Chapter XIX: Part I: Morphology and Physiology (15)

LITERATURE ON THE SPINNING GLANDS

=Helm, E.= Anatomische und histiologische Darstellung der
Spinndrüsen der Schmetterlingsraupen. (Zeitschr. f. wissens. Zool.,
xxvi, 1876, pp. 434–469, 2 Taf.)

=Lidth de Jeude, Th. W. van.= Zur Anatomie und Physiologie der
Spinndrüsen der Seidenraupe. (Zool. Anzeiger, 1878, pp. 100–102.)

=Engelmann, W.= Zur Anatomie und Physiologie der Spinndrüsen der
Seidenraupe. (Onderz. Phys. Lab. Utrecht, iii, 1880, pp. 115–119.)

=Joseph, G.= Vorläufige Mitteilung über Innervation und Entwickelung
der Spinnorgane bei Insekten. (Zool. Anzeiger, 1880, pp. 326–328.)

=Poletajew, N.= Ueber die Spinndrüsen der Blattwespen. (Zool.
Anzeiger, 1885, pp. 22–23.)

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

=Blanc, Louis.= Étude sur la sécrétion de la soie et la structure du
brin et de la bave dans le _Bombyx mori_. Lyon, 1889, pp. 48, 4 Pls.

—— La tête du _Bombyx mori_ à l’état larvaire. Anatomie et
physiologie. (Extrait du volume des Travaux du Laboratoire d’Études
de la Soie. Années 1889–1890, Lyon, 1891, pp. 180, 95 figs.)

=Gilson, G.= Recherches sur les cellules sécrétantes. La soie et les
appareils séricigènes: I. Lépidoptères. (La Cellule, 1890, vi, pp.
115–182, 3 Pls. I, Lépidoptères (suite); II, Trichoptères. Ibid., x,
pp. 71–93, 1893, 1 Pl.)

=Garman, H.= Silk-spinning dipterous larvæ (Science, xx, 1893, p.
215).

Also the writings of Meckel, Pictet, Duméril, Klapálek,
Wistinghausen, Loew, Hagen, Fritz Müller, Kolbe, McLachlan, de
Selys-Longchamps.

_c._ The cæcal appendages.

These diverticula of the mid-intestine (“stomach”) are appended to the anterior end, and in the living, transparent larva of Sciara, which has two large, long, slender cœca (Fig. 341), the partly digested food may be seen oscillating back and forth from the anterior end of the stomach into and out of the base of each cæcum. In the Locustidæ (Anabrus, Fig. 299) and Gryllidæ (Fig. 344, _e_) there are two large, short cæca, and in the locusts (Caloptenus) there are six cæca, while cockroaches have eight. In the Coleoptera (Carabidæ and Dyticidæ) these large cæca appear to be replaced by very numerous slender, minute villi or tubules, which arise from the anterior part of the stomach (Figs. 317, _r_, also 342).

These cæca differ in structure from the stomach, as shown by Graber, as well as by Plateau and by Minot. The latter states that a single transverse section of one of the diverticula of the locust demonstrates at once that its structure is entirely different from that of the stomach.

FIG. 341.—Larva of Sciara: _s.gl_, salivary gland; _ur.t_, urinary
tubes; _i_ intestine; _st_, stomach; _cae_ cæcal appendages; _t_,
testis.
]

Its inner surface is thrown up into longitudinal folds, generally
twelve in number. These folds shine through the outer walls, and are
accordingly indicated in the drawings of Dufour, Graber, and others.
The entire cæcum has an external muscular envelope, outside of which
are a few isolated longitudinal muscular bands. The folds within are
formed mainly by the high cylindrical epithelium which lines the
whole interior of the cavity. Tracheæ ramify throughout all the
layers outside the epithelium. There are appearances of glandular
follicles in the bottom of the spaces between the folds. (Minot.)

Burmeister supposed that these cæca were analogous to the pancreas, and this view has been confirmed by Hoppe Seyler, Krukenberg, Plateau, and others, who claim that the digestive properties of the fluid secreted in them agrees with the pancreatic fluid of vertebrates.

FIG. 342—Cross-section of mid-intestine of _Acilius sulcatus_, showing
the arrangement of the cæca, two tracheæ passing into each
cæcum.—After Plateau.
]

_d._ The excretory system (urinary or Malpighian tubes)

The excretory matters or waste products of the blood tissue of worms are carried out of the body by segmentally arranged tubes called _nephridia_. As a rule they arise in the blood sinuses of the body and open externally through minute openings in the skin. As there is a pair to each segment (in certain oligochete worms two or three pairs to a segment), they are often called segmental organs. In the annulate worms each segment of the body, even the cephalic or oral segment, originally contains a pair of these excretory organs. These vessels may have survived in myriopods and perhaps do exist in insects as urinary tubes, and also occur in many of the Arachnida, and thus are characteristic of each important class of land arthropods, but are either wanting or are very rudimentary or much modified in the marine classes, notably the Crustacea and Merostomata (Limulus), where they are represented by the shell-glands of Copepoda, green glands of the lobster, and the brick-red glands of Limulus.

FIG. 343.—Digestive canal of _Perla maxima_: _l_, upper lip; _mh_,
buccal cavity; _ap_, common end of salivary ducts (_ag_); _o_,
œsophagus; _s_, _s_, salivary glands, arranged segmentally; _b_,
cæca of chyle-stomach; _lg_, their ligaments of attachment; _mp_,
urinary tubes; _r_, rectum; _af_, anal orifice.—After Imhof, from
Sharp.
]

In the earliest tracheate arthropod, Peripatus, these tubes are well developed and are highly characteristic, each segment behind the head bearing a pair (Fig. 4, _so_{4}_-_so_{9}_). It has been suggested by some, but not yet proved, that the urinary tubes of insects are morphologically the same as the segmental organs of worms and of Peripatus; but there are no facts directly supporting this view, and, as Sograff states, it is a pure hypothesis and can only be confirmed or disproved by very detailed researches on the development of the urinary tubes of myriopods and of insects. Others regard them as probably homologous with the tracheæ, since they have a similar origin. As, however, they arise in the embryo as outgrowths of the proctodæum they may have arisen in myriopods and insects independently, and not be vermian heirlooms.

While in worms and in Peripatus a pair of these segmental organs occur in each segment, in insects this serial arrangement is not apparent; those with a purely excretory function are not segmentally arranged, with outlets opening externally, but arise as outgrowths of the hind-intestine or proctodæum of the embryo, not being segmentally arranged. The place of their origin is usually the dividing line between the mid and hind intestine (Fig. 343, _mp_); this applies to Scolopendrella (Fig. 15, _urt_) as well as to insects.

The urinary tubes are usually long, slender, blind, tubular glands varying in number from two to over a hundred, which generally arise at the constriction between the mid and hind intestine, and which lie loosely in the cavity of the body, often extending towards the head, and then ending near the rectum (Figs. 301, 310, _vm_). They were first discovered by the Italian anatomist Malpighi, after whom they were called the Malpighian tubes. While at first generally regarded as “biliary” tubes, they are now universally considered to be exclusively excretory organs, corresponding to the kidneys of the higher animals.

FIG. 344.—Digestive canal and appendages of the mole-cricket; _a_,
head: _b_, salivary glands and receptacle; _c_, lateral pouch; _d_,
stomatogastric nerves; _e_, anterior lobes of stomach; _f_, peculiar
organ; _g_, neck of stomach; _h_, plicate part of same; _i_, rectum;
_k_, anal gland; _m_, urinary tubes.—After Dufour, from Sharp.
]

Usually arising from the anterior end of the hind-intestine where it passes into the mid-intestine, in certain forms they shift their position, in some Hemiptera (Lygæus, Cimex) opening into the rectum, while in the Psyllidæ they arise from the slender hinder part of the mid-intestine, being widely separated at their origin. (Fig. 321.)

The length varies in different groups; where they are few in number (two to four, six to eight), they are very long, but where very numerous they are often short, forming dense tufts, each tuft connecting with the intestine by a common duct (ureter), or, as in the mole-cricket, the numerous tubes empty into a single duct (Fig. 344); in the locusts (Acrydiidæ), however, they are arranged in 10 groups, each group consisting of about 15 tubes, making about 150 in all; and are much convoluted and wound irregularly around the digestive canal, and when stretched out being about as long as the entire body.

The urinary tubes occur in twos, or in multiples of two, though a remarkable exception is presented in the dipterous genera Culex and Psychodes, in which there are five tubes; the young and fully grown larvæ, as well as the pupa and imago of Culex, having this number (Fig. 433, _mg_.)

In many insects (Pentatoma, Cimex, Velia, Gerris, Haltica, Donacia,
and often in caterpillars), the vessels open into a sort of urinary
bladder connecting with the intestine on one side.

FIG. 345.—_A_, section of urinary tube of Periplaneta; _B_, part of
tube of Perla; _p_, peritoneal membrane; _c_, cavity or lumen;
_n_, nucleus of a secreting cell.—After Schindler.
]

In the larvæ of some insects the blind ends of the tubes are often
externally bound to the rectum, in the silkworms being attached by
fine threads to the intestine, while in some flies (Tipula and
Ctenophora), two vessels may unite to form a loop. In all larval
Cecidomyiæ, the two tubes are united to form a loop which curves
backward, opening near the vent, the proctodæum being very short.
(Giard.)

FIG. 346.—Portion of a urinary tube of _Calliphora vomitoria_: _tr_,
trachea; _l_, lumen; _k_, nucleus.—After Gegenbaur.
]

While usually the urinary vessels form simple tubes, in many species
of Lepidoptera and Diptera they are branched, thus resembling those
of spiders and scorpions. Moreover, in many Lepidoptera and Diptera
(Fig. 308), the tubes are not simple, but are lobulated, and in some
Hemiptera (Pentatoma, Notonecta, and Tettigonia) are twisted or
lace-like. In rare cases there are two kinds of urinary tubes; in
_Melolontha vulgaris_, two of them are partly lobulated and yellow,
while the other two are simple and white. Their color in beetles
varies, some being whitish or yellowish; in Geotrupes, Dyticidæ,
Hydrophilidæ, etc., reddish brown; in Gryllotalpa as well as
_Locusta viridissima_, there are two different kinds of vessels,
differing in contents and in color (white or yellow), as well as
histologically. (Schindler.)

The exterior of the tubes is richly provided with tracheæ, which
often form a web around them, and the fine branches often seem to
attach them to the intestine. In Acheta they are enveloped by a very
delicate, loose network of muscular fibres. (Schindler.)

The urinary tubes consist, according to Schindler, of at least three
cellular layers (Fig. 345):—

1. An external, connective, nucleated membrane, the peritoneal
membrane.

2. A very delicate homogeneous basal membrane, the _tunica propria_.

3. A single layer of large polygonal excretory cells.

4. Lining the internal canal a chitinous layer penetrated by
pore-canals, the _intima_ often wanting.

The secretory cells are usually of the same size, but in many cases
are relatively small; sometimes four to six or more form the
periphery of the canal, sometimes three or only two. In some insects
the cells are so very large that a single cell forms the entire
periphery. The nuclei in the Lepidoptera (Papilio, Pontia, Cossus)
are large and irregularly branched.

The excretions of the Malpighian vessels, derived from the blood and
from the fat-body, are more or less fluid and granular, sometimes
pulpy. From the cells they pass into the canal, thence into the
intestine, and thence out of the body. How, says Kolbe, the
secretion passes into the intestine, whether by the contraction of
the fine fibrillæ of the peritoneal membrane, or by the external
pressure of the other organs, or by the pressure of the secretory
matter behind, is not yet known. Grandis observed in living
Hydrophilus that the urinary tubes moved, without the muscles
seeming to show what caused the motion. Moreover, the cells
incessantly changed their form. At a lower temperature such motions
ceased. The tracheæ, ending freely in the cells, did not anastomose.
(Kolbe.)

The different colors of the tubes (white, yellow, red, brown, or
green) is due to the hue of the excretions, and is independent of
the color of the blood and of the urinary substances held in the
secreted matter.

Schindler found that insects of different stages, collected in
winter, differed very much in their urinary secretions, the tubes in
the adults being entirely empty, while in the larvæ they were filled
full, so that he concluded that in the former the process of
excretion during the winter hibernation is very slow, but in the
latter very rapid.

As to the activity of the urinary vessels the following experiments
will throw some light. Tursini fed a Pimelia with fuchsin; its
urinary tubes were consequently colored red. Schindler fed insects
with indigo-carmine, which was excreted by the urinary tubes;
Kowalevsky arrived at the same results, which seems to prove that
these vessels are analogous to the kidneys of vertebrates. Moreover,
Schindler injected through the side of the first abdominal segment
into the cavity of the body of a Gryllotalpa a concentrated solution
of sodium salt of indigotin-disulphonic acid. After one or two hours
the external portion of the epithelium of the urinary vessels was
stained deep blue, while the inner portion remained of the normal
transparency; the nuclei being for the most part deeply stained.
Between one and two days after, the staining matter had not yet
wholly passed through the central canal, the surface recently
stained still appearing light blue.

The solid contents of the urinary tubes consist partly of crystals, which occur singly in the epithelial cells, or form scattered masses when situated in the central canal. Besides tabular rhombic crystals, there occur concretions which contain uric acid, and probably consist of urate of soda, also octahedral crystals of chloride of soda, and quadro-pyramidal crystals of oxalate of lime. Also acicular prisms occur; besides chloride of soda, phosphates, carbonate of lime, oxalate of lime in quantity, leucine, coloring matters, etc.; while the fluid secretion also contains urea (?), uric acid, and abundant urates; uric acid crystals were precipitated by the addition of acetic acid, and by adding hydrochloric acid crystals belonging to the dimetric system were formed. The often numerous spheroidal small granules are biurate of soda and biurate of ammonia. Pale, concentrically banded concretions are leucine pellets.

According to Kölliker the contents of the urinary vessels[55] in
general are: (1) round granules of urate of soda and urate of
ammonia; (2) oxalate of lime; and (3) pale transparent concretions
of leucine. Crystals of taurin are also said to occur. (Claus’
Zoölogy, p. 531.)

Although uric acid is characteristic of the urinary tubes, yet
sometimes it is wanting in them, while uric acid substances in
quantity occur in the fat-body or in the mid-intestine.

In the living insect the urinary tubes remove urates from the blood;
“the salts are condensed and crystallized in the epithelial cells,
by whose dehiscence they pass into the central canals of the tubules
and thence into the intestine.” (Miall and Denny.)

The process of excretion is carried on not only by the urinary
tubes, but also, as Cuénot has recently shown (1896) in Orthoptera,
by the pericardial cells and certain cells of the fat-bodies. In the
last-named cells urates are stored throughout life; the pericardial
cells apparently secrete but do not store waste products, which are
finally eliminated by the urinary tubes, the latter constantly
eliminating waste.

=Primitive number of tubes.=—Wheeler considers the primitive number
of urinary tubules to be six, other authors regarding two pairs as
the primary or typical number; and while Wheeler agrees that the
more ancestral tracheate arthropods had but a single pair,
Cholodkowsky supposes the primitive number in insects themselves to
be a single pair. This view is strengthened by the fact that
Scolopendrella has but a single pair (Fig. 15).

While Peripatus has no urinary tubes, in Myriopods a single pair
arises, as in insects, from the hind-intestine.

FIG. 347.—Section of proctodæum of embryo locust, showing origin of
urinary tubes (_ur.t_); _ep_, epithelial or glandular layer; _m_,
cells of outer or muscular layer; _a_, section of a tube.
]

When in insects the number of these tubes is few, they are, with
rare exceptions, arranged in pairs, so that Gegenbaur and others
have considered this paired arrangement as the primitive one. When
the tubules are very numerous in the adult, as in Orthoptera, the
embryos and larvæ have a much smaller number, Wheeler stating that
“in no insect embryo have more than three pairs of these vessels
been found.” We have observed 10 primary tubes in the embryo of
Melanopus (Fig. 347), from each of which afterwards arise 15
secondary tubules. In the Termites, only, do the young forms have
more urinary tubes than the adults.

In Campodea there are about 16 urinary tubes and in Machilis either
12 (Grassi) or 20 (Oudemans); but in other Thysanura the number is
much less, Lepisma having either four, six, or eight, according to
different authors, and both Nicoletia and Lepismina having six,
opening separately into the hind-intestine. On the other hand, these
organs have not yet been detected in Japyx. Whether they exist at
all in the Collembola, which are degenerate forms, is doubtful. The
weight of opinion denies their existence, though they may yet be
found existing in a vestigial condition. They are said by Tullberg
and by Sommer to exist in Podura, but are of peculiar shape.

Coming now to the winged insects, in what on the whole is perhaps
the lowest or most generalized order, the Dermaptera, the number is
over 30, and their insertions regularly encircle the intestine.
(Schindler.) In the most ancient and generalized family of
Orthoptera, the Blattidæ, Schindler detected from 60 to 70 tubes,
but in a nymph of Periplaneta not quite 10 mm. in length he found
from 16 to 18, and in nymphs 4 to 5 mm. long there were only eight
vessels; while Wheeler has found in the embryo of _Phyllodromia
germanica_ but four tubes. In the adult Acrydiidæ there are as many
as 150, in the Locustidæ between 40 and 50, and in the Gryllidæ
about 100.

The Ephemeridæ with about 40, the Odonata with 50 to 60 tubules, the
Perlidæ with from 50 to 60, are polynephrious; while the Termitidæ
and Psocidæ are oligonephrious, the former having from six to eight
and the Psocidæ only four tubes. So also all the other orders not
mentioned, except the Hymenoptera, have few of these tubes. The
Hemiptera, with none in Aphidæ, a single pair in the Coccidæ, and
two in all the rest of the order, have the fewest number.

In the Neuroptera there are from six to eight, while in a larva,
possibly that of Chauliodes, Wheeler finds the exceptional number of
seven.

The closely allied order Mecoptera (Panorpidæ), and also the
Trichoptera, agree with the Neuroptera (Sialis) in having six.
According to Cholodkowsky all Lepidoptera have six of these vessels,
except Galleria, which has but four. He finds that in _Tinea
biselliella_ (also _T. pellionella_ and _Blabophanes rusticella_)
the larva has six vessels, which, however, undergo histolysis during
pupation, a single pair arising in their stead. On this account he
regards the primitive number of urinary tubes as two, or a single
pair, this return from six vessels in the larva to two in the imago
being considered a case of atavism.

In the Coleoptera, the number of urinary tubes is from four to six;
in what few embryo beetles have been examined (Doryphora,
Melolontha), there are six vessels, but in the embryo of _Dyticus
fasciventris_, Wheeler has detected only four, this number being
retained in the adult. He thinks that in beetles in general, a pair
of vessels must be “suppressed during post-embryonic development,
presumably in early larval life.”

In Diptera and Siphonaptera, the number four is very constant, there
being, however, a fifth one in Culex and Psychoda (Fig. 400.)

The number of these vessels is very inconstant in the Hymenoptera,
varying from six (Tomognathus, an ant, worker) to 12 (Myrmica), and
in Apis reaching the number of 150.

In the embryo of the honey-bee and wall-bee (Chalicodoma), there are
only four; we still lack any knowledge of the number in embryo
saw-flies.

The following is a tabular view of insects with few urinary tubes (Oligonephria) and many (Polynephria). It will be seen that the number has little relation to the classification or phylogeny, insects so distantly related as the Orthoptera and Hymenoptera being polynephrious:—

_Oligonephria_

Collembola, 2 (Podura), Tullberg and also Sommer. Thysanura, 4 (Lepisma); in Campodea, 16; in Machilis, 12 or 20; wanting in Japyx. Psocidæ, 4. Termitidæ, 6 (many in the young, Rathke). Mallophaga, 4. Physapoda, 4. Hemiptera, 2 (Coccidæ, none in Aphidæ). Neuroptera, 6–8. (In Sialidæ and Rhaphididæ 6; in Myrmeleonidæ and Hemerobiidæ, 8). Trichoptera, 6. Mecoptera, 6. Lepidoptera, 2–4–6 (2 in Tinea, Tineola, and Blabophanes; in Pterophorus and Yponomeuta, 4).

_Coleoptera_, 4–6; never more.

4
Carabidæ,
Dyticidæ,
Staphylinidæ,
Gyrinidæ,
Palpicornes,
Lamellicornes,
Cantharidæ,
Buprestidæ
(in larva, 6; in beetle, 4).

6
Byrrhidæ,
Nitidulidæ,
Dermestidæ,
Cleridæ,
Meloidæ,
Pyrochroidæ,
Bruchidæ,
Bostricidæ,
Cerambycidæ
Chrysomelidæ,
Coccinellidæ.

Diptera, branching into 4 (Gegenbaur); in Culicidæ, and Psychoda, 5.
Siphonaptera, 4.

_Polynephria_

Orthoptera, 100–150. (In embryo Blattids, 4; in embryo locust, 10; in nymph of Gryllotalpa, 4.) Dermaptera, “over 30” (Schindler). Perlidae, 50–60. Plectoptera (Ephemeridæ), 40. Odonata, 50–60. Hymenoptera, 20–150. (In embryo bees only 4; Cynipidæ, Ichnenumonidæ, and Formicidæ have the smallest number, 6–12.)

Here should be mentioned the singular fact discovered by Koulaguine
that in the larva of Microgaster, the urinary tubes have no
connection with the intestine, but open dorsally on the outside of
the body on each side of the anus. Ratzeburg had stated that the
last segment of the body was in the form of a vesicle. Koulaguine
now shows that this vesicle is in reality the end of the intestine
opening upwards; as the result of this dorsal opening of the
intestine the Malpighian vessels open on the sides of the oval vent,
and have no connection with the intestinal canal. Whether this is of
morphological import, or is only a secondary adaptation, Koulaguine
does not state, his paper being a preliminary abstract.

Wheeler thus sums up our present knowledge regarding the number and homologies of the Malpighian or urinary tubes:

1. It is very probable that the so-called Malpighian vessels of
Crustacea and Arachnida are not the homologues of the _vasa
Malpighi_ of the Eutracheata (insects and myriopods).

2. The Malpighian vessels of the Eutracheata arise as paired
diverticula of the hind-gut and are, therefore, ectodermal.

3. In no insect embryo are more than six vessels known to occur;
although frequently only four are developed.

4. The number six occurs either during embryonic or post-embryonic
life in members of the following groups: Apterygota, Orthoptera,
Corrodentia; Neuroptera, Panorpata, Trichoptera, Coleoptera,
Lepidoptera, and Hymenoptera.

5. The number four seems to be typical for the Corrodentia,
Thysanoptera, Aphaniptera, Rhynchota, Diptera, and Hymenoptera.

6. The embryonic number in Dermaptera, Ephemeridea, Plecoptera, and
Odonata has not been ascertained, but will probably be found to be
either four or six.

7. There is evidence that in at least one case (Melolontha), the
tetranephric is ontogenetically derived from the hexanephric
condition by the suppression of one pair of tubules.

8. It is probable that the insects which never develop more than
four Malpighian vessels have lost a pair during their phylogeny.

9. The post-embryonic increase in the number of Malpighian vessels
in some orders (Orthoptera, Odonata, Hymenoptera) is secondary and
has apparently arisen to supply a demand for greater excreting
surface.[56]

LITERATURE ON THE EXCRETORY (URINARY) ORGANS

=Malpighi, M.= Dissertatio epistolica de Bombyce, Societati regiæ
Londini ad scientiam naturalem promovendam institutæ dicata.
(Londini, 1669, 12 Pls.)

=Herold, M. J. D.= Entwicklungeschichte der Schmetterlinge. 1815.

=Rengger, J. R.= Physiologische Untersuchungen über den tierischen
Haushalt der Insekten. Tübingen, 1817, pp. 82.

=Wurzer.= Chemische Untersuchungen des Stoffes in den Gallgefässen von
_Bombyx mori_. (Meckel’s Archiv f. Physiol., iv, 1818, pp. 213–215.)

=Gaede, H. M.= Physiologische Bemerkungen über die sogenannten
Gallgefässe der Insekten. (Nova Acta Acad. Caes. Leopold.-Carolin.,
1821, x, Pars II, pp. 186–196.)

=Meckel, J. F.= Ueber die Gallen- und Harnorgane der Insekten.
(Meckel’s Archiv, i, 1826, pp. 21–36.)

=Audouin, J. V.= Calculs trouvés dans les canaux biliaires d’un cerf
volant. (Ann. sc. nat., 2 Sér., 1836, v, pp. 129–137.)

=Frey und Leuckart.= Anatomie und Physiologie der Wirbellosen. 1843.

=Dufour, L.= Mémoire sur les vaisseaux biliaires ou le foie des
Insectes. (Ann. sc. nat., 1848, Sér. 2, xix, pp. 145–182, 4 Pls.)

=Karsten, H.= Harnorgane von _Brachinus complanatus_. (Müller’s Archiv
f. Anat. und Physiol., 1848, pp. 367–374.)

=Fabre, J. L.= Étude sur l’instinct et les metamorphoses des
Sphégiens. (Ann. d. sc. nat., 4 Sér., 1856, vi, pp. 137–189.)

—— Étude sur le rôle du tissu adipeux dans la sécrétion urinaire chez
les Insectes. (Ibid., 4 Sér., xix, pp. 351–382.)

=Schlossberger, J. E.= Untersuchungen über das chemische Verhalten der
Krystalle in den Malpighischen Gefässen der Raupen. (Archiv f. Anat.
und Physiol., 1857, pp. 61–62.)

=Leydig, F.= Lehrbuch der Histiologie. 1857.

=Sirodot, S.= Recherches sur les sécrétions chez les Insectes. (Ann.
sc. nat., 4 Sér., Zool., 1858, x, pp. 141–189, 251–334, 12 Pls.)

=Kölliker, A.= Zur feineren Anatomie der Insekten (Ueber die
Harnorgane, u.s.w.) (Verhandl. d. Physikal.-medizin. Gesellsch. in
Würzburg, viii, 1858, pp. 225–235.)

=Schindler, E.= Beitrage zur Kenntnis der Malpighischen Gefässe der
Insekten. 3 Taf. (Zeitschr. f. wiss. Zool., xxx, 1878, pp. 587–660.)

=Chatin, G.= Note sur la structure du noyau dans les cellules
marginales des tubes de Malpighi chez les Insectes et les
Myriapodes. (Ann. d. sc. nat., 6 Sér., xiv., 1882, pp. 7, 1 Pl.)

=Witlaczil, E.= Zur Anatomie der Aphiden. (Arbeiten a. d. Zool.
Instit. d. Univers. Wien., iv, 1882, pp. 397–441, 3 Taf.)

=Cholodkowsky, N.= Sur les vaisseaux de Malpighi chez les
Lépidoptères. (Compt. rend. Acad. d. Sc., Paris, xcix, 1884, pp.
631–633.)

—— Sur la morphologie de l’appareil urinaire des Lépidoptères.
(Archives de Biologie, 1887, vi, pp. 497–514, 1 Pl.)

=Loman, J. C. C.= Ueber die morphologische Bedeutung der sogenannten
Malpighischen Gefässe der echten Spinnen. (Tijdschr. Nederl. Dierk.
Ver. (2) Deel 1, 1887, pp. 109–113, 4 Fig.)

=Marchal, P.= Contribution à l’étude de la désassimilation de l’azote.
L’acide urique et la fonction rénale chez les Invertébrés. (Mém.
Soc. Zool. de France, 1889, iii, pp. 42–57.)

=Kowalevsky, A. O.= Ein Beitrag zur Kenntnis der Exkretionsorgane.
(Biol. Centralbl., ix, 1889–90, pp. 33–47, 65–76, 127–128.)

—— Sur les organes excréteurs chez les arthropodes terrestres.
(Congrès international de Zool., 2^{me} Session à Moscou, 1892, Pt.
I, pp. 186–235, 4 Pls.)

=Griffiths, A. B.= On the Malpighian tubules of _Libellula depressa_.
(Proc. Roy. Soc., Edinburgh, 1889, xv, pp. 401–403, Figs.)

=Grandis, V.= Sulle modificazioni degli epitelii ghiandolari durante
la secrezione. (Atti Accad. Torino, 1890, xxv, pp. 765–789, 1 Pl.;
Archiv Ital. Biol., 1890, xiv, pp. 160–182, 1 Pl.)

=Koulaguine, N.= Notice pour servire à l’histoire du développement des
hyménoptères parasites. (Congrès internat. de Zool., 2^{me} Session
à Moscou, 1892, Pt. I, pp. 253–277.)

=Sograff, Nicolas.= Note sur l’origine et les parentés des
Arthropodes, principalement des Arthropodes trachéates. (Congrès
internat. de Zool., 2^{me} Session à Moscou, 1892, Pt. I, pp.
278–302.)

=Giard, Alfred.= (Note on the urinary tubes of larval Cecidomyia.
Annals Ent. Soc., France, lxii, 1893, pp. lxxx-lxxxiv, 1 Fig.)

=Wheeler, William M.= The primitive number of Malpighian vessels in
insects. (Psyche, vi, May-December, 1893, Parts 1–6, pp. 457–460,
485–486, 497–498, 509–510, 539–541, 545–547, 561–564.)

=Metalnikoff, C. K.= Organes excréteurs des insectes. (Bull. Acad.
imp. Sci. St. Pétersbourg, 1896, iv, pp. 57–72, in Russian, 1 Pl.)

See also the works of Straus-Dürckheim, Will (Müller’s Archiv. 1848,
p. 502), Brugnatelli, Leidy, Dufour, Ramdohr, Basch, Davy, Grassi,
Minot, Berlese, Adlerz, Marchal (Bull. Ent. Soc. France, 1896, p.
257); Bordas (Appareil glandulaire des Hyménoptères, 1894), also C.
R. Acad. Sc. Paris, 1897.

_e._ Poison-glands

Poison-glands are mainly confined to the stinging Hymenoptera, _i.e._ certain ants, and the wasps and bees, but also occur in the mosquito, while many, if not most bugs, seem to instil a drop of poison into the punctured wounds they make.

In the honey and other bees the poison apparatus consists of two poison-glands whose secretion passes by a single more or less convoluted efferential duct into the large poison-sac, and thence by the excretory duct, which is enlarged at the base of the sting (Figs. 194, 195), out through the sting by the same passage as the eggs. According to Carlet, the poison apparatus of bees consists of two kinds of glandular organs, of which one kind secretes a feebly alkaline fluid, the other an acid product. The poison is only effective when both fluids are mixed. The resultant venom is always acid. The action of this venom upon some animals, as rabbits, frogs, and certain beetles, is slight; but the domestic fly and the flesh-fly are immediately killed by it. The inoculation of a fly with the secretion of one of the glands does not produce death until after a considerable time, but death follows very quickly if the same fly is subjected to a second inoculation, this time with the secretion of the other gland. The alkaline glands are in bees and all poisonous Hymenoptera strongly developed, but become vestigial in those forms which sting their prey to serve as food for their larvæ. The poison which the solitary sand and wood wasps and Pompilidæ inject into their victims only paralyzes them.

FIG. 348.—The poison apparatus of Ichneumon: _T_, sting; _GA_ acid
gland; _TG_, _R′_, its tubes opening into the common poison-sac or
reservoir; _ce_, its efferent canal; _Ga_, the tubular alkaline
gland; _R_, the glandular end; _a_, the reservoir; _ce_, its duct;
_Gac_, the accessory gland—After Bordas.
]

FIG. 349.—Cephalic gland of Belostoma.
]

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.

FIG. 350.—View from above of the cephalic gland of Belostoma, ×
20.—This and Fig. 349 after Locy.
]

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.”

FIG. 351.—_A_, median section of head, showing (_du_) the
venomo-salivary duct, with its insertion in (_hy_) the hypopharynx;
_cb_, brain; below is the pharyngeal pump, leading from (_œ_) the
œsophagus; _lre_, base of labrum-epipharynx; _m_, muscle; _n_,
commissure (other parts removed). _B_, the venomo-salivary duct,
showing its bifurcation, and the three glands on one of its
branches; _pg_, poison gland; _sg_, the upper of the two salivary
glands. _C_, the bifurcation of the duct, with its nucleated
hypodermis.—After Macloskie.
]

_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.

FIG. 352.—Under side of worker honey-bee, carrying wax scales, ×
3.—After Cheshire.
]

FIG. 353.—Nymph of Lachnus, showing position of wax-glands.—Gissler
_del._
]

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.

FIG. 354.—Young nymph and developing scale of _Aspidiotus
perniciosus_: _a_, ventral view of nymph, showing sucking beak with
setæ separated, with enlarged tarsal claw at right; _b_, dorsal view
of same, somewhat contracted, with the first waxy filaments
appearing; _c_, dorsal and lateral views of same, still more
contracted, illustrating further development of wax secretion; _d_,
later stage of same, dorsal and lateral views, showing matting of
wax secretions and first form of young scale; all greatly
enlarged.—After Howard and Marlatt, Bull. 3, N. S., Div. Ent., U. S.
Dept. of Agr.
]

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.)

FIG. 355.—Wax disks of social bees: _a_, _Apis mellifica_, worker;
_b_, do., queen; _c_, Melipona, worker; _d_, Bombus, worker.—From
Insect Life, U. S. Dept. Agr.
]

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.)

FIG. 356.—Wax-secreting larva of a saw-fly.
]

FIG. 357.—_Lachnus strobi_, and its two “honey” warts.—Gissler
_del._
]

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.)

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.