(Quart. Journ. Micr. Soc., 1855, iii, pp. 97–102, 1 Fig.)
=Hicks, Braxton.= On a new organ in insects. (Journ. Linn. Soc. Zool.,
London, 1857, pp. 130–140, 1 Pl.)
—— Further remarks on the organ found on the bases of the halteres and
wings of insects. (Trans. Linn. Soc., London, 1857, xxii, pp.
141–145, 2 Pls.)
=Hensen, V.= Ueber das Gehörorgan von Locusta. (Zeitschr. f. wissens.
Zool., xvi, 1866, pp. 190–207.)
=Graber, V.= Bemerkungen über die Gehör- und Stimmorgane der
Heuschrecken und Cicaden (Wiener Sitzungsber. Math.-natur-wiss. Cl.,
lxvi, 1 Abt., 1872, pp. 205–213, 2 Figs.)
—— Die tympanalen Sinnesapparate, der Orthopteren. (Denkschr. d. k.
Akad. d. wissens. Wien, xxxvi, 1876, 2 Abt., pp. 1–140, 10 Taf.)
—— Die abdominalen Tympanalorgane der Cicaden und Gryllodeen. (Ibid.,
1870, xxxvi, pp. 273–290, 2 Taf.)
—— Ueber neue, otocystenartige Sinnesorgane der Insekten. (Archiv f.
mikroskop. Anat., 1878, pp. 35–57, 2 Taf.)
—— Die chordotonalen Sinnesorgane und das Gehör der Insekten. (Archiv
f. mikroskop. Anat., 1882, xx, pp. 506–640; 1883, xxi, pp. 65–145,
Taf.)
=Mayer, Alfred Marshall.= Researches in Acoustics No. 5. 3.
Experiments on the supposed auditory apparatus of the culex
mosquito. (Amer. Jour. Sc. and Arts, Ser. 3, viii, 1874, pp. 81–103;
also Amer. Naturalist, viii, pp. 577–592.)
=Schmidt, Oscar.= Die Gehörorgane der Heuschrecken. (Archiv f.
mikroskop. Anat., xi, 1875, pp. 195–215, 3 Taf.)
=Ranke, J.= Beiträge zu der Lehre von den Uebergangssinnesorganen, das
Gehörorgan der Acridier und das Sehorgan der Hirudineen. (Zeitschr.
f. wissens. Zool., xxv, 1875, pp. 143–164, 1 Taf.)
=Lee, A. Bolles.= Les balanciers des Diptères, leurs organes
sensifères et leur histologie. (Recueil Zool. Suisse, ii, 1885, pp.
363–392, 1 Pl.)
—— Bemerkungen über der feineren Bau der Chordotonalorgane. (Archiv f.
mikroskop. Anat., 1883, xxiii, pp. 133–140, 1 Taf.)
—— Les organes chordotonaux des Diptères et la méthode du chlorure
d’or. (Observations critiques.) (Recueil Zool. Suisse, 1884, ii, pp.
685–689, 1 Pl.)
=Weinland, E.= Ueber die Schwinger (Halteren) der Dipteren (Zeitschr.
f. wissens. Zool., 1890, li, pp. 55–166, 5 Taf.)
=Adelung, N. v.= Beiträge zur Kenntnis des tibialen Gehörapparates der
Locustiden. Inaug. Diss., Leipzig, 1892, 2 Taf.
=Child, Ch. M.= Ein bisher wenig beachtetes antennales Sinnesorgan der
Insekten, mit besonderer Berücksichtigung der Culiciden und
Chironomiden. (Zeitschr. f. wissens. Zool., lviii, 1894, pp.
475–528, 2 Taf.; also, Zool. Anzeiger, xvii Jahrg., pp. 35–38, and
in Annals and Mag. Nat. Hist., 1894 (6), xiii, pp. 372–374).
Also the writings of J. Müller, Kirby and Spence, Burmeister, Gilbert
White, Westwood, Guilding, Meinert, Paasch, Leydig, Viallanes,
Minot, Forel, Mayer, Darwin (Descent of Man, i, ch. x.), F. Müller,
Lubbock (Senses of animals), Westring, Köppen, Bates, Vom Rath,
Peckham, Jourdan, Nagel, etc.
_b._ The sounds made by insects
=Scudder, S. H.= Notes on the stridulation of grasshoppers. (Proc.
Bost. Soc. Nat. Hist., xi, 1868, pp. 306–313 and 316.)
—— The songs of the grasshoppers. (Amer. Naturalist, ii, 1868, pp.
113–120, 5 Figs.)
=Riley, C. V.= The song notes of the periodical Cicada. (Proc. Amer.
Assoc. Adv. Science, xxxiv, 1885, pp. 330–332; also in Kansas City
Rev., October, 1885, pp. 173–175.)
=Swinton, A. H.= (Ent. Month. Mag., 1877.) Sound produced in Ageronia
by a modification of the hook and bristle of the wings.
=Hampson, G. F.= On stridulation in certain Lepidoptera, etc. (Proc.
Zool. Soc. London, 1892, ii, pp. 188–193, Fig; also Psyche, vi, p.
491, 1 Fig.)
With the writings of Landois, Lubbock, Graber, Kolbe, Carpenter (Nat.
Science), Bruyant, and others.
THE DIGESTIVE CANAL AND ITS APPENDAGES
[Illustration:
FIG. 297.—Transverse section through an abdominal segment of larva of
_Megalopyge crispata_, showing the relations of the digestive canal
to the other organs: _int_, hind-intestine, with its mucous or
epithelial layer (_ep_), and _ml_ its outer or muscular layer; _ng_,
ventral ganglion; _ht_, heart; _mp_, urinary tubes; _f_, fat-body;
_sc_, thickened portion of the hypodermis (_hy_) containing the
setigenous cells; _m_, muscles; _m′_, a pair of retractor muscles
inserted near the base of the lateral glandular process (_lgp_);
_cut_, cuticula; _l_, legs. Also compare Figs. 142–144 and 234.
]
[Illustration:
FIG. 298.
The alimentary or digestive canal of insects is a more or less
straight tube, which connects the mouth and anus, the latter
invariably
situated in the last segment of the body, under the last
tergite or suranal plate. It lies directly over the ventral nervous
cord and under the dorsal vessel, passing through the middle of the
body (Fig. 297). It is loosely held in place by delicate retractor
muscles (_retractores ventriculi_, found by Lyonet in the larvæ of
Lepidoptera, and occurring in those of Diptera), but is principally
supported by exceedingly numerous branches of the main tracheæ.
FIG. 298.—Internal anatomy of _Melanoplus femur-rubrum_: _at_, antenna
and nerve leading
to it from the “brain” or supraœsophageal ganglion (_sp_); _oc_,
ocelli, anterior and vertical ones,
with ocellar nerves leading to them from the brain; _œ_, œsophagus;
_m_, mouth; _lb_, labium or under
lip; _if_, infraœsophageal ganglion, sending three pairs of nerves to
the mandibles, maxillæ, and
labium respectively (not clearly shown in the engraving); _sm_,
sympathetic or vagus nerve, starting
from a ganglion resting above the œsophagus, and connecting with
another ganglion (_sg_) near the
hinder end of the crop; _sal_, salivary glands (the termination of the
salivary duct not clearly
shown by the engraver); _nv_, nervous cord and ganglia; _ov_, ovary;
_ur_, origin of urinary tubes;
ovt, oviduct; _sb_, sebaceous gland; _bc_, bursa copulatrix; _ovt_,
site of opening of the oviduct (the
left oviduct cut away); 1–10, abdominal segments. The other organs
labelled in full.—Drawn from
his original dissections by Mr. Edward Burgess.
]
[Illustration:
FIG. 299.—Digestive canal of _Anabrus_: _m_, mouth: _œ_, œsophagus;
_sm_, the sympathetic nerve passing along the crop; _t_, tongue;
_fg_, frontal ganglion; _br_, brain, the nervous cord passing
backward from it; _sr_, salivary reservoir; _sg_, salivary gland;
_pv_, proventriculus; _ur_, origin of urinary tubes; _sb_, sebaceous
gland; 1–10, the ten abdominal segments.—Burgess _del._
]
It is in the higher adult insects differentiated into the mouth and
_pharynx_, the _œsophagus_ or gullet, supplementary to which is the
_crop_ (_ingluvies_) or “_sucking stomach_” of Lepidoptera, Diptera, and
Hymenoptera; the _proventriculus_ or gizzard; the _ventriculus_,
“chyle-stomach,” or, more properly, mid-intestine, and the
hind-intestine, which is divided into the _ileum_, or short intestine,
the long intestine, often slender and coiled, with the colon and the
rectum. Morphologically, however, the digestive or enteric canal is
divided into three primary divisions, which are indicated in the embryo
insect; _i.e._, the _fore-intestine_ (_stomodæum_ of the embryo),
_mid-intestine_ or “chyle-stomach,” and _hind-intestine_ or _proctodæum_
(Fig. 300). The three primary regions, with their differentiations, may
be tabulated thus:—
_Fore-intestine_ (Stomodæum). Mouth and pharynx.
Pumping apparatus of Hemiptera,
Lepidoptera, and Diptera.
Œsophagus.
Crop or ingluvies, food reservoir, or
“sucking stomach.”
Proventriculus.
_Mid-intestine_ (Mesenteron). Mid-intestine, “chylific stomach,” or
ventriculus (with cœcal glands).
_Hind-intestine_ (Proctodæum). Ileum, or short intestine (with the
urinary tubes).
Long intestine.
Colon.
Rectum (with rectal glands).
Anus (with anal glands).
[Illustration:
FIG. 300.—The three primary divisions of the alimentary canal of an
embryonic orthopterous insect: _br_, brain; _sbg_, subœsophageal
ganglion; _ng_, nervous cord; _st_, stomodæum; _pr_, proctodæum;
_mv_, malphigian tubes; _mesen_, mid-intestine; _ht_, heart; _md_,
mandibles; _mx_, _mx′_, 1st and 2d maxillæ.—After Ayers, with some
changes.
]
The appendages of the alimentary canal are: (1) the salivary and poison
glands, which arise from the stomodæum in embryonic life; (2) while to
the chylific stomach a single pair of cœcal appendages (Orthoptera and
larval Diptera, _e.g._ Sciara), or many cœca may be appended; (3) the
urinary tubes, also the rectal glands and the paired anal glands. In a
Hemipter (_Pyrrhocoris apterus_) appendages arise from the intestine in
front of the origin of the urinary tubes. In certain insects a single
cœcal appendage (Nepa, Dyticus, Silpha, Necrophorus, and the
Lepidoptera) arises from the proctodæum.
[Illustration:
FIG. 301—Larva of honey-bee: _g_, brain; _bm_, ventral nervous cord;
_œ_, œsophagus; _sd_, spinning-gland; _cd_, mid-intestine or
chyle-stomach; _ed_, hind-intestine, not yet connected with the
mid-intestine; _vm_, urinary tube; _an_, anus; _st_, stigmata.—After
Leuckart, from Lang.
]
In certain larval insects, as those of the Proctotrypidæ (first larval
stage), the higher Hymenoptera (ichneumons, ants, wasps, and bees, Fig.
301), in the Campodea-like larvæ of the Meloidæ and Stylopidæ, the larva
of the ant-lion (Myrmecoleo), and those of _Diptera pupipara_
(Melophagus), the embryonic condition of the separation of the
proctodæum and mid-gut (mesenteron) persists, the stomach ending in a
blind sac; in such cases the intestine, together with the urinary tubes,
is entirely secretory.
The anus is wanting in the larva of the ant-lion, as also in the wasps
(in which there is a rudimentary colon) and in freshly hatched bees,
though it becomes perfectly formed in the fully grown larvæ (Newport,
art. Insecta, p. 967, and H. Müller).
In the larvæ of lamellicorn Coleoptera (_Melolontha vulgaris_) the
digestive tube is nearly as simple as in bees, though there is a large
colon, which at its beginning forms an immense cœcum, and has also one
anal aperture (Newport).
The length and shape of the digestive canal is dependent on the nature
of the food and also on the mode of life, especially the ease or
difficulty with which the food is digested.
Newport, while stating that the length of the alimentary canal in
larvæ is not in general indicatory of the habits of the species,
makes this qualification after describing the digestive canal of
Calandra as compared with that of Calosoma: “The length and
complication of the intestines, therefore, appear to have some
reference to the quality of the food to be digested, since it is
well known that the food of these latter insects (weevils) is of
difficult assimilation, being as it is chiefly the hard ligneous
fibres of vegetable matter; but they cannot be received as always
indicatory of a carnivorous [or] vegetable feeder, since, as above
remarked, the length of the canal is considerable in one entirely
carnivorous larva, while it is much shorter in some herbivorous, and
particularly in pollenivorous larvæ, as in the Melolontha and the
apodal Hymenoptera.”
[Illustration:
FIG. 302.—Digestive canal of a carabid beetle: _b_, œsophagus; _c_,
crop; _d_, proventriculus; _f_, mid-intestine, or “chyle-stomach,”
with its cœeca; _g_, posterior division of the stomach; _i_, the
two pairs of urinary tubes; _h_, intestine; _k_, rectum; _l_, anal
glands.—After Dufour, from Judeich and Nitsche.
]
Newport also contends that the length of the alimentary canal is not
more indicative in the perfect insect of the carnivorous or
phytophagous habits of the species than in the larva. It is nearly
as long (being from two to three times the length of the whole
body), and is more complicated, in the rapacious Carabidæ (Fig. 302)
than in the honey-sipping Lepidoptera, whose food is entirely
liquid. Referring to the digestive canal of Cicindelidæ, which is
scarcely longer than the body, he claims that “we cannot admit that
the length of the digestive organs, and the existence of a gizzard
and gastric vessels, are indicatory of predacity of habits in the
insect, because a similar conformation of parts exists often in
strictly vegetable feeders. The existence and length of these parts
seem rather to refer to the comparative digestibility of the food
than to its animal or vegetable nature.” Newport then refers to the
digestive canal of Forficulidæ (in which the gizzard is present, the
canal, however, passing in an almost direct line through the body,
making but one slight convolution), “a farther proof that the
_length_ of the canal must not be taken as a criterion whereby to
judge of the habits of a species.” He adds this will apply equally
well to the omnivorous Gryllidæ, in which there exists a short
alimentary canal, but a gizzard of more complicated structure than
that of the Dytiscidæ.
In larval insects and others (Synaptera, Orthoptera, etc.), in which the
digestive canal is simplest, it is scarcely longer than the body, and
passes through it as a straight tube.
In the caterpillar, which is a voracious and constant feeder, the
digestive canal is a large straight tube, not clearly differentiated
into fore-stomach, stomach, and intestine; but in the imago, which only
takes a little liquid food, it is slender, delicate, and highly
differentiated. In the larva the mid-gut forms the largest part of the
canal; in the imago, the intestine becomes very long and coiled into
numerous turns; at the same time the food-reservoir (the “sucking
stomach”) develops, and the excretory tubes are longer.
_a._ The digestive canal
[Illustration:
FIG. 303.—Interior view of the bottom of the head of _Danais
archippus_, the top having been cut away, showing, in the middle,
the pharyngeal sac with its five muscles: the frontal (_f.m_),
dorsal pair (_d.m_), and the lateral pair (_l.m_); _cl_, clypeus;
_cor_, cornea; _œ_, œsophagus; _p.m_, one of the large muscles which
move the labial palp.—After Burgess.
]
It will greatly simplify our conception of the anatomy of the digestive
canal if we take into account its mode of origin in the embryo, bearing
in mind the fact that during the gastrula condition the ectoderm is
invaginated at each pole to form the primitive mouth and fore-gut
(stomodæum) and hind-gut (proctodæum). The cells of the ectoderm secrete
a chitinous lining (intima), which forms the continuation of the outer
chitinous crust, and thus the lining of each end of the digestive canal
is cast whenever the insect molts; while the mid-intestine (mesenteron),
arising independently of the rest of the canal much later in embryonic
life from the mesoderm, is not the result of any invagination, being
directly derived from the mesoderm, and is not lined with chitin.
=The mouth, or oral cavity, and pharynx.=—This is the beginning of the
alimentary bounded above by the clypeus, and labrum, with the
epipharynx, and below by the hypopharynx, or tongue, as well as the
labium. Into it pour the secretion of the salivary glands, which passes
out through an opening at the base of the tongue or hypopharynx. On each
side of the mouth are the mandibles and first maxillæ.
=The sucking or pharyngeal pump.=—This organ has been observed by Graber
in flies and Hemiptera, but the fullest account is that by Burgess, who
was the first to discover it in Lepidoptera. In the milk-weed butterfly
(_Danais archippus_) the canal traversing the proboscis opens into a
pharynx enclosed in a muscular sac (Figs. 303, 304, and 310).
The pharyngeal sac, says Burgess, serves as a pumping organ to suck the
liquid food through the proboscis and to force it backwards into the
digestive canal.
[Illustration:
FIG. 304.—Longitudinal section through the head of Danais, showing the
interior of the left half: _mx_, left maxilla, whose canal leads
into the pharynx; _hph_, floor of the latter, showing some of the
taste-papillæ; _oe_, œsophagus; _ep_, epipharyngeal valve; _sd_,
salivary duct; _d.m_, _f.m_, and _cl_, as in Fig. 302.—After
Burgess.
]
Meinert (“Trophi Dipterorum”) has made elaborate dissections of the
mouth and its armature, including the pharynx of several types of
Diptera, with its musculature. He describes the pharynx as the
principal, and in most Diptera, as the only part of the pump (antlia),
and says: “By the muscles of the pump (_musculis antliæ_) the superior
lamina of the pharynx is varied that the space between the two laminæ
may be increased, and the liquid is thus led through the siphon formed
by the mouth-parts into the mouth” (Fig. 81).
=The œsophagus.=—This is a simple tube, largest in those insects feeding
on solid, usually vegetable, food, and smallest in those living on
liquid food. It usually curves upwards and backwards, passing directly
under the brain, and merges into the crop or proventriculus either at
the back part of the head or in the thorax, its length being very
variable. Its inner walls longitudinally are folded and lined with
chitin.
According to Newport, in the œsophagus of the Gryllidæ, of the two
layers of the mucous lining the second is distinctly glandular and
secretory, and in it there are many thousands of very minute granular
glandular bodies, which probably secrete the “molasses” or repellent
fluid often ejected by these and other insects when captured.
=The crop or ingluvies.=—This, when present, is an enlargement of the
end of the œsophagus, and lined internally with a muscular coat. It is
very large in locusts (Fig. 298), Anabrus (Fig. 299), and other
Orthoptera (the Phasmidæ excepted), in the Dermaptera, and most adult
Coleoptera. A crop-like dilatation in front of a spherical gizzard is
also present in the Synaptera (Poduridæ and Lepismidæ), as well as in
the Mallophaga (Nirmidæ).
[Illustration:
FIG. 305.—Digestive canal of Calandra: _H_, pear-shaped œsophagus;
_I_, crop; _K_, gastric cœca _L_, ilium; _MN_, colon; _P_, urinary
tubes.—After Newport.
]
[Illustration:
FIG. 306.—Section of the crop (_H_), gizzard (_I_), and stomach (_K_)
of Athalia.-After Newport.
]
[Illustration:
_Fig. 307._—Upper side of head and digestive canal of Myrmeleon larva:
_a_, crop; _b_, “stomach”; _c_, free ends of two urinary tubes;
_c′_, common origin of other six tubes; _d_, cœcum; _e_, spinneret;
_ff_, muscles for protruding its sheath; _gg_, maxillary
glands.—After Meinert, from Sharp.
]
In the larvæ of weevils (_Calandra sommeri_) there is a crop (Fig. 305),
but not in the larva of Calosoma; also, according to Beauregard, in the
pollen-eating beetles Zonitis, Sitaris, and Malabris it is wanting,
while in Meloe it is highly developed (Kolbe).
The crop forms a lateral dilatation of the end of the œsophagus in the
larvæ of weevils and of saw-flies (_Athalia centifoliæ_, Fig. 306).
=The “sucking stomach” or food-reservoir.=—This is a thin muscular pouch
connected by a slender neck with the end of the œsophagus or the crop,
when the latter is present. There is no such organ in Orthoptera, except
in Gryllotalpa. It is wanting in the Odonata and in the Plectoptera
(Ephemeridæ); in Platyptera (Perlidæ and Termitidæ), in Trichoptera, and
in Mecoptera (Panorpidæ). In most adult Neuroptera (Myrmeleonidæ,
Hemerobiidæ, and Sialidæ), but not in Rhaphidiidæ, the long œsophagus is
dilated posteriorly into a kind of pouch or crop, and besides there is
often a long “food-reservoir” arising on one of its sides, that of
Myrmeleon (Fig. 307) and Hemerobius being on the right side.
[Illustration:
FIG. 308.—Digestive canal of _Sarcophaga carnaria_: _a_, salivary
gland; _b_, œsophagus; _c_, food reservoir; _f-g_, stomach; _h_,
intestine; _i_, urinary tubes; _k_, rectum.—From Judeich and
Nitsche.
]
A true food-reservoir is present in most Diptera (Fig. 308) as well as
in the larvæ of the Muscidæ, but according to Dufour it is wanting in
some Asilidæ and in _Diptera pupipara_, and according to Brauer in the
Œstridæ. The food-reservoir in Diptera is always situated on the left
side of the digestive canal; there is usually a long neck or canal,
while the reservoir is either oval or more usually bilobed, and often
each lobe is itself curiously lobed.
In Lepidoptera (Figs. 309, 310) the so-called “sucking stomach” is, as
Graber has proved, simply a reservoir for the temporary reception of
food; though generally found to contain nothing but air, Newport has
observed that in flies it is filled with food after feeding. He has
found this to be the case in the flesh fly, and in Eristalis he has
found it “partially filled with yellow pollen from the flowers of the
ragwort upon which the insect was captured,” the pollen grains also
occurring in the canal leading to the bag, in the gullet, and in the
stomach itself. Graber has further proved by feeding flies with a
colored sweet fluid that this sac is only a food-receptacle. As he says:
“It can be seen filling itself fuller and fuller with the colored fluid,
the sac gradually distending until it occupies half the hind-body.”
The food-reservoir of the Hymenoptera is a lateral pouch at the end of
the long, slender œsophagus, and has been seen in the bee to be filled
with honey.
[Illustration:
FIG. 309.—Digestive canal of _Sphinx ligustri_: _h_, œsophagus; _i_,
rudiment of the gizzard; _k_, “stomach”; _q_, its pyloric end; _t_,
food reservoir; _p_, urinary tubes; _l_, ilium; _m_, cœcum of colon;
_n_, rectum; _v_, vent.—After Newport.
]
In the mole-cricket the hinder part of the crop is armed within with
hook-like bristles directed backwards so as not to prevent the energetic
pressure of the food backwards into the proventriculus, and to obviate
the possibility of a regurgitation. (Eberli.)
=The fore-stomach or proventriculus.=—This is especially well developed
in the Dermaptera, in the Orthopterous families Locustidæ, Gryllidæ, and
Mantidæ, while in the Thysanura (Lepisma) there is a spherical gizzard
provided with six teeth. It also occurs in many wood-boring insects, and
in most carnivorous insects, notably the Carabidæ, Dyticidæ, Scolytidæ,
in the Mecoptera (scorpion-flies), in the fleas, and in many kinds of
ants, as well as Cynips, Leucospis, and Xyphidria. It is very muscular,
lined within with chitin, which is usually provided with numerous teeth
arising from the folds. These folds begin in the œsophagus or crop, and
suddenly end where the mesenteron (“chylific stomach”) begins. It has
been compared with the gizzard of birds, and is usually called by German
authors the chewing or masticating stomach. (Kaumagen.)
The proventriculus is best developed in the Gryllidæ (_Acrida
viridissima_), where the six folds at the end of the crop close
together to form a valve between the crop and proventriculus. “They
are each armed with five very minute hooked teeth; and, continued
into the gizzard, develop many more in their course through that
organ. These first teeth are arranged around the entrance to the
gizzard, and seem designed to retain the insufficiently comminuted
food and to pass it on to that organ.
[Illustration:
FIG. 310.—Anatomy of _Danais archippus_ after removal of right half
of the body. _Lettering of the head_: _a_, antenna; _ph_, pharynx;
_pl_, labial palpi; _r_, proboscis; _g_, brain; _usg_,
subœsophageal ganglion. _Lettering of the thorax_: I. II. III.
thoracic segments; _b_{1}_, _b_{2}_, _b_{3}_, the coxal joints of
the three pairs of legs; _bm_, muscles of the wings; _ac_ cephalic
aorta with its swelling; _œ_, œsophagus; _bg_, thoracic ganglia of
the ventral cord; _sd_, salivary glands of one side, those of the
other side cut off near their entrance into the common salivary
duct. _Lettering of the abdomen_: 1–9. abdominal segments; _h_,
heart; _sm_, so-called sucking-stomach (food-reservoir); _cm_,
chyle-stomach; _ag_, abdominal ganglia: _ed_, hind intestine with
colon (_c_) and rectum (_r_); _rm_, urinary vessels; _ov_, ovarial
tubes, those of the right side cut off; _ove_, terminal filaments
of the ovaries; _bc_, bursa copulatrix; _obc_, its outer aperture;
_od_, oviduct; _vag_, vagina; _wo_, its outer aperture; _ad_,
glandular appendages of the vagina partly cut away; _vk_,
connective canal between the vagina and bursa copulatrix with
swelling (receptaculum seminis); _an_, anus.—After Burgess, from
Lang.
]
[Illustration:
FIG. 311.—Transverse section of the proventriculus of _Gryllus
cinereus_: _muc_, muscular walls; _r_, horny ridge between the
large teeth (_sp_).—After Minot.
]
[Illustration:
FIG. 312.—Transverse section of the proventriculus of the
cockroach.—After Miall and Denny.
]
[Illustration:
FIG. 313.—Digestive canal of the honey-bee: _A_, horizontal section
of the body; _lp_, labial palpus; _mx_, maxilla; _e_, eye; _pro.
t_, prothorax; _mesa. t_, mesothorax; _meta. t_, metathorax; _dv_,
dorsal vessel; _v_, _v_, ventricles of the same; No. 1, No. 2, No.
3, salivary gland systems; _œ_ œsophagus; _g_, _g_, ganglia of
chief nerve-chain; _n_, nerves; _hs_, honey-sac; _p_, petaloid
stopper or calyx of honey-sac or stomach-mouth; _c. s_, chyle
stomach; _bt_, urinary tubes; _si_, small intestine (ilium); _l_,
“lamellæ or gland-plates of colon,” rectal glands; _li_, rectum.
_B_, cellular layer of stomach; _gc_, gastric cells, × 200. _C_,
urinary tube; _bc_, cells; _t_, trachea. _D_, inner layer, with
gastric teeth (_gt_).—After Cheshire.
]
“Next to these in succession on each of the longitudinal ridges are
four flat, broad, somewhat quadrate teeth, each of which is very
finely denticulated along its free margin. These extend about
half-way through the gizzard. They appear to be alternately elevated
and depressed during the action of the gizzard, and to serve to
carry on the food to the twelve cutting teeth, with which each ridge
is also armed, and which occupy the posterior part of the organ.
These teeth are triangular, sharp-pointed, and directed posteriorly,
and gradually decrease in size in succession from before backward.
Each tooth is very strong, sharp-pointed, and of the color and
consistence of tortoise shell, and is armed on each side by a
smaller pointed tooth. These form the six longitudinal ridges of the
gizzard, between each two of which there are two other rows of very
minute teeth of a triangular form, somewhat resembling the larger
one in structure, occupying the channels between the ridges. The
muscular portion of the gizzard is equally interesting. It is not
merely formed of transverse and longitudinal fibres, but sends from
its inner surface into the cavity of each of the large teeth other
minute but powerful muscles, a pair of which are inserted into each
tooth. The number of teeth in the gizzard amounts to 270, which is
the same number in these Gryllidæ as found formerly by Dr. Kidd in
the mole-cricket. Of the different kinds of teeth there are as
follows: 72 large treble teeth, 24 flat quadrate teeth, 30 small
single-hooked teeth, and 12 rows of small triangular teeth, each row
being formed of 12 teeth. This is the complicated gizzard of the
higher Orthoptera.” (Newport.)
In the more generalized cockroach, there are six principal folds,
the so-called teeth, which project so far inwards as to nearly meet
(Fig. 312). The entire apparatus of muscles and teeth is, as Miall
and Denny state, “an elaborate machine for squeezing and straining
the food, and recalls the gastric mill and pyloric strainer of the
crayfish. The powerful annular muscles approximate the teeth and
folds, closing the passage, while small longitudinal muscles, which
can be traced from the chitinous teeth to the cushions, appear to
retract these last, and open a passage for the food.”
As in the fore-stomach or proventriculus of the lobster, the solid,
rounded teeth do not appear to triturate the solid fragments found in
the organ, but act rather as a pyloric strainer to keep such bodies out
of the chylific stomach. We accept the view of Plateau that this section
of the digestive canal in insects, which he compares to the psalterium
of a ruminant, is a strainer rather than a masticatory stomach, and both
Forel and Emery, as well as Cheshire, take this view.
The proventriculus of the honey-bee (Fig. 313, _hs_) is called by
apiarians the “honey-sac” or “honey-stomach.” Cheshire states that if it
be carefully removed from a freshly killed bee, its calyx-like
“stomach-mouth” may be seen to gape open and shut with a rapid snapping
movement. The entrance to the stomach is guarded by four valves, each of
which is strongly chitinous within, and fringed along its edge with
downward-pointing fine stiff bristles. By the contraction of the
longitudinal muscles (_lm_), the valves open to allow the passage of
food from the honey-sac to the “chyle-stomach.” It is closed at will by
circular muscles (_tm_). Then the bee can carry food for a week’s
necessities, either using it rapidly in the production of wax, or eking
it out if the weather is unfavorable for the gathering of a new store.
[Illustration:
FIG. 314.—“Honey-sac stopper,” “stomach-mouth,” or calyx-bell of
honey-bee, × 50. _A_, front view of one of the lobes of the
calyx-bell; _l_, lip-like point, covered by down-turned bristles
(_b_); _sm_, side membrane. _B_, longitudinal section of the
stomach-mouth, with continuations into entrance of chyle-stomach;
_l_, _l_, lip-like ends of leaflets; _s_, setæ; _lm_, longitudinal
muscles; _tm_, transverse muscles in cross-section; _cl_, cell-layer
of honey-sac; _LM_, _TM_, longitudinal and transverse muscles of
same; _nc_, nucleated cells of tubular extension of stomach-mouth
into chyle-stomach; _lm′_, _tm′_, longitudinal and transverse
muscles of chyle stomach; _c_, _c_, cells covered within by an
intima. _C_, cross-section of stomach-mouth; _m_, cross-section of
muscles seen at _lm_ in _B_; _tm_, transverse muscles surrounding
stomach-mouth. _D_, cross-section through small intestine; _a_ and
_m_, longitudinal and surrounding muscles.—After Cheshire.
]
Cheshire also shows that when bees suck up from composite and other
flowers nectar together with much pollen, the outside wrinkled
membrane (_sm_, _A_, Fig. 314) “is seen to continually run up in
folds, and gather itself over the top of the stomach-mouth, bringing
with it, by the aid of its setæ, the large pollen-grains the nectar
contains.” The lips (_l_, _l_, _B_, Fig. 314), now opening, take in
this pollen, which is driven forwards into the cavity made between
the separating lips by an inflow of the fluid surrounding the
granules. The lips in turn close, but the down-pointing bristles are
thrown outwards from the face of the leaflet, in this way revealing
their special function, as the pollen is prevented from receding
while the nectar passes back into the honey-sac, strained through
between the bristles aforesaid, the last parts escaping by the
loop-like openings seen in the corners of _C_, Fig. 314. The whole
process is immediately and very rapidly repeated, so that the pollen
collects and the honey is cleared. “Three purposes, in addition to
those previously enumerated, are thus subserved by this wondrous
mechanism. First, the bee can either _eat or drink_ from the mixed
diet she carries, gulping down the pollen in pellets, or swallowing
the nectar as her necessities demand. Second, when the collected
pollen is driven forwards into the chyle-stomach, the tube
extension, whose necessity now becomes apparent, prevents the
pellets forming into plug-like masses just below _p_, Fig. 313, for,
by the action of the tube, these pellets are delivered into the
midst of the fluids of the stomach, to be at once broken up and
subjected to the digestive process. And third, while the little
gatherer is flying from flower to flower, her stomach-mouth is busy
in separating pollen from nectar, so that the latter may be less
liable to fermentation and better suited to winter consumption. She,
in fact, carries with her, and at once puts into operation, the most
ancient, and yet the most perfect and beautiful, of all
‘honey-strainers.’”
Forel’s experiments on the proventriculus of ants prove that through
its valvular contrivance it closes the passage from the crop to the
mid-intestine (“chylific stomach”), and allows the contents of the
former to pass slowly and very gradually into the latter. Emery
confirms this view, and concludes that the organ in the Camponotidæ
and in the Dolichoderidæ provided with a calyx-bell, usually
regarded as a triturating stomach (Kaumagen), but more correctly as
a pumping stomach, consists of parts which perform two different
functions. Under the operation of the muscles of the crop the
entrance to the pumping stomach becomes closed, in order by such
spasmodic contraction to prevent the flow of the contents of the
crop into the proventriculus. By the pressure of the transverse
muscles of the proventriculus its contents are emptied into the
mid-intestine, while simultaneously a regurgitation into the crop is
prevented. In the Dolichoderidæ and Plagiolepidinæ the closure in
both cases is effected by the valves. In the true Camponotidæ there
are two separate contrivances for closing; the calyx belonging to
the crop-musculature, while the valves essentially belong to the
proventricular pumping apparatus.
Opinions vary as to the use of this portion of the digestive canal.
Graber compares it to the gizzard of birds, and likens the action of
the rosette of teeth to the finer radiating teeth of the sea-urchin,
and styles it a chopping machine, which works automatically, and
allows no solid bits of food to pass in to injure the delicate walls
of the stomach (mid-gut).
He also states that the food when taken from the proventriculus is
very finely divided, while that found in the œsophagus contains
large bits.
Kolbe says that this view has recently been completely abandoned,
and that the teeth are used to pass the food backwards into the
chylific stomach. “But Goldfuss had denied the triturating action of
the proventriculus of the Orthoptera (Symbolæ ad Orthopterorum
quorundam Œconomiam, 1843), stating that the contents of the same
are already fluid in the gullet, so that the fore-stomach (Kaumagen)
does not need to comminute the food” (Kolbe). In the Gryllidæ and
Locustidæ, just before the posterior opening of the proventriculus
into the stomach the chitinous lining swells into a ring and
projects straight back as the inner wall of the cylindrical chylific
stomach. The muscular layer forms two sac-like outgrowths or folds,
which separate on the circular fold from the chitinous membrane.
This apparatus only allows very finely comminuted food to pass into
the stomach.
In the Acrydiidæ (_Eremobia muricata_) at the end of the
proventriculus, where it passes into the stomach, is a small
circular fold which hangs down like a curtain in the stomach.
=The œsophageal valve.=—Weismann[50] states that the origin of the
proventriculus in the embryo of flies (Muscidæ) shows that it should be
regarded as an intussusception of the œsophagus. While in the embryo the
invaginated portion of the œsophagus is short, after the hatching of the
larva it projects backwards into the mid-intestine. Kowalevsky also
observed in a young muscid larva, 2.2 mm. in length, that the œsophagus,
shaped like a tube, extends back into the expanded portion
(proventriculus) and opens into the stomach (Fig. 315, _A_). In a larva
10 mm. long the funnel is shorter, the end being situated in the
proventriculus (Fig. 315, _B_, _pr_). In the cavity between the outer
(_o_) and inner wall (_i_) no food enters, and the use of this whole
apparatus seems to be to prevent the larger bits of food from passing
into the chylific stomach (Kowalevsky).
[Illustration:
FIG. 315.—Œsophageal valve of young muscid larva: _m_, its opening:
_t_, thickening of the cells; _mes_, mesoderm.—After Kowalevsky.
]
Beauregard has found a similar structure in the Meloidæ, and calls
it the “cardiac valvule” (Fig. 318, _Kl_). It was observed by
Mingazzini in the larvæ of phytophagic lamellicorn beetles, and
Balbiani described it in a myriopod (Cryptops) under the name of the
“œsophageal valvule.”
Gehuchten describes a homologous but more complicated structure in a
tipulid larva (_Ptychoptera contaminata_), but differing in
containing blood-cavities, as a tubular prolongation of the
posterior end of the œsophagus which passes through the
proventriculus and opens at various positions in the anterior part
of the chylific stomach (Fig. 316).
The three layers composing this funnel are distant from each other
and separated by blood-cavities, the whole forming “an immense
blood-cavity extended between the epithelial proventricular lining
and the muscular coat.”
According to Schneider the longitudinal muscular fibres of the fore
and hind gut in insects pass into the stomach (mid-gut). The
anterior part of the fore-gut has generally only circular fibres.
When, however, the longitudinal fibres arise behind the middle, then
they separate from the digestive canal and are inserted a little
behind the beginning of the chylific stomach. Hence there is formed
an invagination of the proventriculus, which projects into the
cavity of the stomach.
Schneider describes this process, which he calls the “beak,” as an
invagination of the fore-stomach which projects into the cavity of
the stomach. The two layers of the invagination in growing together
form a beak varying in shape, being either simple or lobed and armed
with bristles or teeth. This beak is tolerably large in Lepisma,
Dermaptera (Forficula), Orthoptera, and in the larvæ and adults of
Diptera, but smaller in the Neuroptera and Coleoptera, while in
other insects it is wanting.
=Proventricular valvule.=—Gehuchten also describes in Ptychoptera
what he calls “the proventricular valvule,” stating that it is “a
circular fold of the intestinal wall” (Fig. 310, _vpr_). He claims
that it has not before been found, the “proventricular beak” of
Schneider being regarded by him as the œsophageal valvule.
[Illustration:
FIG. 316.—Digestive canal of _Ptychoptera contaminata_: _gs_, salivary
glands; _ra_, œsophagus; _pr_, proventriculus; _gt_, crown of eight
small tubular glands; _im_, mid-intestine; _ga_, two accessory white
glands; _vm_, urinary vessels; _ig_, small intestine; _gi_, large
intestine; _r_, rectum; _A_, the proventriculus in which the hinder
end of the œsophagus extends as far as the chyle-stomach. _B_,
longitudinal section of the proventricular region; _sph_, muscular
ring or œsophageal sphincter; _ppr_, wall of the proventriculus;
_e_, circular constriction dividing the cavity of the proventriculus
in two; _vpr_, circular fold of the wall of the mid-intestine
forming the proventricular valve; _vœ_, œsophageal valve.—After
Gehuchten.
]
=The peritrophic membrane.=—This membrane appears first to have been
noticed by Ramdohr in 1811 in _Hemerobius perla_. It has been found by
Schneider, who calls it the “funnel.” On the hinder end of the
fore-stomach, he says, the cuticula forms a fold enclosing the outlet of
the fore-stomach, and extending back like a tube to the anus. This
“funnel,” he adds, occurs in a great number of insects. It has been
found in Thysanura, but is wanting in Hemiptera. In the Coleoptera it is
absent in Carabidæ and Dyticidæ. It is generally present in Diptera and
in the larvæ of Lepidoptera, but not in the adults. In Hymenoptera it
has been found in ants and wasps, but is absent in Cynipidæ,
Ichneumonidæ, and Tenthredinidæ. All those insects (including their
larvæ) possessing this funnel eat solid, indigestible food, while those
which do not possess it take fluid nourishment. It is elastic, and
firmly encloses the contents of the digestive tract. Until Schneider’s
discovery of its general occurrence, it had only been known to exist in
the viviparous Cecidomyia larvæ (Miastor). Wagner, its discoverer,
noticed in the stomach of this insect a second tube which contained
food. Pagenstecher was inclined to regard the tube as a secretion of the
salivary glands. Metschnikoff, however, more correctly stated that the
tube consisted of chitin, but he regarded it as adapted for the removal
of the secretions. (Schneider.) Plateau, however, as well as Balbiani,
the latter calling it the “peritropic membrane,” considers this membrane
as a secretion of the chylific stomach, and that it is formed at the
surface of the epithelial cells. It surrounds the food along the entire
digestive tract, forming an envelope around the fæcal masses. On the
other hand, Gehuchten states that in the larva of Ptychoptera its mode
of origin differs from that described by Plateau and by Schneider, and
that it is a product of secretion of special cells in the
proventriculus.
=The mid-intestine.=—This section of the digestive canal, often, though
erroneously, called the “chylific stomach” or ventriculus, differs not
only in its embryonic history, but also in its structure and physiology
from the fore and hind intestine of arthropods, and also presents no
analogy to the stomach of the vertebrate animals. In insects it is a
simple tube, not usually lined with chitin, since it is not formed by
the invagination of the ectoderm, as are the fore and hind intestine,
the absence of the chitinous intima promoting the absorption of soluble
food. Into the anterior end either open two or more large cœcal tubes
(Fig. 299), or its whole outer surface is beset with very numerous fine
glandular filaments like villi (Fig. 317 and Fig. 329).
The mid-intestine varies much in size and shape; it is very long in the
lamellicorn beetles (Melolontha and Geotrupes), and while in Meloë it is
very large, occupying the greatest part of the body-cavity, in the
longicorn beetles and in Lepidoptera it is very small. The pyloric end
consists of an internal circular fold projecting into the cavity. In the
Psocidæ (Cæcilius) the pyloric end is prolonged into a slender tube
nearly as long as the larger anterior portion.
The limits between the mid and hind intestine are in some insects
difficult to define, the urinary tubes sometimes appearing to open into
the end of the mid-intestine (“stomach”). The latter also is sometimes
lined with an intima. The limits are also determined by a circular
projection, directly behind which is an enlargement of the intestine in
the shape of a trench (_rigole_), or circular _cul-de-sac_ (the “pyloric
valvule” of some authors, including Beauregard), while the walls of the
small intestine contract so as to produce a considerable constriction of
the cavity of the canal. This constriction exactly coincides with the
beginning of the double layer of circular muscles in the wall of the
small intestine. An internal layer, which is the continuation of the
circular muscles of the chylific stomach, and an external layer much
more developed probably belong to this part of the alimentary canal.
Since the homologue of the circular fold occurs in the locust as well as
in Diptera, it is probably common to insects in general.
[Illustration:
FIG. 317.—Digestive canal of _Carabus monilis_: _h_, œsophagus; _i_,
gizzard or proventriculus; _k_, “stomach,” with its cœca (_r_); _p_,
urinary tubes; _q_, their point of insertion; _m_, _n_, colon, with
cœcal glands; _s_, anal glands; _a_, _b_, _c_, a gastric cœcum; _a_,
_b_, portion of lining of gizzard.—After Newport.
]
[Illustration:
FIG. 318.—Digestive canal of Meloe: _sch_, œsophagus; _Kl_, œsophageal
valve; _mD_, mid-intestine; _eD_, hind-intestine; _Ei_, eggs; _g_,
sexual opening.—After Graber.
]
Gehuchten adds that the limit set by the circular projection does
not exactly coincide with the opening into the intestine of the
urinary tubes and the two annexed glands. He shows by a section (his
Fig. 133) that the tubular glands open into the alimentary canal in
front of the circular fold. It is the same with the Malpighian
tubes. They are not, therefore, he claims, dependences of the
terminal intestine, but of the mid-intestine. Beauregard has
observed the same thing in the vesicating insects (Meloidæ). The
Malpighian tubes, he says, open into the “chylific stomach” before
the valvular crown. This arrangement does not seem to be general,
because, according to Balbiani, the Malpighian vessels open into the
beginning of the intestine in Cryptops. Compare also Minot’s account
of the valve in locusts separating the stomach from the intestine,
and in front of which the urinary or Malpighian tubes open.
=Histology of the mid-intestine.=—The walls of the stomach are composed
of an internal epithelium, a layer of connective tissue, with two
muscular layers, the inner of which is formed of unstriated circular
muscular fibres, and the outer of striated longitudinal muscular fibres.
In the cockroach short processes are given off from the free ends of the
epithelial cells, as in the intestine of many mammals and other animals.
“Between the cells a reticulum is often to be seen, especially where the
cells have burst; it extends between and among all the elements of the
mucous lining, and probably serves, like the very similar structure met
with in mammalian intestines, to absorb and conduct some of the products
of digestion.” (Miall and Denny.)
Gehuchten shows that the epithelial lining of the mesenteron (chylific
stomach) of the dipterous larva Ptychoptera is composed of two kinds of
cells, _i.e._ secreting or glandular cells and absorbent cells, the
former situated at each end of the stomach, and the absorbent cells
occupying the middle region. The part played by these cells in digestion
will be treated of beyond in the section on digestion. (See p. 327.)
=The hind-intestine.=—In many insects this is divided into the ileum, or
short intestine, and the long intestine. The limit between the intestine
and stomach is externally determined by the origin of the urinary tubes,
which are outgrowths of the anterior end of the proctodæum. Like the
fore-intestine the hind-intestine is lined with a thick muscular layer,
and, as Gehuchten states, the passage from the epithelial lining of the
stomach (mid-intestine) to the muscular lining of the intestine is
abrupt.
=Large intestine.=—In Ptychoptera, as described by Gehuchten, there are
no precise limits between the small and large intestine; the epithelium
of the large intestine has a special character, and its constituents
present a close resemblance to the absorbed cells of the chylific
stomach, being like them large and polygonal. The muscular layer is not
continuous, and is formed of longitudinal and circular fibres, the
latter being the larger.
=The ileum=—Though in most insects slender, and therefore called the
small intestine, the ileum is in locusts (Fig. 298) and grasshoppers
(Anabrus, Fig. 299) as thick as the stomach. In many carnivorous beetles
(Dyticus, Fig. 320, _il_, and Necrophorus) it is very long, but rather
slender and short in the Carabidæ and Cicindelidæ, as well as those
insects whose food is liquid, such as Diptera. In the Lepidoptera it
varies in length, being in Sphinx quite long and bent into seven folds
(Fig. 309), while it is very short in the Psocidæ, Chrysomelidæ, and
Tenthredinidæ.
In the locust the ileum is traversed by six longitudinal folds with
intervening furrows; outside of each furrow is a longitudinal muscular
band. Seen from the inner surface the epithelium has an unusual
character, the cells in the middle of each of the flat folds being quite
large, polygonal in outline, while towards the furrows the cells become
very much smaller. The walls are double when seen in transverse section,
the inner layer consisting of epithelial cells resting on connective
tissue, the outer layer formed of circular muscles. The cuticula is
thin, but probably chitinous; it resembles that on the gastro-ileal
folds, except that there are no spinules, but unlike the cuticula of the
stomach it extends equally over the folds and the furrows. (Minot.) In
the cockroach the junction of the small intestine with the colon is
abrupt, a well-developed annular fold assuming the nature of a circular
valve. (Miall and Denny.)
=The gastro-ileal folds.=—In the locust the intestine is separated from
the chylific stomach by what Minot calls “the gastro-ileal folds,” which
form a peculiar valve. The urinary vessels open just underneath and in
front of this valve. In Melanoplus, and probably in the entire family of
Acrydiidæ, they are indicated as “dark spots, round in front and lying
at the anterior end of the ileum so as to form a ring around the
interior of the intestine.” They are 12 in number, and all alike. They
are pigmented and round in front where they are broadest and stand up
highest; they narrow down backwards, the pigment disappears, and they
gradually fade out into the ileal folds. Directly beneath them, and just
at the posterior end of the stomach, there is a strong band of circular
striated muscular fibres. The epithelium of these folds is covered with
minute conical spines, which are generally, but not always, wanting
between the folds. (Minot.)
=The colon.=—This section of the intestine (Fig. 319) is sometimes
regarded as a part of the rectum. In the locust the six longitudinal
folds of the ileum are continued into the colon, but their surface,
instead of being smooth as in the ileum, is thrown up into numerous
irregular curved and zigzag secondary folds. The cells of the epithelium
are of uniform size, and the layer is covered by a highly refringent
cuticula without spines; and, like that in the ileum, it rests on a
layer of connective tissue, beyond which follows (1) an internal coat of
longitudinal, and (2) an external coat of circular striated muscular
fibres. (Minot.)
[Illustration:
FIG. 319.—Digestive canal of _Lucanus cervus_: _G_, anterior muscles
of the pharynx; _H_, œsophagus; _I_, gizzard; _K_, chyle-stomach;
_L_, ilium; _M_, colon (cœcal part of); _N_, colon; _O_, rectum;
_a_, frontal ganglion of the vagus; _b_, vagus nerve; _c_, anterior
lateral ganglion connected with the vagus.—After Newport.
]
In butterflies (_Pontia brassicæ_), in _Sphinx ligustri_, and probably
in most Lepidoptera the colon is distinct from the rectum, and is
anteriorly developed into a very large more or less pyriform or
bladder-like cæcum (Figs. 309, 310), which in certain Coleoptera
(Dyticus, Fig. 320, _d_; Silpha, Necrophorus, etc.) is of remarkable
length and shape; it also occurs in Nepidæ (Fig. 327). In the cockroach
a lateral cæcum “is occasionally, but not constantly, present towards
its rectal end,” and a constriction divides the colon from the rectum.
(Miall and Denny.)
=The rectum.=—The terminal section of the hind-gut varies in length and
size, but is usually larger than the colon, and with thick, muscular
walls. In Lepidoptera it is narrow and short.
The rectum is remarkable for containing structures called rectal glands
(Fig. 298). Chun describes those of _Locusta viridissima_ as six flat
folds, formed by a high columnar epithelium and a distinct cuticula;
there is a coat of circular bands corresponding to the furrows between
the glands. Minot states that this description is applicable to the
locusts (Acrydiidæ) he has investigated, the only difference being in
the structural details of the single layers. He claims that the rectal
folds “do not offer the least appearance of glandular structure,”
neither is their function an absorbent one, as Chun supposed. From their
structure and position Fernald regards the rectal glands of Passalus as
acting like a valve, serving to retain the food in the absorptive
portions of the digestive tract till all nutriment is extracted.
[Illustration:
FIG. 320.—_Dyticus marginalis_, ♂ opened from the back: _a_, crop;
_b_, proventriculus; _c_, mid-intestine beset with fine cœcal
glands; _d_, long cœcal appendage of the colon; apodemes;
_B_{1}_-_B_{3}_, apodemes; _vhm_, coxal extensor muscle, moving the
hind leg; _ho_, testis; _dr_, accessory gland; _r_, penis; _e_,
reservoir of the secretion of the anal gland.—After Graber.
]
The epithelial folds of the larvæ of dragon-flies serve as organs of
respiration, the water being admitted into this cavity, and when
forcibly expelled serving to propel the creature forward. Paired and
single anal glands (repugnatorial) enter the rectum of certain
Coleoptera (Figs. 302, _l_; 317, _s_; 320, _e_).
=The vent (anus).=—The external opening of the rectum is situated in the
end of the body, in the vestigial 10th or 11th abdominal segment, and is
more or less eversible. It is protected above in caterpillars, and other
insects with 10 free abdominal segments, by the suranal plate. It is
bounded on the sides by the paranal lobes, while beneath is the
infra-anal lobe.
The anus is wanting in certain insects, and where this is the case the
hind-gut, owing to a retention of the embryonic condition, is usually
separated from the mid-intestine. (See p. 300.)
[Illustration:
FIG. 321.—Enteric canal of _Psyllopsis fraxinicola_: _œ_, œsophagus;
_md_, mid-intestine; _ed_, hind-intestine; _vm_, urinary vessels;
_s_, the coil formed by the hind-intestine and the most anterior
part of the mid-intestine.—After Witlaczil, from Lang.
]
Some remarkable features of the digestive canal in hemipterous
insects are noteworthy. In the Coccidæ, according to Mark, the
anterior end of the long mid-intestine forms, with the hinder end of
the œsophagus, a small loop, whose posterior end is firmly grown to
the wall of the rectum, and forms a cup-like invagination of the
latter. Then the rest of the tube-like stomach turns sidewise and
forms a large loop, which turns back on itself and occupies a large
part of the body-cavity. This loop receives on the anterior end,
near the œsophagus, the two urinary vessels, and forms just below
the opening into the rectum a short cæcum.
In other homopterous genera (Psyllidæ and some Cicadidæ) Witlaczil
describes nearly the same peculiarity, the mid-gut and part of the
intestine forming a loop growing together for a certain distance and
winding round each other (Fig. 321).
=Histology of the digestive canal.=—In all the divisions of the
digestive canal of insects the succession of the cellular layers
composing it is the same: 1st, a cuticula; 2d, an epithelium; 3d,
connective tissue; 4th, muscular tissue. In the locust, the first
division of the canal (fore-gut), there are two muscular coats, an
internal longitudinal and an external circular coat; the fibres are all
striated. The lining epithelium is not much developed, but forms a
thick, hard, and refringent cuticula, which is thrown up into spiny
ridges. In the second division (mid-gut, “stomach”) the epithelium is
composed of very high columnar cells, which make up the greater part of
the thickness of the walls, while the cuticula is very delicate,
slightly refringent, with no ridges, and is probably not chitinous; the
fibres of the muscular coats are not striated, while this division is
also distinguished by the presence of glandular follicles and folds. The
stomach and the cæcal appendages have all these peculiarities in common,
while no other part of the canal is thus characterized.
The third division (intestine and rectum) is composed of an epithelium,
the cells of which are intermediate in size between those of the fore
and mid gut. The cells are often pigmented, and they are covered by a
much thicker cuticula than that of the stomach, but which is not so
thick and hard as that of the œsophagus and proventriculus. The very
refringent cuticula is not thrown up into ridges, though in some parts
it is covered with delicate conical spines, which are very short. “The
epithelium and underlying connective tissue (_tunica propria_) are
thrown up into six folds, which run longitudinally, being regular in the
ileum and rectum (as the rectal glands), but very irregular in the
colon. Outside the depression between each two neighboring folds there
is a longitudinal muscular band, these making six bands. This peculiar
disposition of the longitudinal muscles does not occur in any other part
of the canal; it is, therefore, especially characteristic of the third
division.” (Minot.)
LITERATURE ON THE ORGANS OF DIGESTION
=Treviranus, G. R.= Resultate einiger Untersuchungen über den inneren
Bau der Insekten. (Verdauungsorgane von _Cimex rufipes_.) (Annal. d.
Wetterau. Gesells., 1809, i, pp. 169–177, 1 Taf.)
=Ramdohr, C. A.= Abhandlungen über die Verdauungswerkzeuge der
Insekten. 1811, vii, pp. 221, 30 Taf.
=Dutrochet, R. J. H.= Mémoire sur les métamorphoses du canal
alimentaire dans les insectes. (Journal de Physique, 1818, lxxxvi,
pp. 130–135, 189–204; Meckel’s Archiv, 1818, iv, pp. 285–293.)
=Suckow, F. W. L.= Verdauungsorgane der Insekten. (Heusinger’s
Zeitschr. f. organ. Physik., 1828, iii, pp. 1–89.)
=Doyère, L.= Note sur le tube digestif des Cigales. (Ann. Sc. nat.
Zool., 1839, 2 Sér., xi, pp. 81–85.)
=Grube, A. E.= Fehlt den Wespen- oder Hornissenlarven ein After oder
nicht? 1 Taf. (Müller’s Archiv für Physiol., 1849, pp. 47–74.)
=Sirodot.= Recherches sur les sécrétions chez les insectes. (Ann. Sc.
nat. Zool., 1858, 4 Sér., x, pp. 141–189, 251–328, 12 Pls.)
=Milne-Edwards, H.= Leçons sur la physiologie et l’anatomie comparée,
v, 1859, pp. 498–536, 581–638.
=Dufour, L.= Recherches anatomiques sur les Carabiques et sur
plusieurs autres insectes Coléoptères. Appareil digestif. (Ann. Sc.
nat., ii, 1824, pp. 462–482, 2 Pls.; iii, 1824, pp. 215–242, 5 Pls.,
pp. 476–491, 3 Pls.; iv, 1824, pp. 103–125, 4 Pls.; iv, 1825, pp.
265–283.)
—— Recherches anatomiques sur l’Hippobosque des chevaux. (Ann. Sc.
nat., 1825, vi, pp. 299–322, 1 Pl.)
—— Description et figure de l’appareil digestif de l’_Anobium
striatum_. (Ibid., xiv, 1828, pp. 219–222, 1 Pl.)
—— Recherches anatomiques sur les Labidoures. Appareil de la
digestion. (Ibid., xiii, 1828, pp. 348–354, 2 Pls.)
—— Recherches anatomiques et considerations entomologiques sur
quelques insectes Coléoptères, compris dans les familles des
Dermestins, des Byrrhiens, des Acanthopodes et des Leptodactyles.
Appareil digestif. (Ibid., Sér. 2, Zool., i, 1834, pp. 67–76, 2
Pls.)
—— Résumé des recherches anatomiques et physiologiques sur les
Hémiptères. (Ibid., pp. 232–239.)
—— Mémoire sur les métamorphoses et l’anatomie de la _Pyrochroa
coccinea_. Appareil digestif. (Ibid., Sér. 2, Zool., xiii, 1840, pp.
328–330, 334–337, 2 Pls.)
—— Histoire comparative des metamorphoses et de l’anatomie des
_Cetonia aurata_ et _Dorcus parallelepipedus_. Appareil digestif.
(Ibid., Sér. 2, Zool., 1824, xviii, pp. 174–176, 2 Pls.)
—— Anatomie générale des Diptères. Appareil digestif. (Ibid., Sér. 3,
Zool., i, 1814, pp. 248, 249.)
—— Histoire des métamorphoses et de l’anatomie du _Piophila
petasionis_. Appareil digestif. (Ibid., Sér. 3, Zool., i, 1844, pp.
372–377, 1 Pl.)
—— Études anatomiques et physiologiques sur les insectes Diptères de
la famille des Pupipares. Appareil digestif. (Ibid., Sér. 3, Zool.,
iii, 1845, pp. 67–73, 1 Pl.)
—— Recherches sur l’anatomie et l’histoire naturelle de l’_Osmylus
maculatus_. Appareil digestif. (Ibid., Sér. 3, Zool., ix, 1848, pp.
346–349, 1 Pl.)
—— Études anatomiques et physiologiques, et observations sur les
larves des Libellules. Appareil digestif. (Ibid., Sér. 3, Zool.,
xvii, 1852, pp. 101–108, 1 Pl.)
—— Recherches anatomiques sur les Hyménoptères de la famille des
Urocerates. Appareil digestif. (Ibid., Sér. 4, Zool., i, 1854, pp.
212–216, 1 Pl.)
—— Fragments d’anatomie entomologique. Sur l’appareil digestif du
_Nemoptera lusitanica_. (Ibid., Sér. 4, viii, 1857, pp. 6–9, 1 Pl.)
—— Recherches anatomique et considerations entomologiques sur les
Hémiptères du genre Leptopus. Appareil digestif. (Ibid., Sér. 4,
Zool., 1858, x, pp. 352–356, 1 Pl.)
—— Recherches anatomiques sur l’_Ascalaphus meridionalis_. Appareil
digestif. (Ibid., Sér. 4, xiii, 1860, pp. 200–202, 1 Pl.)
=Leydig, F.= Zur Anatomie von _Coccus hesperidum_. (Zeitschr. f.
wissens. Zool., v, 1853, pp. 1–12, 1 Taf.)
=Lubbock, J.= On the digestive and nervous System of _Coccus
hesperidum_. (Proc. Roy. Soc., ix, 1886, pp. 480–486; also Ann. Mag.
Nat. Hist., 1859, Ser. 3, iii, pp. 306–311.)
=Scheiber, S. H.= Vergleichende Anatomie und Physiologie der
Œstriden-Larven. V. Das chylo- und uropœtische System. (Sitzber. d.
k. Akad. d. Wissens. Wien. Math.-naturwiss. Cl., 1862, xlv, pp.
39–64, 1 Taf.)
=Gerstaecker, A.= Bronn’s Klassen und Ordnungen des Tierreichs. V.
Gliederfüssler. (Ernährungsorgane, pp. 87–105.)
=Graber, V.= Zur naheren Kenntnis des Proventriculus und der
Appendices ventriculares bei den Grillen und Laubheuschrecken.
(Sitzber. d. k. Akad. d. Wissensch. Wien. Mathem.-naturwiss. Cl.,
lix, 1869, pp. 29–46, 3 Taf.)
—— Ueber die Ernährungsorgane der Insekten und nächstverwandten
Gliederfüssler. (Mitteil. d. naturwiss. Vereins für Steiermark.
Graz, 1871, ii, pp. 181, 182.)
—— Verdauungssystem des Prachtkäfers. (Ibid., Graz, 1875.)
—— Die Insekten., i, 1877. (Verdauungsapparat, pp. 308–328.)
=Wilde, K. F.= Untersuchungen über den Kaumagen der Orthopteren.
(Archiv f. Naturgesch., xliii Jahrg., 1877, pp. 135–172, 3 Taf.)
=Simroth, H.= Ueber den Darmkanal der Larven von _Osmoderma eremita_
mit seinen Anhängen. (Giebel’s Zeitschr. f. d. ges. Naturwiss.,
1878, li, pp. 493–518, 3 Taf.)
=Müller, H.= Ueber die angebliche Afterlösigkeit der Bienenlarven.
(Zool. Anzeiger, 1881, pp. 530, 531.)
=Schiemenz, Paulus.= Ueber das Herkommen des Futtersaftes und die
Speicheldrüsen der Bienen, nebst einem Anhänge über das Riechorgan.
(Zeitschr. f. wissens. Zool., xxxviii, 1883, pp. 71–135, 3 Taf.)
=Rovelli, G.= Alcune ricerche sul tubo digerente degli Atteri,
Ortotteri e Pseudo-Neurotteri. (Como, 1884, p. 15.)
=Beauregard, H.= Structure de l’appareil digestif des Insectes de la
tribu des Vésicants. (Compt. rend. Acad. Paris, 1884, xcix, pp.
1083–1086.)
—— Recherches sur les Insectes vésicants., 1 Part, Anatomie. (Journ.
Anat. Phys. Paris, 1885, xxi Année, pp. 483–524, 4 Pls.; 1886, xxii
Année, pp. 85–108, 242–284, 5 Pls.)
—— Les Insectes vésicants, Paris, 1890, Chap. III, Appareil digestif,
pp. 63–99; (Phénomènes digestifs, pp. 161–170; Pls. 6–9.)
=Wertheimer, L.= Sur la structure du tube digestif de l’_Oryctes
nasicornis_. (Compt. rend. Soc. Biol. Paris, 1887, Sér. 8, iv, pp.
531, 532.)
=Kowalevsky, A.= Beitrage zur Kenntniss der nachembryonal Entwicklung
der Musciden. (Zeitschr. f. wissens. Zool., xlv, 1887, pp. 542–594,
5 Taf.)
=Schneider, A.= Ueber den Darm der Arthropoden, besonders der
Insekten. (Zool. Anzeiger, 1887, x Jahrg., pp. 139, 140.)
—— Ueber den Darmkanal der Arthropoden. (Zool. Beiträge von A.
Schneider, ii, 1887, pp. 82–96, 3 Taf.)
=Fritze, A.= Ueber den Darmkanal der Ephemeriden. (Berichte der
Naturforsch.-Gesellsch. zu Freiburg i. Br., 1888, iv, pp. 59–82, 2
Taf.)
=Emery, C.= Ueber den sogenannten Kaumagen einiger Ameisen. (Zeitschr.
f. wissens. Zool., 1888, xlvi, pp. 378–412, 3 Taf.)
=Meinert, F.= Contribution à l’anatomie des Fourmilions. (Overs.
Danske Vidensk. Selsk. Forhandl. Kjöbenhavn, 1889, pp. 43–66, 2
Pls.)
=Mingazzini, P.= Richerche sul canale digerente dei Lamellicorni
fitofage (Larve e Insetti perfetti). (Mitteil. Zool. Station zu
Neapel, ix, 1889–1891, pp. 1–112, 266–304, 7 Pls.)
=Fernald, Henry T.= Rectal glands in Coleoptera. (Amer. Naturalist,
xxiv, pp. 100, 101, Jan., 1890.)
=Visart, O.= Digestive canal of Orthoptera. (Atti Soc. Toscana Scient.
Natur., vii, 1891, pp. 277–285.)
=Eberli, J.= Untersuchungen an Verdauungstrakten von _Gryllotalpa
vulgaris_. (Vierteljahresschr. d. Naturforsch. Gesells. Zurich,
1892, Sep., p. 46, Fig.)
=Holmgren, Emil.= Histologiska studier öfver några lepidopterlarvers
digestionskanal och en del af deras Körtelartade bildningar. (Ent.
Tidskr. Årg. xiii, pp. 129–170, 1892, 6 Pls.)
=Ris, F.= Untersuchung über die Gestalt des Kaumagens bei den Libellen
und ihren Larven. (Zool. Jahrb. Abth. Syst., ix, 1896, pp. 596–624,
13 Figs.)
See also the works of Straus-Dürckheim, Newport, Mark, Witlaczil,
Vayssière, Landois, Jordan, Oudemans, Berlese, List, Grassi, Verson,
Miall and Denny, Leidy, Cheshire, Kowalevsky, Gehuchten, Locy, etc.
_b_. Digestion in insects
For the most complete and reliable investigation of the process of
digestion, we are indebted to Plateau, whose results we give, besides
the conclusions of later authors:
In mandibulate or biting insects, the food is conducted through the
œsophagus by means of the muscular coating of this part of the digestive
canal. Suctorial insects draw in their liquid food by the contractions
followed by the dilatations of the mid-intestine (chylific stomach).
Dragon-flies, Orthoptera, and Lepidoptera swallow some air with their
food.
Where the salivary glands are present, the neutral alkaline fluid
secreted by them has the same property as the salivary fluid of
vertebrates of rapidly transforming starchy foods into soluble and
assimilable glucose. In such forms as have no salivary glands, their
place is almost always supplied by an epithelial lining of the
œsophagus, or, as in the Hydrophilidæ, a fluid is secreted which has the
same function as the true salivary fluid.
Nagel states that the saliva of the larva of Dyticus is powerfully
digestive, and has a marked poisonous action, killing other insects, and
even tadpoles of twice the size of the attacking larva, very rapidly.
The larvæ not only suck the blood of their victims, but absorb the
proteid substances. Drops of salivary juice seem to paralyze the victim,
and to ferment the proteids. The secretion is neutral, the digestion
tryptic. Similar extra-oral digestion seems to occur in larvæ of
ant-lions, etc. (Biol. Centralbl., xvi, 1896, pp. 51–57, 103–112; Journ.
Roy. Micr. Soc., 1896, p. 184.)
In carnivorous insects and in Orthoptera, the œsophagus dilates into a
crop (ingluvies) ended by a narrow, valvular apparatus (or gizzard of
authors). The food, more or less divided by the jaws, accumulates in the
crop, which is very distensible; and, when the food is penetrated by the
neutral or alkaline liquid, there undergoes an evident digestive action
resulting, in carnivorous insects, in the transformation of albuminoid
substances into soluble and assimilable matter analogous to peptones,
and, in herbivorous insects, an abundant production of sugar from
starch. This digestion in the crop, a food-reservoir, is very slow, and,
until it is ended, the rest of the digestive canal remains empty.
“Any decided acidity found in the crop is due to the injection of acid
food; but a very faint acidity may occur, which results from the
presence in the crop of a fluid secreted by the cæcal diverticula of the
mesenteron.” (Miall and Denny.)
When digestion in the crop is accomplished, the matters are subjected to
an energetic pressure of the walls through peristaltic contractions, and
then, guided by the furrows and chitinous teeth, pass along or gradually
filter through the valvular apparatus or proventriculus, whose function
is that of a strainer.
At the beginning of the “chyle-stomach” (mesenteron) of Orthoptera are
glandular cæca which secrete a feebly acid fluid. This fluid emulsifies
fats, and converts albuminoids into peptones. It passes forwards into
the crop, and there acts upon the food.
In the mesenteron (mid-intestine) the food is acted upon by an alkaline
or neutral fluid, never acid, either secreted, as in Orthoptera, by
local special glands, or by a multitude of minute glandular cæca, as in
many Coleoptera, or by a simple epithelial layer. It has no analogy with
the gastric juices of vertebrates; its function differs in insects of
different groups; in carnivorous Coleoptera it actively emulsionizes
greasy matters; in the Hydrophilidæ it continues the process of
transformation of starch into glucose, begun in the œsophagus. In the
Scarabæidæ, it also produces glucose, but this action is local, not
occurring elsewhere; in caterpillars, it causes a production of glucose,
and transforms the albuminoids into soluble and assimilable bodies
analogous to peptones, and also emulsionizes greasy matters. Finally, in
the herbivorous Orthoptera there does not seem to be any formation of
sugar in the stomach itself, the production of glucose being confined to
the crop (jabot).
When digestion in the crop is finished, the proventriculus relaxes, and
the contents of the crop, now in a semi-fluid condition, guided by the
furrows and teeth, passes into the mesenteron, which is without a
chitinous lining, and is thus fitted for absorption.
The contents of the mid-intestine (chylific stomach) then slowly and
gradually pass into the intestine, the first anterior portion of which,
usually long and slender, is the seat of an active absorption. The
epithelial lining observed in certain insects seems, however, to
indicate that secondary digestion takes place in this section. The
reaction of the contents is neutral or alkaline.
The second and larger division of the intestine only acts as a stercoral
reservoir. (The voluminous cæcum occurring in Dyticidæ, Nepa, and
Ranatra, whether full or empty, never contains gas, and it is not, as
some have supposed, a swimming-bladder.) The liquid product secreted by
the Malpighian tubes accumulates in this division, and, under certain
circumstances, very large calculi are often formed. In his subsequent
paper on the digestion of the cockroach, Plateau states that in the
intestine are united the residue of the work of digestion and the
secretion of the urinary or Malpighian tubes, this secretion being
purely urinary.
These organs are exclusively depuratory and urinary, freeing the body
from waste products of the organic elements. The liquid they secrete
contains urea (?), uric acid and abundant urates, hippuric acid (?),
chloride of sodium, phosphates, carbonate of lime, oxalate of lime in
quantity, leucine, and coloring-matters.
The products of the rectal or anal glands vary much in different groups,
but they take no part in digestion, nor are they depuratory in their
nature.
Insects have nothing resembling chylific substances.[51] The products of
digestion, dissolved salts, peptones, sugar in solution, emulsionized
greasy matters, pass through the relatively delicate walls of the
digestive canal by osmose, and mingle outside of the canal with the
blood.
Whatever substances remain undigested are expelled with the excrements;
such are the chitin of the integuments of insects, vegetable cellulose,
and chlorophyll, which is detected by the microspectroscope all along
the digestive canal of phytophagous insects.
In his experiments in feeding the larvæ of Musca with lacmus, Kowalevsky
found that the œsophagus, food-reservoir, and proventriculus, with its
cæcal appendages, always remained blue, and had an alkaline reaction;
the mid-intestine, also, in its anterior portion, remained blue, but a
portion of its posterior half became deep red, and also exhibited a
strong reaction. The hind-intestine, however, always remained blue, and
also had an alkaline reaction. (Biol. Centralbl., ix, 1889, p. 46.)
=The mechanism of secretion.=—Gehuchten describes the process of
secretion in insects, the following extract being taken from his
researches on the digestive apparatus of the larva of Ptychoptera. The
products of secretion poured into the alimentary canal are more or less
fluid; for this reason, it is impossible to say when an epithelial cell
at rest contains these products. For the secreting nature of these cells
is only apparent at the moment when they are ready for excretion; then
the cellular membrane swells out, and a part of the protoplasmic body
projects into the intestinal cavity.
Before going farther, the terms _secretion_ and _excretion_ should, he
says, be defined. With Ranvier, he believes that the elaboration in the
protoplasm of a definite fluid substance is, _par excellence_, the
secretory act, while the removal of this substance is the act of
excretion.
[Illustration:
FIG. 322.—Different phases of the mechanism of secretion and of
excretion.—After Gehuchten.
]
A glandular cell of the chylific stomach, when at rest, is always
furnished with a striated “platform,” or flat surface, or face, on the
side facing the cavity of the stomach, and the free edge of the
platform, or plateau, is provided with filaments projecting into the
digestive cavity (Fig. 322, _f_). These glandular cells, when active,
differ much in appearance. In a great number, the platform (plateau) has
disappeared, and is replaced by a simple, regular membrane. During the
process of secretion, a finely granular mass, in direct continuity with
the protoplasm, swells, and raises the membrane over the entire breadth
of the cell, causing it to project into the intestinal cavity (Fig. 322,
_A_, _B_). These vesicles, or drops of the secretion, whether free or
still attached by a web to the cells, are clear and transparent in the
living insect, but granular in the portions of the digestive canal fixed
for cutting into sections. Gehuchten then asks: “How does a cell gorged
with the products of secretion empty itself?” Both Ranvier and also
Heidenhain believe that one and the same glandular cell may secrete and
excrete several times without undergoing destruction, but their
researches made on salivary glands have not answered the question.
Gehuchten explains the process thus: when the epithelial cell begins to
secrete, the clear fluid elaborated in the protoplasm of the cell
increases the intracellular tension, until, finally, the fluid breaks
through certain weak places in the swollen basal membrane of the
platform, and then easily passes through the closely crowded filaments,
and projects out into the intestinal cavity as a pear-shaped vesicle of
a liquid rich in albumens at first attached to the free face of the
cell, but finally becoming free, as at Fig. 322, _A_, _B_.
When the elaboration of the substance to be secreted is more active, the
mechanism of the secretion is modified. The basal membrane of the
platform may then be raised at several places at once; instead of a
single vesicle projecting into the intestinal cavity, each cell may
present a great number more or less voluminous. If all remain small and
rapidly detach themselves from the glandular cell, the filaments of the
platform are simply separated from each other at different points of the
free face, as in Fig. 322, _C_. On the other hand, when the different
vesicles of a single cell become larger, the filaments of the platform
are compressed and crowded against each other in the spaces between the
vesicles remaining free, and the undisturbed portions of the platform
appear homogeneous (Fig. 322, _D_). After the excretion of the secretory
products by this process of strangulation, the cell then assumes the
aspect of a glandular cell at rest, and may begin again to form a new
secretion.
To sum up: The process of excretion may occur in two ways:
1. Where the membrane ruptures and the substances secreted are sent
directly out into the digestive cavity. 2. Where the vesicles become
free by strangulation, floating in the glandular or intestinal cavity,
and ending by rupturing and coming into contact with the neighboring
vesicles or with the food.
=Absorbent cells.=—Besides the glandular or secreting cells in
Ptychoptera, there is between the two regions of the chyle-stomach lined
with these cells a region about a centimetre long composed of absorbent
cells. The absorbent cells are very large, polygonal, and contain a
large nucleus, in which is a striated convoluted chromatic cord.
The food on entering the chyle-stomach is brought into contact with the
products secreted in the proventriculus, in the first part of the
chyle-stomach, and in the tubular glands. These products of secretion
act on the food, extracting from them useful substances which they
render soluble. These substances, after having been absorbed by the
absorbent cells in the middle region of the stomach, undergo special
modifications, and are transformed into solid products, which are
situated at the bottom of these cells. Afterwards the alimentary
substances freed from a portion of their useful substances are again
placed in contact with the products of secretion in the distal part of
the chylific ventricle, and reach the terminal part of the intestine.
“The products of secretion,” adds Gehuchten, “diverted into the
intestinal canal do not come into immediate contact with the alimentary
substances; they are separated from it by a continuous, structureless,
quite thick membrane (the peritrophic membrane), which directly envelops
the cylinder of food matters, extending from the orifice of the
œsophageal valvule to the end of the intestine. Between this membrane
and the free face of the epithelial lining there exists a circular
space, into which are thrown and accumulate the excreted substances. The
latter then cannot directly mingle with the aliments; but when they are
liquid they undoubtedly pass through this membrane by osmose, and thus
come into contact with the nutritive substances. It is the same with the
products of absorption. The absorption of soluble products of the
intestinal cavity is not then so simple a phenomenon as it was at first
thought to be, since these products are nowhere brought into immediate
contact with the absorbent cells” (pp. 90, 91).
The most recent authority, Cuénot, states that absorption of the
products of digestion takes place entirely in the mid-intestine, and in
its cæca when these are present. The mid-intestine exercises a selective
action on the constituents of the food comparable to the action of the
vertebrate liver.
LITERATURE ON THE PHYSIOLOGY OF DIGESTION
=Davy, J.= Note on the excrements of certain insects, and on the
urinary excrement of insects. (Edinburgh New Phil. Journ., 1846, xl,
pp. 231–234, 335–340; 1848, xlv, pp. 17–29.)
—— Some observations on the excrements of insects, in a letter
addressed to W. Spence. (Trans. Ent. Soc. London, Ser. 2, iii, 1854,
pp. 18–32.)
=Bouchardat, A.= De la digestion chez le ver à soie. (Revue et Mag. de
Zool., Sér. 2, 1851, iii, pp. 34–40.)
=Lacaze-Duthiers, H., et A. Riche.= Mémoire sur l’alimentation de
quelques insects gallicoles et sur la production de la graisse.
(Ann. Scienc. natur., 1854, Sér. 4, ii, pp. 81–105.)
=Basch, S.= Untersuchungen über das chylopoetische und uropoetische
System von _Blatta orientalis_. (Sitzungsber. d. math.-naturwiss.
Classe d. Akad. d. Wissensch. Wien., 1858, xxxiii, pp. 234–260, 5
Taf.)
=Lambrecht, A.= Der Verdauungsprozess der stickstoffreichen
Nährmittel, welche unsere Bienen geniessen, in den dazu geschaffenen
Organen derselben. (Bienenwirtschaftl. Centralbl., viii Jahrg.,
1872, pp. 73–78, 83–89.)
=Plateau, F.= Recherches sur les phénomènes de la digestion chez les
insectes, (Mém. Acad. roy. de Belgique, Sér. 2, xli, 1 Part, 1873,
pp. 124, 3 Pls.)
—— Note additionelle au mémoire sur les phénomènes de la digestion
chez les insectes. (Bull. Acad. roy. de Belgique, Sér. 2, xliv,
1877, pp. 710–733.)
=Tursini, G. Fr.= Un primo passo nella ricerca dell’ assorbimento
intestinale degli artropodi. (Rend. d. R. Accadem. di Sc. fis. e
matemat. di Napoli, 1877, xvi, pp. 95–99, 1 Pl.)
=Jousset de Bellesme=, Physiologie comparée. Recherches expérimentales
sur la digestion des insectes et en particulier de la blatte. Paris,
1876, vii, and 96 pp., 3 Pls.
—— Recherches sur les fonctions des glandes de l’appareil digestif des
Insectes. (Compt. rend., lxxxii, Paris, 1876, pp. 97–99.)
—— Travaux originaux de physiologie comparée. (i, Insectes, Digestion,
Métamorphoses.) Paris, 1878, 5 Pls.
=Simroth, H.= Einige Bemerkungen über die Verdauung der Kerfe.
(Zeitschr. f. d. gesammten Naturwiss., xli, 1878, pp. 826–831.)
=Krukenberg, C. Fr. W.= Versuche zur vergleichenden Physiologie der
Verdauung und vergleichende physiologische Beiträge zur Kenntnis der
Verdauungsvorgange. (Untersuch, a. d. physiolog. Institut d.
Universität Heidelberg, 1880, i, 4, pp 327, Figs.; ii, 1, p. 1,
Figs.)
=Metschnikoff, E.= Untersuchungen über die intrazelluläre Verdauung
bei wirbellosen Tieren. (Arb. d. zool. Instit. Wien., 1883, v, pp.
141–168, 2 Taf.)
=Locy, William A.= Anatomy and physiology of the family Nepidæ. (Amer.
Naturalist, xviii, 1884, pp. 250–255, 353–367, 4 Pls.)
=Vangel, E.= Beiträge zur Anatomie, Histiologie, und Physiologie des
Verdauungsapparates des Wasserkäfers, _Hydrophilus piceus_.
(Termész. Füzet., x, 1886, pp. 111–126 (in Hungarian); pp. 190–208
(in German), 1 Pl.)
=Schönfeld.= Die physiologische Bedeutung des Magenmundes der
Hönigbiene. (Archiv f. Anat. u. Physiol., Physiol. Abt., 1886, pp.
451–458.)
=Faussek, V.= Beiträge zur Histiologie des Darmkanals der Insekten.
(Zeitschr. f. wiss. Zool., 1887, xl, pp. 694–712, 1 Taf.; Abstract
in Zool. Anz., Jahrg. x, pp. 322, 323, 1 Taf.)
=Frenzel, J.= Ueber Bau und Thätigkeit des Verdauungskanals der Larve
des _Tenebrio molitor_, mit Berücksichtigung anderer Arthropoden
(Berlin. Ent. Zeitschr., 1882, pp. 267–316, 1 Taf.); Inaug.-Diss.
Göttingen, 1882.
—— Einiges über den Mitteldarm der Insekten, sowie über
Epithel-regeneration. (Archiv f. Mikrosk. Anat., 1885, xxvi, pp.
229–306, 3 Taf.)
—— Zum feineren Bau des Wimperapparates. (Ibid., 1886, xxviii, pp.
53–80, 1 Taf.)
—— Die Verdauung lebenden Gewebes und die Darmparasiten. (Archiv f.
Anat., 1891.)
=Gehuchten, A. van.= Recherches histologiques sur l’appareil digestif
de la _Ptychoptera contaminata_, I Part. Étude du revêtment
épithélial et recherches sur la sécrétion. (La Cellule, 1890, vi,
pp. 183–291, 6 Pls.)
=Cuénot, L.= Études physiologiques sur les Orthoptères. (Arch. Biol.,
xiv, 1895, pp. 293–341, 2 Pls.)
=Needham, James G.= The digestive epithelium of dragon-fly nymphs.
(Zool. Bull., i, 1897, Chicago, pp. 104–113, 10 Figs.)
With the writings of Mingazzini (see p. 323), Kowalevsky, Ranvier,
Haidenhain, Beauregard (p. 323), Sadones.
THE GLANDULAR AND EXCRETORY APPENDAGES OF THE DIGESTIVE CANAL
Into each primary division of the digestive canal open important glands.
The salivary and silk-glands are offshoots of the œsophagus (stomodæum);
the cœcal appendages open into the stomach (mesenteron), while the
urinary tubes grow out in embryonic life from the primitive intestine
(proctodæum), and there are other small glands which are connected with
the end of the hind-intestine.
_a._ The salivary glands
We will begin our account of these glands with those of the Orthoptera,
where they are well developed. In the cockroach a large salivary gland
and accompanying reservoir lie on each side of the œsophagus and crop.
The gland is a thin, leaf-like, lobulated mass, divided into two
principal lobes. These open into a common trunk, which after receiving a
branch from a small accessory lobe, and from the salivary reservoir,
unites with its fellow to form the unpaired salivary duct which opens
into the under side of the lingua. Each salivary reservoir is a large
oval sac with transparent walls. (Miall and Denny, also Figs. 299, _sr_,
and 327.) The ducts and reservoirs have a chitinous lining, and the
ducts are, like the tracheæ, surrounded by a so-called spiral thread, or
by separate, incomplete, hooplike bands, which serve to keep the duct
permanently distended. In the locust (Fig. 298) the lobules are more
scattered, forming small separate groups of acinose glands. In the
embryo of Forficula Heymons has observed a pair of salivary glands
opening on the inner angle of the mandibles, a second pair opening in
the second maxillæ, while a third pair of glands, whose function is
doubtful, is situated in the hinder part of the head, opening to the
right and left on the chitinous plate (postgula) behind the submentum.
In Perla, there are two pairs segmentally arranged (Fig. 343).
[Illustration:
FIG. 323.—Left side of the head of the silkworm: _a_, adductor muscle
of the mandible, from which the muscular fibres have been removed;
_b_, upper fibres of the same; _c_, lower fibres cut away to show
the adductor muscle (_e_); _d_, fibres inserted on the accessory
adductor lamella; _f_, œsophagus, much swollen; _g_, salivary gland;
_h_, dorsal vessel; _i_, _l_, tracheæ of the mandibular muscles;
_k_, trachea; _n_, optic nerve.—After Blanc.
]
[Illustration:
FIG. 324.—Lower side of the head of the silkworm exposed, the spinning
apparatus, the œsophageal ganglion, and the adductor of the left
mandible removed: _M_, mandible; _P_, abductor of the mandible; _R_,
adductor; _N_, salivary gland attached at _O_ to the edge of the
adductor muscle; _o_, _o_, transverse portion of the “hyoid”; 3,
masticator nerve and its recurrent branch (7); _L_, tongue cut
horizontally.—After Blanc.
]
Here we might refer to a pair of glands regarded by Blanc as the true
salivary glands. They do not appear to be the homologues of the salivary
glands of other insects, though probably functioning as such. The
functional salivary glands of lepidopterous larvæ have been overlooked
by most entomotomists, and the spinning glands have been, it seems to
us, correctly supposed to be modified salivary glands. Lucas also
regards those of case-worms (Trichoptera) as morphologically salivary
glands. Those of the silkworm were figured by Réaumur (Tom. i, Pl. v,
Fig. 1), but not described; while those of Cossus, which are voluminous,
were regarded by Lyonet as “_vaisseaux dissolvans_.” Dr. Auzoux (1849),
in his celebrated model of the silkworm, represented them accurately,
while Cornalia briefly described them as opening into the mouth. The
first satisfactory description is that of Blanc (1891), who states that
in the silkworm “the two salivary glands” are small, flexuous, yellow
tubes, which occupy a variable position on the sides of the œsophagus
(Fig. 323). The glandular portion passes into the head, ending at the
level of the adductor plate of the mandibles (Fig. 324, _o_), and
entering the buccal cavity at the base of the mandible, as seen in Fig.
323. It is plain, when we recognize the direct homology of the
silk-glands of the caterpillars with the salivary glands of other
insects, and of the spinneret with the hypopharynx, that these so-called
“salivary glands” in lepidopterous larvæ are different structures. They
are probably modified coxal glands, belonging to the mandibular segment.
[Illustration:
FIG. 325.—One of the two salivary glands of _Cæcilius burmeisteri_:
_d_, excretory duct; _cn_, the lumen or canal; _cg_, gland-cells;
_ct_, salivary fluid.—After Kolbe.
]
The polygonal epithelial cells of these glands contain branched
nuclei, recalling those of the spinning-glands. In those
caterpillars which feed on leaves, the salivary glands are slightly
developed, but in such as bore into and eat wood, as the Cossidæ,
the glands are, as figured by Lyonet, very large, forming two
sausage-shaped bodies passing back to the beginning of the
mid-intestine, each ending in a long convoluted filament. The
salivary glands of the imago are very long and convoluted (Fig. 310,
_sd_).
In the Panorpidæ these glands differ in the sexes, the males having
three pairs of very long tortuous tubes, while, in the females, they
are reduced to two indistinct vesicles. (Siebold.)
In the Diptera in general there are two pairs, one situated in the beak,
the other in the thorax. In the larvæ there is a single pair (Fig. 341).
Kraepelin describes a third pair in the Muscidæ at the point of
transition from the fulcrum to the œsophagus, but Knüppel has apparently
found only what may be fat cells at this point, so that the supposed
presence of a third pair in Diptera needs confirmation. In the Psocidæ
there are two salivary glands, of simple tubular shape (Fig. 325).
In the Nepidæ the salivary glands are four in number, and of
conglomerate structure, two being long and extending back into the
beginning of the abdomen, while the other two are about one-fourth as
long. (Figs. 327, 328.) In Cicada, besides a pair of simple tortuous
tubes, there is in the head another pair of glands, each composed of two
tufts of short lobes, situated one behind the other. (Dufour.) In many
Hemiptera (Pyrrhocoris, Capsus, etc.) there is but a single pair, each
gland consisting of four lobes; in the Coccidæ each gland is divided
into two lobes (Fig. 326); in the Aphidæ, according to Witlaczil, they
consist of two lobes grown together. In the Psyllidæ they are said to be
absent.
In _Phylloxera vastatrix_ the saliva is forced through a salivary
passage out of the duct and into the mouth by a pumping apparatus
furnished with special muscles. (Krassilstschik.)
In the Odonata acinose glands are present in the imago, but not in the
nymph until in its last stage, Poletaiew accounting for their absence in
the earlier stages by the fact that the larva swallows more or less
water while taking its food.
In the Coleoptera, as we have observed in Anopthalmus, there are three
pairs of salivary glands (Fig. 74). In the Blapsidæ these glands consist
of many ramifying tubes united on each side of the œsophagus into a
single duct; in others they are but slightly developed, while in still
others they are wanting.
The salivary glands are most highly differentiated in the Hymenoptera,
and especially in the bees (Bombus and Apis), where Schiemenz found not
less than five systems of glands (Fig. 329; also 87), of which four
systems are paired. One pair of these glands lies in the tongue, three
in the head, and one in the thorax.
[Illustration:
FIG. 326.—Acinous salivary glands of _Orthezia cataphracta_. In some
acini the nuclei and boundaries of the cells are shown.—After List,
from Field’s Hertwig.
]
System I is situated in the head, and consists of unicellular
glands; the duct from each cell leads into a common, strongly
chitinized duct, opening into the gullet.
System II, composed of acinose glands, lies also in the head; its
duct is united with that of System III, situated in the thorax.
(Fig. 329, 2, 3.)
System IV is situated at the base of the upper surface of the
mandibles, and forms a delicate sac lined within with glandular
cells; its duct opens at the insertion of the mandibles.
System V lies in the beak, and is a single gland consisting of
unicellular glands; it opens into the common opening of Systems II
and III. This system is wanting in the honey-bee, but occurs in
Bombus and other genera.
[Illustration:
FIG. 327.—Appendages of digestive canal of Belostoma.—After Locy.
]
[Illustration:
FIG. 328.—Salivary and other glands of Ranatra.—After Locy.
]
In all the five systems there constantly occur three cellular
layers: the intima, epithelial, and propria. As regards their origin
Schiemenz states that Systems I and IV are new structures, that
System III arises in part, and Systems II and V wholly, from the
silk-glands of the larva. As the glands differ much in the sexes,
and in different species and genera, Schiemenz believes that their
function is very manifold.
In addition to those previously discovered by Schiementz, Bordas has
detected two additional pairs of salivary glands in the worker and
male honey-bee, _i.e._ the internal mandibular and sublingual
glands, so that in Apis there are in all six pairs, and apparently
one unpaired.
The delicate chitinous external layer of the gland is perforated by many
very fine pores through which the salivary fluid secreted by the
epithelial cells passes into the salivary duct. The glands are
externally bathed by the blood.
In many insects, including lepidopterous larvæ, the single median
opening of the salivary duct is converted into a spraying apparatus.
In the adult Lepidoptera, according to Kirbach:—
[Illustration:
FIG. 329.—Salivary glands of the honey-bee: systems No. 1–3, × 15:
_sv_, salivary valve (of systems 2 and 3) at base of tongue; _lp_,
labial palpus; _mx_, maxilla; _so_, salivary opening of system 1 in
hypopharyngeal plate; _no_, openings in plate for termination of
taste-nerve; _œ_, œsophagus; _sd_, salivary duct; _b_, junction of
ducts of system No. 2; _c_, junction of ducts of system No. 3; _sc_,
_sc_, salivary sacs; _fl_, front lobe; _bl_, back lobe; _a_,
chitinous duct, with spiral thread. _B_, single acinus of system No.
1, × 70: _n_, nucleus; _st_, salivary tract; _d_, large duct. _C_,
single pouch, or acinus, from system No. 2: _a_, propria or outer
membrane; _sc_, secreting cells. _D_, termination of system No.
3:_{1},_{2},_{3},_{4}, lines marking end of section; _d_, duct in
section; _sc_, secreting cells in section; _n_, nucleus.—After
Cheshire.
]
“Its lower half forms a thick chitinous gutter, with a concave cover
above, in which the similarly shaped upper half lies encased, so
that between the two only a small semicircular opening remains.
Powerful muscles extend from the cover to the lower side and to the
two ridges of the bottom plate; through their contraction the upper
channel is elevated, and presses out of the hinder part of the ducts
into the space thus formed a great quantity of the saliva, which by
allowing the contraction of the cover-muscle through the
crevice-like opening, which is situated in the lower edge of the
mouth-opening, becomes squeezed out in order either to mix with the
fluid where the 2d maxillæ fuse, passing up into the canal in the
proboscis, or to penetrate into and thus dilute the semi-fluid or
solid substances taken, into the proboscis.”
The morphology and general relations of the salivary glands have
been sketched out by Hatschek, Patten, and by Lucas, from
observations on those of the case-worms or larval Trichoptera.
[Illustration:
FIG. 330.—Eight pairs of glands of Andrena: I, thoracic; II,
postcerebral; III, supracerebral; IV, lateropharyngeal; V,
mandibular; VI, internomandibular; VII, sublingual; VIII, lingual;
_Md_, mandible; _L_, tongue; _o_, eye; _œ_, œsophagus; _J_,
honey-sac.—After Bordas.
]
Patten states that the spinning-glands in Neophylax are formed by a
pair of ectodermal invaginations on the ventral side of the embryo,
between the base of the 2d maxillæ and the nervous cord. They
increase rapidly in length, and “they also unite to form a common
duct, which opens at the end of the upper lip.”
The salivary glands in the same insect are “formed by invagination
of the ectoderm on the inner sides of the mandibles, in the same
manner as are the spinning glands.”
Lucas has shown that in trichopterous larvæ (Anabolia) there are
three pairs of salivary glands in the head, which are serially
arranged. The first pair belong to the mandibular, the second pair
to the 1st maxillary, and the third pair, or spinning glands, to the
2d maxillary segment. The first or mandibular glands open into the
mouth at the base of the mandibles directly behind the dorsal
condyle. The second pair open between the 1st and 2d maxillæ; at the
base of the latter, near the ventral condyle of the mandibles. The
third pair open into the hypopharynx, which is modified to form the
spinneret. Lucas agrees with Korschelt in regarding them as modified
coxal glands, Schiemenz having previously regarded the headglands of
the imago of the bee as belonging to the segments bearing the three
pairs of buccal appendages, so that each segment originally
contained a pair of glands. It is thus proven that the silk-glands
are modified salivary glands adapted to the needs of spinning larvæ,
and indeed in the imago the sericteries revert to their primitive
shape and use as salivary glands.
The serial arrangement of the salivary glands in the Hymenoptera,
where the number varies from five to ten pairs, is clearly proved by
Bordas. He has detected five more pairs than were previously known,
and names the whole series as follows:—1, the thoracic salivary
glands, which are larger than the others, and nine other pairs,
which are all contained in the head as follows: 2, postcerebral; 3,
supracerebral; 4, lateropharyngeal; 5, mandibular; 6,
internomandibular, situated on the inner side of the base of
mandible; 7, sublingual; 8, lingual (these and 1 to 7 common to all
Hymenoptera); 9, paraglossal (in Vespidæ); 10, maxillary (very
distinct in most wasps). These glands do not all occur in the same
species, being more or less atrophied.
Bordas further shows the segmental arrangement of the cephalic
glands by stating that the supracerebral glands correspond to the
antennal segment, the sublingual glands to the labial, the
mandibular glands (external and internal) to the mandibular segment,
the maxillary glands to the 1st maxillary segment, the lingual
glands to the 2d maxillary segment, while the thoracic and
postcerebral salivary glands, he thinks, correspond to the ocular
segment, a view with which we are indisposed to agree, although
conceding that each of the six segments of the head has in it at
least one pair of salivary glands.
=Functions of the different salivary glands in Hymenoptera.=—The
secretion of the thoracic glands is feebly alkaline. The
postcerebral salivary glands, considered by Ramdohr to be organs of
smell, secrete, like the preceding, a distinctively alkaline fluid,
which mingles with the products of the thoracic glands. The
supracerebral glands, also equally well developed in all
Hymenoptera, though much atrophied in the females and especially the
males of _Apis mellifica_, also in the Vespinæ and Polistinæ,
secrete an abundant, feebly acid liquid, which is actively concerned
in digestion.
As to the mandibular glands, which Wolf supposed to be olfactory
organs, their acid secretion, though smelling strongly, acts
energetically on the food as soon as introduced into the mouth.
The sublingual glands, atrophied in most Apidæ, but relatively
voluminous in Sphegidæ, Vespinæ, Polistinæ, Crabronidæ, etc., empty
their secretion into a small prebuccal excavation, where accumulate
vegetable and earthy matters collected by the tongue, and the saliva
secreted by these glands, acts upon them before they pass into the
pharynx. The lingual glands secrete a thick, sticky liquid, which
causes foreign bodies to adhere to the tongue, and also agglutinates
alimentary substances. The uses of the other glands, maxillary and
paraglossal, are from their minuteness undetermined. (Bordas.)
LITERATURE ON THE SALIVARY GLANDS
=Leydig, F.= Zur Anatomie der Insekten. (Archiv Anat. und Phys. 1859.)
—— Untersuchungen zur Anatomie und Histiologie der Tiere. Bonn, 1883,
pp. 174, 8 Taf.
—— Intra- und interzellulare Gänge. (Biolog. Centralblatt, x, 1890,
pp. 392–396.)
=Dohrn, A.= Zur Anatomie der Hemipteren. (Stettin. Entom. Zeit., 1866,
salivary glands, pp. 328–332.)
=Kupffer, C.= Die Speicheldrüsen von _Periplaneta orientalis_ und ihr
Nervenapparat. (Beiträge zur Anatomie und Physiol., 1875.)
=Schiemenz, P.= Ueber das Herkommen des Futtersaftes und die
Speicheldrüsen der Biene. (Zeitschr. f. wissens. Zool., xxxviii,
1883, pp. 71–135, 3 Taf.)
=Korschelt, E.= Ueber die eigentümlichen Bildungen in den Zellkernen
der Speicheldrüsen von _Chironomus plumosus_. (Zool. Anzeiger, 1884,
pp. 189–194, 221–225, 241–246.)
=Hofer, B.= Untersuchungen über den Bau der Speicheldrüsen und des
dazu gehörenden Nervenapparates von Blatta. (Nova Acta d. Kais.
Leopold.-Carol. Deutsch. Akad. d. Naturforscher, li, 1887, pp.
345–395, 3 Taf.)
=Knüppel, A.= Ueber Speicheldrüsen von Insekten. (Archiv für Naturg.,
1887, Jahrg. 52, pp. 269–303, 2 Taf.)
=Blanc, Louis.= La tête du _Bombyx mori_ à l’état larvaire, anatomie
et physiologie. (Extrait des Travaux du Laboratoire d’Études de la
Soie, 1889–1890; Lyon, 1891, p. 180, many figs.)
=Bordas, L.= Anatomie des glandes salivaires des Hyménoptères de la
famille des Ichneumonidæ. (Zool. Anzeiger, 1894, pp. 131–133.)
—— Glandes salivaires des Apides, _Apis mellifica_. ♂ and ♀. (Comptes
rendus Acad. Sc., Paris, cxix, pp. 363, 483, 693–695, 1894; also two
articles in Bull. Soc. Philomath. Paris, 1894, pp. 5, 12, 66.)
—— Appareil glandulaire des Hyménoptères. (Ann. Sc. Nat. Zool., xix,
Paris, 1894, pp. 1–362, 11 Pls.) (See also p. 366.)
=Berlese, Antonio.= Le cocciniglie Italiane viventi sugli agrumi.
Firenze, 1896, 12 Pls. and 200 Figs.
With the writings of Mark, Minot, Locy, List, Krassilstschik, Nagel
(1896).
_b._ The silk or spinning glands, and the spinning apparatus
The larvæ of certain insects, chiefly those of the Lepidoptera, possess
a pair of silk or spinning glands (sericteries) which unite to form a
single duct opening in the upper lip at the end of the lingua, which is
modified to form the spinneret. (See pp. 71, 75.) All caterpillars
possess them, and they are best developed in the silkworms, which spin
the most complete cocoon. Silk-glands also occur in the larvæ of the
Tenthredinidæ, in the case-worms or larval Trichoptera, also in certain
chrysomelid beetles (Donacia, Hæmonia), and in a weevil (Hypera). In a
common caddis-worm (Limnophilus) the glands are of a beautiful pale
violet-blue tint, and two and a half times as long as the larva itself;
viz. the body is 20 mm. and the glands 55 mm. in length.
In caterpillars the glands are of tubular shape, shining white, and much
like the ordinary simple tubular salivary glands of the imago. When only
slightly longer than the body they are twice folded, the folds parallel
and situated partly beneath and partly on the side of the digestive
canal; not usually, when folded in their natural position, extending
much behind the end of the stomach; but in the silkworms they are so
long and folded as to envelop the hinder part of the canal. In geometrid
caterpillars the glands when stretched out only reach slightly beyond
the end of the body; in Datana they are half again as long as the body.
Helm thus gives their relative length in certain Eurasian caterpillars,
and we add that of _Telea polyphemus_:—
_Vanessa io_ length of body 32 mm.; of the silk glands 26 mm.
_Smerinthus tiliæ_ length of body 63 mm.; of the silk glands 205 mm.
_Bombyx mori_ length of body 56 mm.; of the silk glands 262 mm.
_Antheræa yamamaya_ length of body 100 mm.; of the silk glands 625 mm.
_Telea polyphemus_ length of body 60 mm.; of the silk glands 450 mm.
Thus in Telea the silk-glands are about 18.50 inches in length, being
about seven times as long as the body.
For the most complete accounts of the spinning glands of Lepidoptera and
their mechanism we are indebted to Helm and to Blanc, and for that of
the Trichoptera to Gilson.
The unpaired portion, or spinning apparatus (_filière_ of Lyonet), is
divided into two portions; the hinder half being the “thread-press,” the
anterior division the “directing tubes.” The silk material, stored up in
the thickest portion of the glands, passes into the thread-press (Fig.
334, _A_), which is provided with muscles which force the two double
ribbon-like threads through the directing tube, as wire is made by
molten iron being driven through an iron plate perforated with fine
holes. The entire spinning apparatus, or _filator_, as we may call it,
is situated in the tubular spinneret. The opening of the spinneret is
directed anteriorly, and the anterior end of the directing tube passes
directly into this opening so that the directing tube may be regarded as
an invagination of the lingua.
The silk thread which issues from the mouth of the spinneret is, as
Leeuwenhoek discovered, a double ribbon-like band, as may be seen in
examining the silk of any cocoon.
=The process of spinning.=—Since the appearance of Helm’s account,
Gilson, and also Blanc, have added to our knowledge of the way in which
the silk is spun and of the mechanism of the process. Gilson has
arrived, in regard to the function of the press or filator, at the
following conclusions: 1, the press regulates the thread, it compresses
it, gives it its flattened shape; 2, it regulates the layer of gum[52]
(grès) which surrounds the thread; 3, it may render the thread immovable
by compressing it as if held by pincers.
The process of spinning in the silkworm, says Blanc, comprises all the
phenomena by which the mass of silk contained in the reservoir is
transformed into the silk fluid of which the cocoon is spun. The
excretory canals each contain a cylindrical thread of silk having a mean
diameter of 0.2 mm. and surrounded by a layer of gum (_grès_) which in
the fresh living organ exactly fills the annular space situated between
the fibroin cylinder and the wall. Arrived within the common duct, the
two threads receive the secretion of Filippi’s gland, where the silken
fluid is formed, but has not yet assumed its definite external
characters. The two threads press through the common canal and arrive at
the infundibulum (Fig. 334, _c_) of the press, at the bottom of which is
situated the orifice of the spinning canal, almost completely divided
into two by the sharp edge of the rachis (Figs. 334, _a_, 335, _l_). The
threads each pass into one of the two grooves, and the layer of gum
(_grès_) fills the rest of the canal of the press or filator.
[Illustration:
FIG. 331.—Longitudinal section of the spinneret: _a_, horizontal
portion of the tongue; _b_, vertical portion; _c_, _f_, circle of
the tongue; _d_, tongue-pad; _e_, orifice of the spinneret; _g_,
body of the lyre; _h_, prebasilar membrane forming a fold; _i_,
internal canal of the spinneret; _k_, filator.—After Blanc.
]
[Illustration:
FIG. 332.—The lower lip (labium) of _Bombyx mori_, isolated, seen from
the left side: _A_, lyre; _B_, spinneret; _C_, labial palpus; _D_,
vertical part of the labium; _E_, horizontal part of the same; _H_,
_L_, silk-canal; _K_, right gland of Filippi; _L_, canal of the left
gland; _N_, labial nerve; _a_, oblique fibre of the elevator of the
labium; _b_, right fibre of the same; _c_, depressor of the labium;
_d_, superior spinning muscles.
]
[Illustration:
FIG. 333.—The labium in a horizontal position, seen from the side:
_f_, the filator or press situated under the external part of the
spinneret (_d_), between the branches (_b_), of the lyre (_a_); _e_,
labial palp; _c_, tongue.
]
[Illustration:
FIG. 334.—Longitudinal section of the spinneret and press (_filator_):
_A_, filator or press; _B_, spinneret; _C_–_D_, body of the lyre;
_F_, lower part of the labium; _E_, common canal; _eh_, its
epithelium; _G_, superior muscle of the press; _a_, rachis; _b_, its
posterior enlargement; _c_, infundibulum; _d_, cuticle; _o_, orifice
of the spinning canal; _op_, central canal of the lyre and of the
spinneret; _fi_, hypodermis of the lyre; _f_, _f_, hypodermic pad of
the lyre.
]
The silken substance is then pressed by the more or less powerful
contractions of the muscles of the filator, so that the passage of the
threads is facilitated. If the muscles totally contract, the spinning
canal is opened wide, the threads pass easily upwards and assume the
form of a triangular prism (Fig. 336).
[Illustration:
FIG. 335.—Spinning apparatus, seen from above: _A_, opening of the
spinneret; _B_, central canal of the spinneret (_C_); _D_, common
canal; _E_, canal of Filippi; _F_, excretory canal of a silk-gland;
_i_, orifice of the canal of Filippi’s gland; _l_, rachis; _k_, ring
of the infundibulum; _b_, _c_, _d_, _e_, _f_, cavity of the
different canals; _h_, spur which separates the two excretory
canals.—This and Figs. 331–334 after Blanc.
]
If this contraction diminishes, the chitinous wall of the spinneret
comes together, owing to its elasticity; the ceiling of the canal
approaches the floor; the cavity tends to take the form of a
semicircular slit, and the threads are compressed, flattened. As each
mass or thread of silk is much more voluminous than the canal, except
when the latter is extremely dilated, it follows that the two threads
are always compressed, or squeezed together, and that each of them is
compelled to mould itself in the groove it occupies and to take its
shape. Hence the variations in the appearance of the two masses or
divided portions of silk, which as stated present all grades between the
form of an isosceles-triangular prism and that of a nearly flat ribbon;
but this last case is quite rare. The use of the spinneret, then, is to
compress the thread and to change its form more or less considerably, at
the same time as it diminishes its diameter.
[Illustration:
FIG. 336.—Diagram of the press and its muscles: _a_, lower; _b_,
lateral; _c_, upper muscles of the press.—After Blanc.
]
Moreover, this constant compression of the thread as it passes through
the press keeps it in a certain state of tension so as to allow the
caterpillar while spinning to firmly hold its thread.
Finally, when the worm suspends the contraction of its spinning muscles,
the press flattens, vigorously compresses the thread, and arrests its
motion, in such a way that if there was a strain on the silken fluid
(_bave_), it would break rather than oblige the caterpillar to let go
any more of it.
The press does not act directly on the silken thread, but through the
gummy layer (_grès_) which transmits over the whole surface of the
silken fluid (_brin_) the pressure exerted on it. After having overcome
this difficult passage, the silk thread has acquired its definite form;
it rapidly passes out of the spinneret.
=How the thread is drawn out.=—Having seen, says Blanc, how the two
masses of silk (_brins_), in passing through the spinning apparatus (or
press), join each other, constituting the frothy silken fluid, thus
becoming modified in form, it remains to examine the way in which the
thread is drawn out of the spinneret. If we examine a caterpillar while
spinning, it will be seen that in moving its head it draws on the frothy
mass of silk fixed to the web of the cocoon. This traction certainly
aids very much the exit of the thread, but it is not the only cause.
The silk, Blanc affirms, is pushed out by a force _a tergo_, developed
by different agents, such as the pressure of the distended cuticle or
the silky mass contained in the reservoir, as seen in the section of a
worm which has spun its cocoon. But if we consider a caterpillar before
it has begun to spin, it is difficult to explain the mechanism of
spinning. As Blanc has often observed, in making sections of the heads
of silkworms, two cases arise. Sometimes the worm has already spun a
little, and a certain length of the frothy silk (_bave_) issues from the
orifice of the spinneret, where it forms a small twisted bundle. At
other times the worm has not spun since its last moult or the frothy
mass of silk has broken within the head, and we find the end in the
common tube. In the first of these two cases, the worm, dilating its
press, is able by a general contraction to discharge a little of the
gritty material (_grès_) which lines the ball of silk hanging at the end
of the spinneret. It can also reject a certain quantity of the secretion
of Filippi’s glands and thus soften the gritty substance. The little
plug of silk can then adhere to the body with which it comes in contact.
In the same case it is necessary that the two bits or portions of silk
traverse the press, and this normally has a calibre less than their
diameter. The worm should then distend the spinning tube as much as is
practicable, so as to make the openings as large as possible. It has
been stated that the press is, in this condition, at least as large as
the mass of frothy silk. This Blanc believes (although Gilson thinks
otherwise) is pushed by a force _a tergo_, and reaches the funnel of the
spinning canal; its two bits of silk (_brins_) unite there, penetrate
into the canal itself, and, owing to successive impulses produced by the
general contractions of the worm, press through and pass out of the
spinneret.
While the silkworm is engaged in spinning its cocoon, the spinneret and
press execute very varied movements, determined by the elevator,
depressor, retractor, and protractor muscles of the labium, as well as
those of the press. These movements, originally very numerous, may
combine among themselves, so that the spinneret is susceptible of
assuming during the process of spinning still more diverse positions.
[Illustration:
FIG. 337.—Portion of the silk-gland of _Bombyx mori_: _p_, tunica
propria; _i_, tunica intima; _s_, secretion-cell with branched
nuclei; _a_, separate secretion-cell from the anterior part of the
silk-gland of _Amphidasis betularia_; _b_, the same of _Vanessa
urticæ_; _c_, the same in _Smerinthus tiliæ_.—After Helm.
]
[Illustration: