DEFENSIVE OR REPUGNATORIAL SCENT-GLANDS
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.
Fig. 358.—Sternite of Machilis maritima, with the pair of coxal sacs (cb) on the right side everted; hs, coxal appendages; m, retractor muscles.—After Oudemans, from Lang.
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.
Fig. 359.—Under side of end of Aphlebia, showing the two eversible sacs; V-X, five last abdominal segments; A, portion showing the hairs; B, showing origin of a hair in its follicle.—After Krauss.
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.)
Fig. 360.—External flaps (gl) of glands of Platyzosteria.
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).
Fig. 361.—Glands (g) of Lachnus; h, “honey” wart.—Gissler del.
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.)
Fig. 362.—Median section through the femoro-tibial joint of leg of Coccinella, showing at o the opening through which the blood oozes out; f, femur; t, tibia; e, extensor muscle of the tibia: s, sinew of the same; at ch, chitinized; h, articular membrane; v, tibial process.—After Lutz.
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.
Fig. 363.—Freshly hatched larva of Hyperchiria io, with its two pairs of eversible glands (g).
Fig. 364.—Young larva of Megalopyge crispata, enlarged, showing the seven pairs of lateral processes (lp): sp, spiracle; abl′, abl6, six pairs of abdominal legs besides the anal pair.
In the Hemileucidæ there is a pair of lateral osmeteria, on the 1st and on the 7th abdominal segments, which however, are not highly colored (Figs. 363, 366, 10). In Megalopyge (Lagoa, Fig. 364) there is a lateral row of singular pale permanently everted processes which appear to be the homologues of the osmeteria of larvæ of other lepidopterous families. As these are repeated on seven segments, their metameric arrangement is obvious. The relation of these curious glands to the viscera is seen in Fig. 297, lgp, and their minute structure in Fig. 365.
At A, the lumen (l) is a deep narrow cavity, with the secretion (secr.), collected at the mouth of the cavity, composed of a thin, mucus-like, coagulated fluid, containing granules of varying degrees of fineness, which take the stain readily. Outside of these are collected fine nuclei (bc), stained dark, and enveloped in a slight, transparent, pale, protoplasmic envelope, which may be blood corpuscles. The glandular cells themselves are simply modified hypodermal cells, as seen at C. In some of the nuclei, indistinct nucleoli are seen, and deeply stained granules, especially around the periphery of the nuclei. At B is represented a section on one side of the middle, but still showing the spacious lumen. In the section represented by C, the knife passed through the process still nearer the outer edge, and near the base; at C1, three of the glandular cells, with their large, deeply stained nuclei, are drawn. A transverse section at D shows the large lumen or cavity (l).
As to the function and homologies of these structures, it is difficult to decide. We have never noticed that they give off any odor, though they may prove to be repugnatorial; they are not visible in the fully grown, living insect, being concealed by the long, dense hairs clothing the body; they are not spraying organs, as they are imperforate at the end, not ending as the lateral, eversible glands of Hyperchiria io, etc., in a crateriform orifice.
They may be permanently everted glands, or osmeteria, which have, by disuse, lost their power of retraction and their crateriform opening, as well as the power of secreting a malodorous fluid.
Fig. 365.—Section of lateral processes of larva of Megalopyge.
In certain of the butterflies, the Heliconidæ (Colænis, Heliconius, Euides, and Dione), there is thrust from the end of the abdomen a pair of large, irregular, rounded, eversible glands, which give out a disagreeable odor, and are consequently repellent, and which seem to be the homologues of the odoriferous glands of other butterflies.
The large, soft, rounded, eversible glands, looking like puff-balls or a rounded pudding (Fig. 366, 12), are everted, when the butterflies are roughly seized, from the dorsal side of the penultimate segment of the abdomen. The males possess two smaller tubercles on the inside of the anal claspers or lobes. Müller also detected, in the females of various species of the Heliconidæ enumerated above, a pair of club-shaped processes like the balancers of flies, which are thrust out on each side of and under the odoriferous puff-balls of the hinder edge of the penultimate segment (Fig. 366, 13). The club or head is armed with hairs or bristles, which, in Heliconius, are like the scales of a butterfly.
Fig. 366.—Scent-glands of insects: 1. Anal eversible glands of Eleodes.—After Gissler. 2. Anal eversible glands of Blaps.—After Gilson. 3. Anal glands (agl) of Carabus hortensis: rs, reservoir; d, excretory duct; i, intestine; r, rectum.—After Kolbe. 4. Prothoracic spraying apparatus of Cerura vinula: gl, the gland; d, its duct, with tænidia; t, the spraying tubes; m, muscles; rm, retractor muscles.—After Klemensiewicz. 5. The thoracic glandular sac of Macrurocampa marthesia: gl, the glandular sac; d, its duct; e, peritracheal epithelium; t, the spiral threads or tænidia. 6. Irregular separate masses of chitinous ridges on the cuticular lining of the wall of the sacs of Macrurocampa marthesia. 7. Osmeterium (os) of the larva of Papilio machaon at rest: rm, the retractor muscles at the ends; m, the numerous oblique muscles; dm, dorsal longitudinal muscles; t, trachea; oe, œsophagus; gang, brain; 1, head; 2, 3, 4. thoracic segments. 8. Osmeterium (os) of one side, enlarged: g, glandular portion at the base; d, depressions in the cuticula of the glandular portion; t, trachea.—This and Fig. 7 after Klemensiewicz. 9. Eversible dorsal glands (ev. gl) of larva of Orgyia leucostigma in Stage II: gc, glandular cells at bottom of the crater-like depression; m, retractor muscle; p, poison gland-cells of the root of the seta (s); c, cuticula; hyp, hypodermis; A, portion of the cuticle and hypodermis enlarged. 10. Lateral eversible gland of Hyperchiria io, Stage II: rm, retractor muscle; oen, œnocytes. 11. The same as Fig. 10, but representing a section through one side of the eversible gland. 12. A, end of body of Colænis julia; ev, eversible anal gland; oa, odoriferous appendages; B, the same in Heliconius apseudes, side view; C, odoriferous appendages of Colænis dido in fresh condition; D, tested with alcohol and benzine. 13. Odoriferous appendages of Heliconius eucrate, head cleansed.—Figs. 12, 13, after F. Müller. 14. Odoriferous glands (ogl) in the pupa of Vanessa io: r, rectum; h, the folds of hypodermis which forms the terminal papilla of the abdomen; ov, oviduct.—After Jackson.
In the caterpillars of certain blue butterflies (Lycænidæ) is an internal osmeterium, being a very minute sac which is everted from a transverse slit on the top of the 7th abdominal segment. Its function is quite the opposite of those of the caterpillars of other families, since the sac exudes a sweet fluid very attractive to ants, which may be diffused more widely by the delicate spinulose bristles crowning the summit. W. H. Edwards states that in several species of Lycæna, besides that on the 7th abdominal segment, there is on the 8th segment a pair of minute dorsal evaginable tubercles.
A pair of small ramose odoriferous glands are said by Siebold, who regarded them as alluring glands, to occur in Argynnis, Melitæa, and Zygæna, to be situated near the orifice of the oviduct, and Scudder has detected them near the anus of the female pupa of Danais archippus. The appearance of the odoriferous glands in the pupa of Vanessa io is well shown by Jackson (Fig. 366, 14). They develop as two tubular ingrowths of the hypodermis, perfectly distinct one from the other, each having its own separate aperture to the exterior. In Fig. 366, 14 the condition of parts is nearly as in the imago, the glands being situated below the rectum and opening of the oviduct. In both sexes of another Brazilian butterfly (Didonis biblis) on the median line of the abdomen between the 4th and 5th segments are two roundish vesicles covered with short gray hairs, which emit a disagreeable smell.
It is possible that the dark-green fluid in Parnassius, secreted by an evaginable gland, and which is moulded into shape by the scimetar-shaped peraplast (Scudder), is formed by the homologues of the anal glands of other butterflies.
Distribution of repugnatorial or alluring scent-glands in insects[[59]]
A. Larval Insects
a. Each thoracic segment; sternal. Phryganea grandis.
b. Prothoracic, sternal, discharging a lateral jet of spray; with a single large internal sack.