoma.

The osmeterium in Papilio larvæ.—The caterpillars of the swallowtailed butterflies (Papilio, Doritis, and Thais), as is well known, when irritated thrust out from a transverse slit on the upper part of the prothoracic segment a large orange-yellow V-shaped fleshy tubular process (the osmeterium), from which is diffused a more or less melonlike but disagreeable, in some cases insufferable, odor; the secretion is acid and reddens litmus paper. The mechanism has been described and figured by Klemensiewicz.

When at rest, or retracted, the osmeterium lies in the upper part of the body in the three thoracic segments, and is crossed obliquely by several muscular bundles attached to the walls of the body, and by the action of these muscles the evagination of the osmeterium is strongly promoted. After eversion the tubes are slowly retracted by two slender muscles inserted at the end of each fork or tube, and arising from the sides of the 3d segment behind the head, crossing each other in the median line (Fig. 366, 7 r.m.). The secretion is formed by an oval mass of glandular cells at the base of the forks; in the glandular mass is a furrow-like depression about which the secretory cells are grouped. The secretion collects in very fine drops on the side of each furrow opposite the glandular cells.

According to C. D. Ash the larva of an Australian Notodontid (Danima banksii Lewin) protrudes from the under side of the prothoracic segment a Y-shaped red organ like that of Papilio; no fluid or odor is given out.

Dorsal and lateral eversible metameric sacs in other larvæ.—The showy caterpillars of Orgyia and its allies have a conspicuous coral-red tubercle on the back of the 6th and also the 7th abdominal segment, which on irritation are elongated, the end of the tubercles being eversible. When at rest the summit is crateriform, but on eversion the end becomes rounded and conical. These osmeteria are everted by blood pressure, and retracted by a muscle. Fig. 366, 9, represents a section of an osmeterium of Orgyia leucostigma when retracted by the muscle (m); at the bottom of the crater are the secreting or glandular cells (gc), being modified hypodermal cells. These doubtless serve as terrifying organs to ichneumons and other insect enemies, and though we have been unable to detect any odor emanating from the tubercles, yet possibly they give out a scent perceived by and disagreeable to their insect assailants.