firefly (_Photinus?_) is perfectly continuous, without trace of lines
either bright or dark. “It extends from a little above Fraunhofer’s line
C, in the scarlet, to about F in the blue, gradually fading out at the
extremities. It is noticeable that precisely this portion of the
spectrum is composed of rays which, while they more powerfully than any
others affect the organs of vision, produce hardly any thermal or
actinic effect. In other words, very little of the energy expended in
the flash of the fire is wasted. It is quite different with our
artificial methods of illumination. In the case of an ordinary gaslight
the best experiments show that not more than one or two per cent of the
radiant energy consists of _visible rays_; the rest is either invisible
heat or actinism; that is to say, over 98 per cent of the gas is wasted
in producing rays that do not help in making objects visible.”
Panceri also remarks that while in the spectroscope the light of some
Chætopteri, Beroë, and Pyrosoma exhibit one broad band like that given
by monochromatic light, that of Lampyris and Luciola is polychromatic.
(Amer. Nat., vii, 1873, p. 314.)
The filtered rays of Lampyris pass (like Röntgen and uranium rays)
through aluminium (Muraoka).
The physiology of insect phosphorescence is thus briefly stated by Lang:
“The cells of this luminous organ secrete, under the control of the
nervous system, a substance which is burnt during the appearance of the
light; this combustion takes place by means of the oxygen conveyed to
the cells of the luminous body by the tracheæ, which branch profusely in
it and break up into capillaries.”
Emery states that the males of Luciola display their light in two ways.
When at night time they are active or flying, the light is given out at
short and regular intervals, causing the well-known sparkling or
scintillating light. If we catch a flying Luciola or pull apart one
resting in the day time, or cut off its hind body, it gives out a
tolerably strong light, though not nearly reaching the intensity of the
light waves of the sparkling light. In this case the light is constant,
yet we notice, especially in the wounded insect, that the phosphorescent
plate in its whole extent is not luminous, but glows at different places
as if phosphorescent clouds passed over it.
It is self-evident that a microscopic observation of the light of the
glow-worm or firefly is not possible, but an animal while giving out its
light, or a separated abdomen, may readily be placed under the
microscope and observed under tolerably high powers. By making the
experiment in a rather dark room Emery saw clear shining rings on a dark
background. “All the rings are not equally lighted. Comparing this with
the results of anatomical investigation, it is seen that the rings of
light correspond with the previously described circular tracheal
capillaries, _i.e._ the limits between the tracheal-cell cylinder and
the parenchym-cells. The parenchym-cells are never stained of a deep
brown; this proves that its plasma may be the seat of the
light-producing oxidation. Hence this process of oxidation takes place
in the upper surface of the parenchym-cells, but outside of their own
substance. The parenchym-cells in reality secrete the luminous matter;
this is taken up by the tracheal end-cells and burnt or oxidized by
means of the oxygen present in the tracheal capillaries. Such a
combustion can only take place when the chitinous membrane of the
tracheæ is extraordinarily fine and easily penetrable, as is the case in
the capillaries of the photogenic plate; therefore the plasma of the
tracheal cells only oxidizes at the forking of the terminal tracheal
twigs and in the capillaries.” (Emery.)
The color of the light of Luciola is identical in the two sexes, and the
intensity is much the same, though that of the female is more
restricted. The rhythm of the flashes of light given out by the male is
more rapid, and the flashes briefer, while those of the female are
longer, more tremulous, and appear at longer intervals.
Emery then asks: What is the use of this luminosity? Is it only to
allure the females of Luciola, which are so much rarer than the males?
Contrary to the general view that it is an alluring act, he thinks that
phosphorescence is a means of defence, or a warning or danger-signal
against insectivorous nocturnal animals. If we dissect or crush a
Luciola, it gives out a disagreeable cabbage-like smell, and perhaps
this is sufficient to render it inedible to bats or other nocturnal
animals. An acrid taste they certainly do not possess.
It has long been known that the eggs of fireflies, both Lampyridæ and
Pyrophorus, are luminous. Both Newport and more recently Wielowiejski
attributes the luminosity not to the contents of the egg, but to the
portions of the fat-body cells or fluid covering on the outside of the
eggs, due to ruptures of the parts within the body of the female during
oviposition. The larvæ at different ages are also luminous.
The position of the luminous organs changes with age. In the larvæ of
Pyrophorus before moulting, according to Dubois, the luminous organs are
situated only on the ventral side of the head and prothoracic segment.
In larvæ of the second stage there are added three shining spots on each
of the first eight abdominal segments, and a single luminous spot on the
last segment. These spots are arranged in a linear series and thus form
three luminous cords. In the adult beetles there is a luminous spot in
the middle of the first abdominal sternite, but the greatest amount of
light is produced by the two vesicles on the hinder part of the
prothorax, the position of which varies according to the species.
LITERATURE ON PHOSPHORESCENCE
=Peters, W.= Ueber das Leuchten der _Lampyris italica_. (Müller’s
Archiv f. Anatomie, 1841, pp. 229–233.)
=Kölliker, A.= Die Leuchtorgane von Lampyris, eine vorläufige
Mittheilung. (Verhandl. d. phys. medizin. Gesellsch. Würzburg, 1857,
viii, pp. 217–224.)
=Schultze, Max.= Ueber den Bau der Leuchtorgane der Männchen von
_Lampyris splendidula_. (Sitzber. d. niederrhein. Gesellsch. f.
Natur. u. Heilkunde zu Bonn, 1864, Sep. p. 7.)
—— Zur Kenntniss der Leuchtorgane von _Lampyris splendidula_. (Archiv
f. mikroskop. Anat., 1865, i, pp. 124–137, 2 Taf.)
=Wielowiejski, H. Ritter von.= Studien über die Lampyriden. (Zeits. f.
wissens. Zool., 1882, xxxvii, pp. 354–428, 2 Taf.)
=Emery, Carlo.= Untersuchungen über _Luciola italica_ L. (Zeits. f.
wissens. Zool., 1884, xl, pp. 338–355, 1 Taf.)
—— La luce della _Luciola italica_ osservata col microscopio. (Bull.
Soc. Ent. Ital. Anno xvii, 1885, pp. 351–355, 1 Taf.)
=Wheeler, William Morton.= Concerning the blood tissue of the Insecta,
III; Psyche, vi, 1892, p. 255. (Structure of the light organ of
_Photuris pennsylvanica_.)
=Heinemann, C.= Zur Anatomie und Physiologie der Leuchtorgane
Mexikanischer Cucujos, Pyrophorus. (Archiv f. mikroskop. Anat.,
1886, xxvii, pp. 296–383.)
=Dubois, R.= Contribution à l’étude de la production de la lumière par
les êtres vivants. Les Elaterides lumineux. (Bull. Soc. Zool.
France, 1886, Année ii, pp. 1–275, 9 Pls.)
=Wielowiejski, H. Ritter von.= Beiträge zur Kenntniss der Leuchtorgane
der Insekten. (Zool. Anzeiger, 1889, Jahrg. xii, pp. 594–600.)
=Hudson, G. V.= The habits and life-history of the New Zealand
glowworms. (Trans. N. Zealand Inst., xviii, 1890, pp. 43–49, 1 P1.)
=Dubois, R.= Sur le mechanisme de la production de la lumière chez
l’_Orya barbarica_ d’Algérie. (Comptes rend. Acad. Sc. France, 1892,
cxvii, pp. 184–186. Also in Ann. and Mag. Nat. Hist., 1893 (6), xii,
pp. 415–416.)
=Schmidt, P.= Ueber das Leuchten der Zuckmücken (Chironomidæ). (Zool.
Jahrb. Morph., Abth. viii, 1894, pp. 191–216, 2 Taf. Also Ann. and
Mag. Nat. Hist., xv, pp. 133–141, 1895. The light is due to
bacteria. Nature, 1897.)
=Chun, C.= Leuchtorgane und Facettenaugen. (Biol. Centralbl., 1896,
pp. 315–320.)
=Muraoka, H.= (On the filtered rays of Lampyris, Wiedemann’s Annalen,
Dec., 1896.)
See also the writings of Audouin, Carus, D. Turner, Thompson.
THE RESPIRATORY SYSTEM
While land vertebrates breathe by inhaling the air through the mouth
into the lungs, insects respire by internal air-tubes (_tracheæ_), which
ramify throughout every part of the body and its appendages. The air
enters these tubes through a few openings, called spiracles or
_stigmata_, arranged segmentally in the sides of the body. These tracheæ
are everywhere bathed by the blood, and thus the latter is constantly
aërated or kept fresh; the blood not, as in vertebrates or as in
molluscs, seeking the lungs or gills, or any specialized respiratory
portion of the body where the oxygen combines with the hæmoglobin, but
the respiratory tubes, so to speak, themselves seek out the blood and
the blood-tissue in every part of the insect body, penetrating to the
tips of the antennæ and of the legs, entering the most delicate tissues,
even perhaps passing through the walls of epithelial cells. As Lang
remarks, the want of an arterial vascular system is compensated for as
well as conditioned by the extremely profuse branching of the tracheæ.
[Illustration:
FIG. 389.—Rat-tailed larva of Eristalis.
]
The aquatic larvæ of certain dragon-flies (Agrionidæ), may-flies,
case-worms, etc., respire by means of tracheal gills or branchiæ, which
are either filamental or leaf-like appendages containing tracheæ.
Somewhat similar structures appended to the thorax of pupal aquatic
Diptera, as in the mosquito and its allies, enable them to breathe while
stationed a little beneath the surface of the water. Other larvæ, as the
rat-tail larva of Eristalis, etc., lying at the bottom of shallow pools
or in ditches, etc., can breathe by raising slightly above the surface a
long appendage with two spiracles at the end, through which the air
enters the tracheal system. (See p. 461.)
Although Aristotle, as well as the natural philosophers of the Middle
Ages, supposed that insects did not breathe, one can easily see that
they do by holding a grasshopper or dragon-fly in one’s hand and
observing the rhythmical rise and fall of the upper and lower walls of
the abdomen, during which the air enters and passes out of the
air-openings or spiracles on each side of the body.
It is plain that insects consume very little air, since caterpillars may
be confined in very small, almost air-tight tin boxes, and continue to
eat and undergo their transformations without suffering from the
confinement. According to H. Müller an insect placed in a small,
confined space absorbs all the oxygen. Insects can survive for many
hours when placed in an exhausted receiver, or in certain irrespirable
gases. “Cockroaches in carbonic acid speedily become insensible, but
after twelve hours’ exposure to the pure gas they survive and appear
none the worse.” (Miall and Denny, p. 165.) Insects of the swiftest
flight breathe most rapidly, their great muscular activity requiring the
absorption of an abundance of oxygen.
[Illustration:
FIG. 390.—Section of Sphinx embryo, showing at _s_ the ectoderm
invaginated, and forming the germ of a stigma and trachea
(_t_).—After Kowalevsky.
]
Warmth, plenty of food, besides muscular activity, increases the demand
for oxygen and the quantity of carbonic acid exhaled.
_a._ The tracheæ
[Illustration:
FIG. 391.—Portion of a trachea of a caterpillar, with its branches
_B_, _C_, _D_: _a_, peritracheal membrane; _b_, nucleus.—After
Leydig, from Gegenbaur.
]
[Illustration:
FIG. 392.—Structure of a trachea, diagrammatic: portions of the
peritracheal membrane (_hy_) and chitinous intima (_cc_) removed to
show the structure; in the chitinous intima or endotrachea (cc) can
be seen the spiral thickenings or tænidia.—After Lang.
]
It will much simplify our conception of the nature of the air-tubes when
we learn that they originate in the embryo as tubular ingrowths of the
integument (ectoderm), these branching and finally reaching every part
of the interior of the body. They are elastic tubes, and being filled
with air are silvery in color, though at their origin near the spiracles
they are reddish or violet bluish; or, in the larva of Æschna, reddish
brown, this tint being due to a finely granular pigment situated in the
peritoneal membrane.
[Illustration:
FIG. 393.—Longitudinal section of the trachea of _Hydrophilus piceus_:
_ep_, epithelium; _cu_, cuticula; _f_, spiral threads.—After Minot.
]
In their essential structure the tracheæ consist of the chitinous
intima, which is a continuation of the cuticle of the integument, and of
a cellular membrane or outer layer of cells (a continuation of the
hypodermis) called the peritoneal membrane, or ectotrachea (Figs. 392,
393).
Leydig discovered that the spiral filaments are not distinct and
separate, but intimately connected with the inner membrane (intima), and
he detected the outer or peritoneal membrane, which Chun afterwards
found to be epithelial in its nature, Minot stating that it is a true
pavement epithelium.
Figure 393 represents a longitudinal section of a large trachea of
Hydrophilus, showing the peritoneal membrane (_ectotrachea_, _ep_) and
the intima or _endotrachea_, divided into the cuticula (_cu_), with the
darker colored inner layer, in which are embedded the dark-colored
tænidia (_f_).
[Illustration:
FIG. 394.—Testis of Anabrus, showing the ramifications of the
tracheæ.—After Minot.
]
=Distribution of the tracheæ.=—The distribution of the air-tubes, as
Lubbock and also Minot state, depends first upon the shape of the
organs, and upon the size of those whose size is variable. Around the
large, hollow organs (digestive canal, sexual organs) the tracheæ ramify
in all directions, forking so that the branches diverge at a wide angle.
In the organs which have muscular walls, like the oviduct, the tracheæ
run straight when the walls are distended, but have a sinuous course
when the walls are contracted. (Minot.)
“Around the organs of more elongated form the branches of the
tracheæ run more longitudinally, as is shown by the air-tubes of the
muscles, which also present some peculiarities worthy of especial
notice.
“A short, thick trunk arrives at the muscular bundle, and dividing
very rapidly, breaks up into a large number of delicate tubes, which
penetrate between the muscular fibres, then terminating in tubes of
exceeding fineness, which at first sight seem to form a network that
might well be called a _rete mirabile_. A closer examination,
however, reveals that it is not a real network, but rather an
interlacing confusing to the eye. The longitudinal direction of the
tracheæ of the muscles presents a striking contrast to the system of
divarication represented in Figs. 13 and 14. The course of the
tracheæ of the Malpighian tubes is also very curious. There is one
large trachea which winds around the tube in a long spiral, giving
off numerous small branches which run to the surface of the tube,
upon which they form delicate ramifications. Each tube has but a
single main trachea, and I think the trachea continues the whole
length of the tube, but of this last point I am not quite sure.”
(Minot.)
While in the nymphs of Orthoptera the tracheæ very closely resemble
those of the adult, in larvæ of insects with a complete metamorphosis
the tracheæ differ very much in distribution from those of the adult.
The larval tracheæ are also more generalized and more like those of the
original type than the tracheæ of perfect insects. (Lubbock.)
In general there are two main tracheæ, one passing along each side of
the body, near the digestive canal, connected with its mate by a few
transverse anastomosing branches, and sending off a branch to each
spiracle, this arrangement being most simple and apparent in the maggots
of Diptera. From these two main branches smaller twigs branch off into
every part of the body with its appendages, passing among the different
organs, often serving as cables to hold them loosely in place; they also
penetrate into the component parts of the organ themselves, passing into
the fat-bodies, and among the fibres of muscles, where they become
finely attenuated and refined like the capillaries of the vascular
system of vertebrates. (Figs. 395, 396.)
[Illustration:
FIG. 395.—_Melanoplus femur-rubrum_, showing distribution of air-tubes
(tracheæ) and air-sacs; _V_, main ventral trachea (only one of the
two shown); _S_, left stigmatal trachea, connecting by vertical
branches with _D_, the left main dorsal trachea; _c_, left cephalic
trachea; _oc_, ocular dilated trachea. From the first, second,
third, and fourth spiracles arise the first four abdominal air-sacs,
which are succeeded by the plexus of three pairs of dilated tracheæ,
I, II, III, in Fig. 396. Numerous air-sacs and tracheæ are
represented in the head and thorax. The two thoracic spiracles are
represented, but not lettered.
]
[Illustration:
FIG. 396.—_D_, left dorsal trachea; _S_, left stigmatal trachea; I,
II, III, first, second, and third pairs of abdominal dilated
tracheæ, forming a plexus behind the ovaries; 1, pair of enormous
thoracic air-sacs; 2, pair of smaller air-sacs; 3–7, abdominal
air-sacs; _oc_, ocular dilated trachea and air-sacs; _c_, cephalic
trachea. The relations of the heart to the dorsal tracheæ are
indicated.—Drawn by Emerton from dissections by the author.
]
In the youngest larva of _Corethra plumicornis_ Weismann ascertained
the thickness of the longitudinal stem to be 0.0017 mm. That of the
finest tracheal endings in the silk-glands of the silkworm was found
by Von Wistinghausen to be 0.0016 mm. (Zeits. f. Wiss. Zool. xlix,
1890, p. 575.) Weismann states that in the larvæ of Corethra and
Chironomus the tracheal system is only incompletely developed; the
tracheæ are not united with each other, and in the youngest larvæ
they do not contain air.
[Illustration:
FIG. 397.—Tracheal system of the right side of _Machilis maritima_:
_k_, head; I, II, III, thoracic segments; 1–10, abdominal
segments; _s_, stigma.—After Oudemans, from Lang.
]
Each of the two main tracheæ, as Kolbe states, sends off into each
segment of the body three branches.
1. An upper or dorsal branch, which supplies the muscles of the
dorsal region.
2. A middle (visceral) branch, whose twigs pass to the digestive
canal and back to the organs of reproduction.
3. A lower (ventral) branch, whose twigs are distributed to the
ganglia and to the muscles of the ventral region.
In certain Thysanura, as a species of Machilis (Fig. 397), we
probably have the primitive condition of the tracheal system, the
longitudinal and transverse anastomoses being absent, but in other
Thysanura (Japyx, Nicoletia, Lepisma, and a few species of Machilis)
they are present.
As Kolbe remarks, whether the fine ends of the tracheæ are closed or
open, whether after the analogy of the blood capillaries of
vertebrates they anastomose with each other, whether the ends of the
air-tubes pass between the cells or penetrate into them, these
questions are not fully settled. According to Leydig’s[61] latest
views the tracheæ penetrate into the cells and unite with the
hyaloplasma. Hence the process of respiration in the last instance
takes place in the hyaloplasma. This assumption accords with the
fact that in the tracheate Arthropods the terminations of the
tracheæ carry the atmospheric air into the space bounded by the
cellular network, also to the hyaloplasma filling the spaces.
Leydig[62] also thinks that the finest tracheal endings penetrate
into the muscular tissue and unite with the primitive muscular
fibres.
Kupffer is likewise of the opinion that the fine tracheæ penetrate into
the cells, and Lidth de Jeude asserts that they enter the epithelial
cells, “each cell containing several branches.” Kölliker, Emery, etc.,
maintain, however, that the tracheal endings lie between the cells.
Wielowiejski,[63] in describing the line tracheæ of the phosphorescent
organs, thinks that the tracheal endings (tracheal capillaries) rarely
end blindly, but anastomose with one another, forming an irregular
network. The latest observer, Gilson (1893), asserts that tracheal twigs
penetrate deeply into the epithelial cells of the silk glands of larval
Trichoptera as well as of caterpillars, passing through their
protoplasm.
[Illustration:
FIG. 398.—Tracheal network of the male glands of _Lampyris
splendidula_: _tec_, tracheal end-cells; _cap_, tracheal
capillaries; at _a_, an expanded matrix.—After Wielowiejski.
]
[Illustration:
FIG. 399.—Tracheal capillary end-network (_tr. c. n._) of silk glands
of _Ocneria dispar_: _p_, peritoneal (peritracheal) membrane.—After
Wistinghausen.
]
A late investigator, C. von Wistinghausen, finds in the tracheæ of
the spinning-glands of caterpillars a completely formed network
between the terminal branches of two or several tracheal groups. The
tracheal tubes of this series of terminal branches pass into this
network, which he calls the tracheal capillary end-network (Figs.
398, 400). This last varies in thickness and spreads out under the
membrana propria of the glandular mass over the entire surface of
the large gland-cells and on a level with the tracheal capillaries.
The tracheal endings do not penetrate into the cells, but are
separated from the plasma of the cells by a thin membrane. The
tracheal capillary end-network appears as a system of fine tubes
like the tracheal capillaries, consisting of a peritoneal layer and
a chitinous intima (Fig. 400). The walls of these tubes are
homogeneous, not porous, though readily permeable by the
parenchymatous fluid. The interchange of gases consequently may go
on easier and more vigorously in a system of richly anastomosing
tubules of the net-like mass of tracheal capillaries, than in tubes
ending blindly.
While the diameter of the tracheal capillaries is 0.0016 mm. or 1 µ,
that of the tubules composing the tracheal capillary end-network is
scarcely measurable, but is less than 1 µ.
[Illustration:
FIG. 400.—Tracheal end-cells of _Lampyris splendidula_: _tr_,
trachea with tænidia; _tre_, tracheal capillaries.—After
Wielowiejski.
]
These tracheal capillaries also occur on the seminal and other
sexual tubes, on the intestine, on the urinary tubes, on the
fat-bodies, but are most easily detected on the silk-glands.
The latest researches are those of E. Holmgren, who has studied the
branching of the tracheæ in the spinning-glands of caterpillars. He
prefers to call the end-cells “transition cells,” as they lead from
the tracheal tubes proper to the capillary network. This latter is
formed by slender nucleated cells, often with an intracellular
lumen, and, according to the author, probably constituting a
respiratory epithelium. He finds that both large and small tracheæ
may penetrate the gland-cells. (Anat. Anzeiger, xi, 1895, pp. 340–6,
3 figs.; Jour. Roy. Micr. Soc., 1896, p. 182.)
_b._ The spiracles or stigmata
The spiracles are segmentally arranged openings in the sides of the
thorax and abdomen, through which the air passes into the air-tubes. In
its essential structure a spiracle, or _stigma_, is a slit-like opening
surrounded by a chitinous ring, the lips or edges of the opening being
membranous and closed by a movable valve of the spiracle attached by its
lower edge, which is closed by an occlusor muscle (Fig. 401). The
aperture when open forms a narrow oval slit; and in most insects the
slit is within guarded by a row of projecting spines or setæ, which form
a lattice work or grate to keep out dust, dirt, fluids, etc.
[Illustration:
FIG. 401.—Horizontal section of left third stigma and trachea of
_Melolontha vulgaris_, showing the chamber or drum leading into the
trachea: _a_, _a_, external frame or valve protecting the outer
opening of the stigma; _b_, _c_, _c_, inner frame closing the
entrance into the trachea (_l_, _k_); _m_, occlusor muscle closing
the inner orifice.—After Straus-Dürckheim.
]
Krancher[64] has described five leading types of stigmata, not,
however, taking into account those of the Synaptera.
I. _Stigmata without lips_ (Primitive or generalized stigmata).
_a._ The simplest stigma is an aperture which is kept open by a
chitinous ring (Acanthia). The opening may be round or elliptical.
There are no lips nor any movement of the edges to be observed. Such
air-holes occur in the abdomen of bugs (Hemiptera) and beetles
(Coleoptera); within the opening of the stigmata in the same insects
is a funnel-like contraction. Also in the Diptera the abdominal
stigmata are of the same type.[65] The stigmata of the Pulicidae
(Siphonaptera) are more complicated, as the edges of the openings
are provided with setæ (Fig. 402).
[Illustration:
FIG. 402.—First abdominal spiracle with a part of the trachea of the
cat-flea: _sp_, spiracle; _t_, trachea.
]
[Illustration:
FIG. 403.—Stigma of Melolontha larva, seen from without: _b_, bulla;
_s_, sieve-like plate; _o_, curved slit-like opening.—After Boas.
]
_b._ The stigma consists of a series of minute single stigmata,
which are usually surmounted by a common chitinous ring, and whose
tubular continuations unite within in a common trachea, so that the
single tubes pass off from the stigma like the fingers on the hand.
This form is found in the larvæ and puparia of Diptera.
II. _Stigmata with lips_ (Secondary more specialized stigmata).
_c._ The lips are represented by a single chitinous ring, with
sparse spines. One side of the stigma is a little higher, and partly
overlaps the other posteriorly; this form is peculiar to the
Orthoptera and Libellulidae.
_d._ The lips are roof-like, bent inwards and densely hairy, forming
a peculiar kind of felting. The setæ of the lips are in most beetles
and many Lepidoptera separate, and more or less branched. In
caterpillars, the setæ are so finely branched as to form a loose
felt, or sieve-like arrangement.
_e._ The stigmata are round, with a very broad border and a
concentric middle portion, the structure being complicated. The
concentric middle portion is pouch-like and bears the occlusor
muscle. This form occurs in the larvæ of lamellicorn beetles, and
can be seen with the naked eye, or with a lens, in Oryctes, Cetonia,
and Melolontha (Fig. 403).
_f._ Over the outer opening of the spiracle is an incurved chitinous
projection, on one side of which the trachea takes its origin. It is
thus in the Hymenoptera.
The remarkable grate-like stigma of the lamellicorn larvæ has the
appearance as if the outer closing plate or valve were impenetrable.
The earlier observers considered these stigmata to be open, but
Meinert regards them as closed; Schiödte, however, has observed by
pressing a preserved specimen of a Melolontha larva the alcohol
within passing out in drops, through the grate-like plate, and hence
he considers this a proof that the stigma is permeable (Kolbe).
More recently (1893) Boas has examined the same structure in the
same species of larva as examined by Schiödte, and he finds it to be
open only during the process of moulting. He finds that on each side
of the larva there are nine short and wide stigmatic branches, each
of which is shut off from the exterior by a brown plate; this
consists of a reniform sieve-plate, and of a curved bulla which fits
into the cavity of the plate. The stigmatic branch, however, is
provided with a large external opening, which is homologous with the
stigma, but which is usually closed by the plate and bulla, and is
only open during the moulting; at first it is circular, but later
becomes a cleft. A transverse section shows that the bulla is a
simple tegumentary fold, the outer chitinous layer of which has
become especially firm. The plate forms a horizontal half-roof,
which springs from one side of the tracheal orifice, and is
supported by obliquely set bases, which spring from the adjoining
part of the inner side of the tracheæ. The plate and bars are purely
cuticular structures. (Zool. Anz., 1893; also Journ. Roy. Micr.
Soc., p. 54.)
The tracheal system of libellulid nymphs is not closed; on the other
hand, in the fully-grown nymphs the anterior stigmata occurring on
the dorsal side are large, and the tracheæ arising from them are
thick. These stigmata are permeable by the air. In half-grown and
still younger stages of Æschna the two anterior thoracic stigmata
are undeveloped. In order to breathe, the fully-grown nymph either
rises up on the upper side and elevates the end of the body to the
surface in order to take the air into the rectum, or it rests with
the back of the thorax at the surface in order to breathe through
the large stigmata. The young nymphs take in air only through the
rectum. The young nymphs of Libellula and its allies, on the other
hand, possess large thoracic stigmata, but they prefer to breathe
through the rectum. The fully-grown nymphs of Agrion breathe through
the thoracic stigmata. (Dewitz, in Kolbe.)
=The position and number of pairs of stigmata.=—The spiracles are
usually situated in the soft membrane between the tergites and
pleurites, but their exact position varies in different groups. In the
Coleoptera they occupy on the thorax a more ventral position, and on the
abdomen are placed near the edge of the dorsal side, under the elytra.
In the dragon-flies, the first pair is situated much more dorsally than
the second and third pairs; the following seven pairs are almost wholly
ventral and lie concealed in the membranous fold near the external
plate. In the Hemiptera, also, the abdominal stigmata, though entirely
free and visible, are situated ventrally.
Primarily, in the embryo a pair of stigmata appear on each segment of
the thorax and abdomen, except the 10th and 11th, and even possibly in
the head, for a pair of stigmata are said to occur in the head of
Podurids (Smynthurus) (Lubbock), though this statement needs
confirmation. Scolopendrella, however, is known to possess a pair of
cephalic spiracles.
From the foregoing statement it will be seen that while in existing
winged insects no more than 10 (in Japyx 11) pairs of stigmata are to be
found in any one species, yet that 12 segments of the body, in different
groups taken collectively, bear them. The primitive number of pairs of
spiracles, therefore, in winged insects, was 12, _i.e._ a pair in each
thoracic segment, and a pair in each of the first nine abdominal
segments. Insects were originally all holopneustic, and gradually as the
type became differentiated into the different orders they became
peripneustic or amphipneustic, and, in certain aquatic forms, apneustic.
(See pp. 459, 461.)
In the still more primitive, probably wingless, ancestors of insects
there was a larger number of stigmata. Hatschek, in 1877, discovered a
pair of tracheal invaginations in each of the three posterior
head-segments of the embryo of a moth, with stigmatal openings in the
1st and 2d maxillary segments.
Thus early in embryonic life every segment of the body, except those
bearing the eyes and the last abdominal, bore a pair of stigmata, so
that the primitive insect had at least 15, and perhaps more, pairs of
stigmata.
The position of the stigmata is subject to much variation, the result of
adaptation to this or that mode of life. Examples are those insects
which live in dusty situations or usually more or less concealed in the
earth, as in most beetles, and in the Hymenoptera. In such beetles, the
stigmata are situated in the thin membrane between the segments; in the
Hymenoptera, on the upper edge of the segments. In the Siphonaptera,
Pediculina, bed-bug, and similar forms, which breathe an air freer from
dust, the spiracles lie free on the outside of the body.
“When the stigmata are free and without any protection on the
abdomen, there are other ways by which the entrance of foreign
bodies into the tracheæ is prevented. In such cases the body is
covered with dense hairs, as in most Diptera and Neuroptera, as well
as many Lepidoptera; or there is situated in front of the stigma
either a small fissure which is covered over by a number of hairs
arising from the edge, as in many Orthoptera; or, as in most
insects, a luxurious growth of hairs on the inside of the stigma
forms a thick filter for the air. Thus we see that also in this
respect each species of insect is completely adapted to its
surroundings.” (Krancher.)
[Illustration:
FIG. 404.—_A_, thoracic stigma of the house-fly: _Sb_, valve which
closes the opening.
]
[Illustration:
FIG. 405.—Diagrammatic figures of the internal apparatus which closes
the trachea, in the stag-beetle: _A_, trachea open; in _B_, closed;
_St_, the stigma, with its grated lips; _Ct_, cuticula of the
body-walls; _Vk_, closing pouch; _Vbü_, closing bow; _Vba_, closing
band; _M_, occlusor muscle.—From Judeich and Nitsche.
]
=The closing apparatus of the stigma.=—Whether the external opening of
the stigma is permanently open or closed, communication with the tracheæ
may be cut off at pleasure during respiration by an internal apparatus
of elastic chitinous bands and rods and the occlusor muscle.
The parts concerned in this operation are: 1. The closing bow; 2. The
closing lever or peg; 3. The closing band; 4. The occlusor muscle (Figs.
405, 406).
[Illustration:
FIG. 406.—Stigma, with the closing apparatus, of _Smerinthus populi_
(imago), seen from within: _b_, closing bow; _c_, closing band: _o_,
stigmatic opening; _r_, external chitinous ring; _l_, closing lever;
_m_, occlusor muscle; _s_, scales which lie like roofing tiles over
the stigma.—After Krancher.
]
“The first three parts are chitinized; they form a ring around the
stigmatic opening, and are united to each other by joints. The bow is
usually crescentic and as a rule surrounds one-half of the trachea. On
the other side is the closing band which, by different contrivances,
representing the closing lever or peg, becomes closely pressed against
the closing bow. This lever is usually of the shape of a slender
chitinous rod, which causes the closure; but it can also bend
rectangularly, become converted into a typical lever as in the
Lepidoptera, or it may assume the form of two peg-like processes, which
press with their base against the closing bow.” (Krancher.)
“The closure of the spiracular opening is effected by the contraction of
the muscles, while the opening is due to the elasticity of the chitinous
parts. When at rest the spiracle is naturally open, so that the air in
the trachea can directly communicate with the external air. Usually one
end of the muscle is attached to the closing peg, and the other end to
the closing bow. Where, as in Melolontha, the closing apparatus is
provided with two levers, then naturally the muscle binds these two
together and brings about by powerful contractions a firm closure of the
trachea”; but, remarks Krancher, “this is not the only kind; there are
numerous modifications. Besides the form just described, the levers
assume the form of valves (Sirex), or of a brush (Pulex); or of a ring
(larvæ of Diptera) with a circular muscle attached to it; or of a ring
which simply becomes compressed (thoracic stigmata of Diptera).”
_c._ Morphology and homologies of the tracheal system
As first shown by Bütschli, the tracheal system is a series of
segmentally arranged tubular invaginations of the ectoderm; a pair of
stigmata primitively occurring on every segment of the body except
perhaps the most anterior, and the last two or last one, a reduction in
their number having since taken place, until in the Podurans none have
survived. In the supposed ancestor of myriopods and insects, Peripatus,
there are tracheæ; but they are very fine, simple, not-branched
chitinous tubes which are united into tufts at the base of a
flask-shaped depression of the integument, the outer aperture of which
depression is regarded as a stigma. In one species (_P. edwardsii_)
these tufts and their openings are scattered irregularly over the body;
but in another kind (_P. capensis_) some of the stigmata at least show
traces of a serial arrangement, being disposed in longitudinal rows—two
on each side, one dorsally and one ventrally, those of each row,
however, being more numerous than the pairs of legs. (See p. 9 and Fig.
4, _D_.)
It should be observed that in Peripatus, which does not possess urinary
tubes, the segmental organs or nephridia are well developed, hence the
tracheal tubes coexisting with them cannot be their homologues. We are
therefore compelled to regard the tracheal system as of independent
origin, arising in the earliest terrestrial air-breathing arthropod, and
not indebted for its origin to any structure found in worms, unless
perhaps, as both Kennell and Lang suggest, to dermal glands, since,
according to Kennell, certain Hirudinea and many Turbellarian worms
possess long, mostly unicellular, glands which spread far through the
parenchyma of the body. (Kennell.)
Thus Kennell supposes that the ancestors of the Tracheates had
spiracles on every segment of the body where the internal
organization allowed them to exist. “The reduction of the breathing
holes to a smaller number, and their restriction of a pair only to a
single segment, was brought about partly by adaptation to a peculiar
mode of life,—as insect larvæ especially teach us,—partly also—I may
say mechanically—as a result of the obstruction to their development
made by the growth or excessive development of other organs.” Among
these he reckons the thick, dense cuticula of the integument, the
internal fusion of several segments to form body-regions, and the
arrangement and great development of the muscles in the head and
thorax, etc. (p. 29.)
[Illustration:
FIG. 407.—Section through a tracheal pit and diverging bundles of
tracheal tubes taken transversely to the long axis of the body:
_tr_, tracheæ, showing rudimentary spiral fibre; _tr. c_, cells
resembling those lining the tracheal pits, which occur at
intervals along the course of the tracheæ; _tr. o_, tracheal
stigma; _tr. p_, tracheal pit.—After Balfour, from Sedgwick.
]
Kennell has suggested the origin of the tracheæ of Peripatus from
the unicellular dermal glands of annelidan ancestors, since he has
found glands in certain land-leaches of tropical America, which are
provided with enormously long tubular passages united into bundles
and opening externally, these tubes appearing to be slightly
chitinized. Fig. 407 will show the appearance of a bundle of fine
tracheal tubes of Peripatus ending at the bottom of a follicle
formed by a deep invagination of the integument, which may be
regarded as a primitive spiracle. (See Kennell, Ueber einige
Landblutegel des tropical America, Zool. Jahrb. ii, 1886; also Die
Verwandtschaftsverhältnisse der Arthropoden, 1891, p. 25.) We may
add that Carrière supposes from his study of the embryology of the
wall-bee (_Chalicodoma muraria_), published in 1890, that not only
the salivary glands, but also the tentorium, are homologues of the
tracheæ, while other structures than tracheæ may have evolved from
unicellular dermal glands, which are widely distributed. It may in
this connection be observed that some authors derive the book-lungs
or book-leaf tracheæ of Arachnida from the gills of Limulus; hence
if those of Arachnida arose from quite different and more
specialized organs than dermal glands, it is not impossible that the
tracheæ of Peripatus, Myriopods, and insects arose _de novo_, and
then we need not look for any primitive structures in worms from
which they arose.
Although Bütschli in 1870 in his embryology of the honey-bee called
attention to the “great similarity which the eleven pairs of
invaginations in the eleven first trunk-segments in their first
indication (_anlage_) have with the spinning-glands, and also with
the segmental organs of Annelids,” he did not go further than this,
and it is now known that in the 2d maxillary segment open not only
spinning-glands, but in the embryo a pair of stigmata.
Paul Mayer, however, regarded the tracheæ and urinary tubes as
homodynamous structures, and this view was advocated by Grassi
(1885) for the reason that while in the embryo honey-bee there are
ten pairs of stigmata, the first thoracic and two last abdominal
segments wanting them, the germs of the urinary tubes arise in a
corresponding situation on the two last abdominal segments. To this
view Emery (Biol. Centralb., 1886, p. 692) objects that in Peripatus
the nephridia and tracheæ “have nothing to do with the segmental
organs,” as Peripatus besides nephridia possesses both coxal glands
and tracheæ.
Both Kennell and Lang derive the coxal glands of Arthropoda from the
setiparous or parapodial glands of annelid worms, and the recent
endeavor of Bernard to show that the tracheæ arose from setiparous
glands seems to be disproved by the fact that in insects as well as
in other Arthropoda coxal glands with their outlets exist in the
same segments as those bearing stigmata. Reasoning by exclusion, we
are led to regard Kennell’s original view as the soundest.
Patten, however, regards the tracheæ as modified ends of nephridia,
remarking: “Since in Acilius some of the abdominal tracheæ at first
communicate with the cavities of the mesoblastic somites, it is
probable that all the tracheæ represent the ectodermic portions of
the nephridia.” (Origin of Vertebrates from Arachnids, p. 355.)
It is probable, therefore, that the tracheæ first arose as modifications
of dermal glands, as in mites and Peripatus, and that at first they were
not provided with tænidia (as in Chilopoda), while in later forms
tænidia were developed. In the earliest tracheate forms the stigmata
were not segmentally arranged, probably appearing irregularly anywhere
in the body, but afterwards in the myriopods and insects became serially
arranged.
_d._ The spiral threads or tænidia
It is generally supposed that the so-called “spiral thread” forms a
continuous thread from one end of a tracheal branch to the other. This
was first shown not to be the case by Platner in 1844. Minot has proved
that “there is not a single spiral thread, but several, which run
parallel to one another and end after making a few turns around the
trachea.”
The tænidia we have found to be in some cases separate, independent,
solid rings, though when there is more than one turn the thread
necessarily becomes spiral. The tænidia of a main branch stop at the
origin of the smaller branches, and a new set begins at the origin of
each branch. The tænidia at the origin of the branch do not pass
entirely around the inside of the peritoneal membrane; in the axils they
are short, separate, spindle-shaped bands (Fig. 409).
At one point in the main trachea of the larva of Datana the tænidia
were seen to end singly on one side (at a considerable distance from
any branch or axil) at intervals, with a tænidium situated between
them, making four or five turns; then there is only one band
situated between two ends; this band or thread is succeeded by a set
with five turns between the two ends, this set being succeeded by
one complete ring situated between two ends; in all cases the ends
vary in length, some threads being short and others long, so that
they apparently end anywhere along the circumference of the trachea,
and this arrangement is seen to apparently extend along the whole
length of the trachea. Hence it is seen that as a rule the tænidia
vary much in length, and never, as generally supposed, pass
continuously from one end to another of a tracheal branch, for there
are many spirals in a branch, each making only from one to five
turns, most usually four turns. Fig. 408, part of a trachea of
_Dyticus marginatus_, shows that at a slight bend in a trachea the
tænidia is interrupted, and short, incomplete, wedge-shaped tænidia
(_e_) are interpolated; at _A_, _d_ is seen a split in one of the
tænidia (compare also MacLeod, Pl. 1, Fig. 9). The threads are quite
irregular in width. In the axils of the branches there is, as seen
in Fig. 409, a basketwork of independent, short, often
spindle-shaped tænidia; these are succeeded by longer ones, until we
have threads passing entirely around near the base of each new
branch; these being succeeded by others which make from two to five
spiral turns.
[Illustration:
FIG. 408.—Tænidia of Dyticus: _d_, a split tænidium; _e_, _e_, ends of
tænidia.
]
The shape of the tænidia appears to vary to a great extent. In
lepidopterous insects we have observed them to be in their general shape
rather flat and slightly concavo-convex, the hollow looking towards the
centre of the trachea. Minot’s section (Fig. 393) shows that in
Hydrophilus they are cylindrical and solid, and Chun states that those
of Stratiomys are round, while in Eristalis they are round, with a ridge
projecting into the cavity of the trachea; in Æschna the thread is
quadrangular. MacLeod states that sometimes it is cylindrical, in other
cases flat, likewise prismatic; Macloskie believes that the spiral
threads of the centipede are “fine tubules, externally opening by a
fissure along their course.”
[Illustration:
FIG. 409.—Tænidia of Dyticus in an axil of two branches: _e_, _e_,
ends of tænidia.
]
Stokes confirms Macloskie’s statements, stating that in the hemipterous
_Zaitha fluminea_ “the tænidia are fissured tubules formed within and
from chitinized folds of the intima, the convexity of the folds looking
towards the lumen of the tracheæ.” In Fig. 414, 1, are represented
portions of several tænidia showing the fissure, which is sometimes
interrupted; at 2 are seen “the formation of what may be called
apertures in a chitinous bridge.” Stokes regards the tænidia as
“inwardly directed folds of the membrane.” Near the spiracles the
tracheal membrane is externally studded with minute papillæ, as shown at
3, where are represented three broad and incomplete tænidia, with the
tapering end, or the beginning, of another. Stokes adds, “Here they are
only broad grooves, with no appearance of the narrow fissure of the
completed tænidium. At 4 is figured a portion of the internal surface of
a large trachea near the external orifice, the tænidia being in an
incipient stage, evidently forming more or less of a network, as is
usually the case next to the stigma” (compare p. 451, and Fig. 414).
[Illustration:
FIG. 410.—End of salivary duct in base of proboscis of _Stomoxys
calcitrans_: _a_, incomplete and irregular tænidia; _b_, two tænidia
making incomplete rings near the distal end of the duct.
]
The tracheæ of chilopod myriopods appear to be like those of
insects. A number of authors have failed to detect the spiral
threads in the Julidæ. As to the Arachnida, several observers,
including Menge and Bertkau, have denied the existence of the spiral
thread in the spiders with the exception of the Attidæ; and MacLeod
finds them “scarcely visible” in Argyroneta.
Besides the tracheæ, the salivary duct is kept permanently distended by
tænidia, which, however, are not spiral. They usually form incomplete
rings, as in Stomoxys, arranged as shown in Fig. 410.
The labella (proboscis) of flies are supported by incomplete chitinous
tubes or “pseudo-tracheæ,” the ends of which form the scraping teeth,
this being, according to Dimmock, their primary function. Dimmock
describes them as cylindrical channels opening on the surface in zigzag
slits. These channels are held open by incomplete rings, one end of
which is forked. “These rings are apparently arranged so that one has
its fork on one side of the opening of the channel, the next ring the
fork on the opposite side of the channel, and so on, in alternation.
Their true structure is revealed when flattened out.”
[Illustration:
FIG. 411.—Abdominal spiracle (left side) of cockroach (_P.
americana_), side view, showing the bow: _p_, lateral pouch of
spiracle (in centre) seen from within. The tessellated structure of
spiracle and trachea shown at _A_, and the margin of the external
aperture at _B_.—After Miall and Denny.
]
The use of the elastic tænidia is to render the tracheæ elastic, and to
keep them permanently open, as is the case with the parallel rings of
the trachea of the higher vertebrates. The tracheæ are thus rendered
firm and solid, at the least expense of chitinous material. The spiral
thread, as MacLeod remarks, “is the realization in nature of what
engineers call a form of the greatest resistance.”
The tænidia are wanting in the fine endings of the tracheæ (tracheal
capillaries); also in the cockroach, according to Miall and Denny,
they are not developed in the large tracheæ close to the spiracles,
and the intima or wall of the tube has a tessellated instead of a
spiral marking (Fig. 411). The same structure is seen in the Perlidæ
(Nemoura, Gerstaecker, Zeit. f. wissen. Zool. xxiv, Taf. xxiii,
Figs. 5 and 7); also in Æschna (Hagen, Zool. Anz. 1880, p. 159). In
certain fine tracheæ of the eyes of the fly no spiral threads are
developed. (Hickson.) The air-sacs or dilated tracheæ are also
without tænidia.
While in the living insect the main and smaller tracheæ are filled, with
air, it is stated by Von Wistinghausen that the fine capillary ends
contain a fluid.
_e._ Origin of the tracheæ and of the “spiral thread”
While we owe to Bütschli the discovery of the mode of origin and
morphology of the tracheæ, which as he has shown[66] arise by
invaginations of the ectoblast; there being originally a single layer of
epiblastic cells concerned in the formation of the tracheæ; we are
indebted to Weismann[67] for the discovery of the mode of origin of the
“intima,” from the epiblastic layer of cells forming the primitive
foundation of the tracheal structure.
Weismann did not observe the earliest steps in the process of
formation of the stigma and main trunk of the tracheæ, which
Bütschli afterwards clearly described and figured.
Weismann, however, thus describes the mode of development of the
intima; after describing the cells destined to form the peritoneal
membrane, he says: “The lumen is filled with a clear fluid and
already shows a definite border in a slight thickening of the
cell-wall next to it.
“Very soon this thickening forms a thin, structureless intima, which
passes as a delicate double line along the cells, and shows its
dependence on the cells by a sort of adherence to the rounded sides
of the cells (Taf. vii, 97 _A_, _a b c_). Throughout the mass, as
the intima thickens, the cells lose their independence, their walls
pressing together and coalescing, and soon the considerably enlarged
hollow cylinder of the intima is surrounded by a homogeneous layer
of a tissue, whose origin from cells is recognized only by the
regular position of the rounded nuclei (Taf. vii, Fig. 97, _B_).
“Then as soon as the wavy bands of the intima entirely disappear,
and it forms a straight, cylindrical tube, a fine pale
cross-striation becomes noticeable (vii, 97, _B_, _int_), which
forms the well-known ‘spiral thread,’ a structure which, as Leydig
has shown, possesses no independence, but arises merely from a
partial thickening of the originally homogeneous intima.
“Meyer’s idea that the spiral threads are fissures in the intima
produced by the entrance of air is disproved by the fact that the
spiral threads are present long before the air enters. Hence the
correctness of Leydig’s view, based on the histological structure of
the tracheæ, is confirmed by the embryological development, and the
old idea of three membranes, which both Meyer and Milne-Edwards
maintain, must be given up.”
Weismann also contends that the elastic membrane bearing the “spiral
thread” is in no sense a primary membrane, not corresponding
histologically to a cellular membrane. On the contrary, the
“peritoneal membrane comprises the primary element of the trachea;
it is nowhere absent, but envelops the smallest branches, as well as
the largest trunks, only varying in thickness, which in the embryo
and the young larva of Musca stands in relation to the thickness of
the lumen.”
The trachea, then, consists primarily of an epithelial layer, the
“peritoneal membrane,” or the invaginated epiblast; from this layer an
intima is secreted, just as the skin or cuticle is secreted by the
hypodermis. We may call the peritoneal membrane the _ectotrachea_, the
intima or inner layer derived from the ectotrachea the _endotrachea_.
The so-called “spiral threads” are a thickening of the endotracheal
membrane, sometimes arranged in a spiral manner. For these chitinous
bands we have proposed the name _tænidia_ (Greek, little bands).
As to the origin of the spiral thread our observations[68] have been
made on the caterpillar of a species of Datana, which was placed in
alcohol, just before pupation, when the larva was in a semipupal
condition, and the larval skin could be readily stripped off. At this
time the ectotrachea of the larva had undergone histolysis, nothing
remaining but the moulted endotrachea, represented by the tænidia, which
lay loosely within the cavity of the trachea. The ectotrachea or
peritoneal membrane of the pupa is meanwhile in process of formation;
the nuclear origin of the tænidia is now very apparent.
[Illustration:
FIG. 412.—Longitudinal section of a trachea, showing the origin of the
tænidia.
]
[Illustration:
FIG. 413.—Origin of the tænidia from nuclei.
]
Fig. 412 represents a longitudinal section through a secondary tracheal
branch, showing the origin of the chitinous bands, or tænidia. At _t′_
are pieces of six tænidia which have been moulted; _ectr_ indicates the
nuclei forming the outer cellular layer, the ectotrachea or peritoneal
membrane. These nuclei send long slender prolongations around the inside
of the peritoneal membrane; these prolongations, as may be seen by the
figure, become the tænidia. The tænidia, being closely approximate, grow
together more or less, and a thin endotracheal membrane is thus
produced, of which the tænidia are the thickened band-like portions. The
endotracheal membrane is thus derived from the ectotrachea, or primitive
tracheal membrane, and the so-called “spiral thread” is formed by
thickenings of the nuclei composing the secondary layer of nuclei, and
which become filled with the chitin secreted by these elongated nuclei.
The middle portion of the tænidia, immediately after the moult, is clear
and transparent, with obscure minute granules, while the nuclear base of
the cell is filled as usual with abundant granules, and contains a
distinct nucleolus.
[Illustration:
FIG. 414.—Tænidia and internal hairs of _Zaitha_.—After Stokes.
]
The origin of the tænidia is also well shown by Fig. 413, which is
likewise a longitudinal section of a trachea at the point of origin of a
branch. The peritracheal membrane or ectotrachea (_ectr_) is composed of
large granulated nuclei; and within are the more transparent
endotracheal cells; at _t′_ are fragments of the moulted tænidia. The
new tænidia are in process of development at _t_; at base they are seen
to be granulated nuclei, with often a distinct nucleolus, each sending a
long, slender, transparent, pointed process along the inside of the
trachea. These unite to form the chitinous bands or spiral threads.
=Internal hair-like bodies.=—In the large tracheæ of Lampyris very fine
chitinous bristles project free into the cavity of the tube
(Gerstaecher), while according to Leydig there are similar chitinous
points in the tracheæ of the Carabid beetle Procrustes. Dugardin had
previously (1849) called attention to such hairs, giving a list of the
insects in which he observed them. Emery figures a section of the
tracheæ of Luciola, “in wendig behaart.”[69] Stokes has described those
of _Zaitha fluminea_ (Fig. 414) as “internal chitinous, hair-like bodies
arising from the fold of the tænidia and projecting into the lumen of
the tubes.” They are hollow, their minute cavity distinctly
communicating with that of the tænidium, from which they arise by an
enlarged base. They end in an exceedingly fine point which is
occasionally bifid or trifid. In Fig. 414, 4, several are shown attached
to the wrinkles of the tracheæ near a spiracle, and at 5 is represented
a transverse section of a trachea with three hairs projecting into its
cavity.[70]
Stokes has also described “certain minute, elliptical bodies in the
tænidia, each with an internal, presumably glandular, appendage, to
all appearance forming part of the tænidium from which it springs.”
These are shown in Fig. 414, at 1, 3, and, more in detail, at 6;
those at 7, whose thickness is about 1⁄8000 of an inch, appear as
collections of exceedingly minute, rounded apertures in a
cushion-like mass. Although not commonly occurring on the tracheal
membrane between the tænidia, they may be found there, as at 4.
_f._ The mechanism of respiration and the respiratory movements of
insects
By holding a locust in the hand one may observe the ordinary mode of
breathing in insects. During this act the portion of the side of the
body between the stigmata and the pleurum contracts and expands; the
contraction of this region causes the spiracles to open. The general
movement is caused by the sternal moving much more decidedly than the
tergal portion of the abdomen. When the pleural portion of the abdomen
is forced out, the soft pleural membranous region under the fore and
hind wings contracts, as does the tympanum, or ear, and the membranous
portions at the base of the hind legs. When the tergum or dorsal portion
of the abdomen falls, and the pleurum contracts, the spiracles open;
their opening is nearly but not always exactly coördinated with the
contractions of the pleurum, but as a rule they are. There were 65
contractions in a minute in a locust which had been held between the
fingers about ten minutes. It was noticed that when the abdomen
expanded, the air-sacs in the first abdominal ring contracted.
For expanding the abdomen no special muscles are required, since it
expands by the elasticity of the parts. For contracting its walls there
are two sets of muscles, viz., special vertical expiratory muscles
serving to compress or flatten the abdomen (Figs. 415–418), and other
muscles which draw together or telescope the segments.
It was formerly supposed that when the abdomen contracted the air was
expelled from the body and the tracheæ emptied; that, when the abdomen
again expanded by its own elasticity, the air-tubes were refilled, and
that no other mechanism was needed. But Landois insisted that this was
not enough; as Miall and Denny state: “Air must be forced into the
furthest recesses of the tracheal system, where the exchange of oxygen
and carbonic acid is effected more readily than in tubes lined by a
dense intima. But in these fine and intricate passages the resistance to
the passage of air is considerable, and the renewal of the air could, to
all appearance, hardly be effected at all if the inlets remained open.
Landois accordingly searched for some means of closing the outlets, and
found an elastic ring or spiral, which surrounds the tracheal tube
within the spiracle.” By means of the occlusor muscle this ring
compresses the tube, “like a spring clip upon a flexible gas-pipe.”
“When the muscle contracts, the passage is closed, and the abdominal
muscles can then, it is supposed, bring any needful pressure to bear
upon the tracheal tubes, much in the same way as with ourselves, when we
close the mouth and nostrils, and then, by forcible contraction of the
diaphragm and abdominal walls, distend the cheeks or pharynx.”
Thus an important point in the respiration of tracheate animals, whether
insects, myriopods, or arachnids, is, as Landois claimed, the closure of
the spiracles, in order that pressure may be brought upon the air in the
tubes, so that it may pass onward into the finest terminations.
The injection of air by muscular pressure into a system of very fine
tubes may, as Miall and Denny remark, appear extremely difficult or even
impossible. Graham (Researches, p. 44) applies the law of diffusion of
gases to explain the respiration of insects, but until physical
experiments have been made, we may, with Miall and Denny, “be satisfied
that an appreciable quantity of air may be made by muscular pressure to
flow along even the finer air-passages of an insect.”
As to the respiratory movements of insects, Plateau is the principal
authority, and the following account of the process is taken from his
elaborate memoir, and from the statements afterwards contributed by him
to Miall and Denny’s “The Cockroach.”
Although many observers have superficially described the respiratory
movements of various insects, Rathke was the first one to state precise
views as to the mechanism of respiration. His posthumous work, treating
of the respiratory movements of the movable chitinous plates of the
abdomen, and of the respiratory muscles characteristic of all the
principal groups, filled an important blank in our knowledge. But,
notwithstanding the skill displayed in this research, many questions
still remain unanswered which require more exact methods than mere
observations with the naked eye or the simple lens.
Plateau, who was followed a year later by Langendorff, conceived the
idea of studying, by such graphic methods as are now familiar, the
respiratory movements of perfect insects.
“He has made use of two modes of investigation. The first, or
graphic method, in the strict sense of the term, consisted in
recording, upon a revolving cylinder of smoked paper, the
respiratory movements, transmitted by means of very light levers of
Bristol board attached to any part of the insect’s exoskeleton.
Unfortunately, this plan is only applicable to insects of more than
average size. A second method, that of projection, consisted in
introducing the insect, carried upon a small support, into a large
magic lantern fitted with a good petroleum lamp. When the
amplification does not exceed 12 diameters, a sharp profile may be
obtained, upon which the actual displacements may be measured, true
to the fraction of a millimetre. Placing a sheet of white paper upon
the lantern screen, the outlines of the profile are carefully traced
in pencil so as to give two superposed figures, representing the
phases of inspiration and expiration respectively. By altering the
position of the insect so as to obtain profiles of transverse
sections, or of the different parts of the body, and, further, by
gluing very small paper slips to parts whose movements are hard to
observe, the successive positions of the slips being then drawn,
complete information is at last obtained of every detail of the
respiratory movements; nothing is lost.”
“This method, similar to that employed by the English physiologist,
Hutchinson,[71] is valuable, because it enables us, with a little
practice, to investigate readily the respiratory movements of very
small arthropods, such as flies or lady-birds. It has this advantage
over all others, that it leaves no room for errors of
interpretation.”
“Not satisfied with mere observation by such means as these, of the
respiratory movements of insects, the writer has also studied the
muscles concerned, and, in common with other physiologists (Faivre,
Barlow, Luchsinger, Dönhoff, and Langendorff), has examined the
action of the various nervous centres upon the respiratory organs.
The result at which he has arrived may be summarized as follows:—
[Illustration:
FIG. 415.—Muscles of right half of the abdomen of _Forficula
auricularia_: _A_, _a_, longitudinal tergal and sternal muscles;
_D_, _E_, oblique muscles; _a_ (in upper figure) vertical
expirator muscles.
]
“1. There is no close relation between the character of the
respiratory movements of an insect and its systematic position.
Respiratory movements are similar only when the arrangement of the
abdominal segments, and especially when the disposition of the
attached muscles, are almost identical. Thus, for example, the
respiratory movements of the cockroach are different from those of
other Orthoptera, resembling those of the heteropterous Hemiptera.
Those of the Trichoptera are like those of the aculeate Hymenoptera,
while the Locustidæ ally themselves in respect to these movements
with the Neuroptera and Lepidoptera.
“2. The respiratory movements of insects, when at rest, are
localized in the abdomen. As graphically stated by Graber, in
insects the chest is placed at the hinder end of the body. If
thoracic respiratory movements exist, they do not depend on the
action of special muscles.
“3. In most cases the thoracic segments do not share in the
respiratory movements of an insect at rest. The respiratory
displacements of the posterior segments of the thorax are, however,
less rare than Rathke believed. Plateau has observed them in certain
Coleoptera (Staphylinus, Chlorophanus, Corymbites), and they are
more feebly manifested in Hydrophilus, Carabus, and Tenebrio. Among
the singular exceptions to this rule is the cockroach (_Periplaneta
orientalis_), in which the terga of the meso- and metathoracic
segments perform movements exactly opposite in direction to those of
the abdomen (Fig. 419).
[Illustration:
FIG. 416.—Muscles of the left half of abdomen of _Staphylinus
olens_; _A_, _B_, longitudinal dorsal muscles; _D_, _E_, oblique
fascia; _a_, longitudinal sternal muscles; _d_, respiratory
muscles (vertical expirators).
]
“4. Leaving out of account all details and all exceptions, the
respiratory movements of insects may be said to consist of the
alternate contraction and recovery of the figure of the abdomen in
two dimensions, viz. vertical and transverse. During expiration both
diameters are reduced, while during inspiration they revert to their
previous amounts. The transverse expiratory contraction is often
slight, and may be imperceptible. On the other hand, the vertical
expiratory contraction is never absent, and usually marked. In the
cockroach (_P. orientalis_) it amounts to one-eighth of the depth of
the abdomen (between segments 2 and 3); in _Eristalis tenax_ to
one-ninth (at the 2d segment).
“5. Three principal types of respiratory mechanism occur in insects,
and these admit of further subdivision:
“_a._ Sterna usually short and very convex, yielding but little.
Terga mobile, rising and sinking appreciably. To this class belong
all Coleoptera, heteropterous Hemiptera, and Blattina (Fig. 420).
“In the cockroach (Periplaneta), the sterna are slightly raised
during expiration (Fig. 421).
“_b._ Terga well developed, overlapping the sterna on the sides of
the body, and usually concealing the pleural membrane, which forms a
sunken fold. The terga and sterna approach and recede alternately,
the sterna being almost always the more mobile. To this type belong
Odonata, Diptera, aculeate Hymenoptera, and acrydian Orthoptera
(Fig. 422).
[Illustration:
FIG. 417.-Muscles of right half of abdomen of _Phryganea striata_,
♀: _A_, _B_, longitudinal dorsal muscles; _a_, _b_, longitudinal
sternal muscles; _D_, _e_, oblique muscles; 1, 2, inspirator
muscles.
]
“_c._ The pleural membrane, connecting the terga with the sterna, is
well developed and exposed on the sides of the body. The terga and
sterna approach and recede alternately, while the pleural zone
simultaneously becomes depressed, or returns to its original figure.
To this type, Plateau assigns the Locustidæ, Lepidoptera, and the
true Neuroptera (excluding Trichoptera) (Fig. 423).
[Illustration:
FIG. 418.—Muscles of left half of abdomen of Melolontha, ♀: _A_,
_B_, longitudinal muscles (prétracteurs of Straus); _a_, _a_, true
respiratory muscles (expirators).—This and Figs. 415–417, after
Plateau.
]
“6. Contrary to the opinion once general, changes in length of the
abdomen, involving protrusion of the segments and subsequent
retraction, are rare in the normal respiration of insects. Such
longitudinal movements extend throughout one entire group only, viz.
the aculeate Hymenoptera. Isolated examples occur, however, in other
zoölogical groups.
“7. Among insects, such as large beetles, Locustidæ, dragon-flies,
etc., sufficiently powerful to give good graphic tracings, it can be
shown that the inspiratory movement is slower than the expiratory,
and that the latter is often sudden.
[Illustration:
FIG. 419.—Profile of trunk of cockroach (_P. orientalis_). The black
surface represents the expiratory contour, while the inspiratory
is indicated by a thin line. The arrows show the direction of the
expiratory movement: _Ms. th_, mesothorax; _Mt. th_, metathorax.
Reduced from a magic-lantern projection.—After Plateau.
]
“8. In most insects, contrary to what obtains in mammals, only the
expiratory movement is active; inspiration is passive, and effected
by the elasticity of the body-wall.
“9. Most insects possess expiratory muscles only. Certain Diptera
(_Calliphora vomitoria_ and _Eristalis tenax_) afford the simplest
arrangement of the expiratory muscles. In these types, they form a
muscular sheet of vertical fibres, connecting the terga with the
sterna, and underlying the soft, elastic membrane which unites the
hard parts of the somites. One of the most frequent complications
arises by the differentiations of this sheet of vertical fibres into
distinct muscles, repeated in every segment, and becoming more and
more separated as the sterna increase in length. Special inspiratory
muscles occur in Hymenoptera, Acridiidæ, and Trichoptera.
“10. The abdominal, respiratory movements of insects are wholly
reflex. Like other physiologists who have examined this side of the
question, Plateau finds that the respiratory movements persist in a
decapitated insect, as also after destruction of the cerebral
ganglia or œsophageal connectives; further, that in insects whose
nervous system is not highly concentrated (_e.g._ Acridiidæ and
dragon-flies), the respiratory movements persist in the completely
detached abdomen; while all external influences which promote an
increased respiratory activity in the uninjured animal, have
precisely the same action upon insects in which the anterior,
nervous centres have been removed, upon the detached abdomen, and
even upon isolated sections of the abdomen.
“The view formerly advocated by Faivre, that the metathoracic
ganglia play the part of special, respiratory centres, must be
entirely abandoned. All carefully performed experiments on the
nervous system of Arthropoda have shown that each ganglion of the
ventral chain is a motor centre, and, in insects, a respiratory
centre, for the somite to which it belongs. This is what Barlow
calls the ‘self-sufficiency’ of the ganglia.” (Miall and Denny.)
[Illustration:
FIG. 420.—Transverse section of abdomen of a lamellicorn beetle. The
position of the terga and sterna after an inspiration is indicated
by the thick line; the dotted line shows their position after an
expiration; and the arrow marks the direction of the expiratory
movement.
]
[Illustration:
FIG. 421.—Cross-section of abdomen of cockroach.
]
[Illustration:
FIG. 422.—Cross-section of abdomen of bee (Bombus).
]
[Illustration:
FIG. 423.—Cross-section of abdomen of Sphinx.—This and Figs. 420–422
after Plateau.
]
Plateau has made similar observations upon the respiration of
spiders and scorpions; but, to his great surprise, he was unable,
either by direct observation, or by the graphic method, or by
projection, to discover the slightest respiratory movement of the
exterior of the body. This can only be explained by supposing that
inspiration and expiration in pulmonate Arachnida are
“intrapulmonary,” and affect only the proper, respiratory organs.
The fact is less surprising because of the wide zoölogical
separation between Arachnida and insects.
_g._ The air-sacs
In flying insects the tracheæ are in certain parts of the body enlarged
into sacs of various sizes. These air-sacs were first observed by
Swammerdam in a beetle (Geotrupes) and afterwards by Sir John Hunter in
the bee, Sprengel subsequently discovering them in other insects. Those
of the cockroach were described and illustrated in a very elaborate and
detailed way by Straus-Dürckheim (Figs. 424 and 425). These vesicles are
without tænidia. In the locust (_M. femur-rubrum_) there is a pair of
very large vesicles in the prothorax (Fig. 396). The five pairs of large
abdominal air-sacs arise, independently of the main tracheæ, directly
from branches originating from the spiracles. All these large sacs are
superficial, lying directly beneath the hypodermis, while the smaller
ones are buried among the muscles. We have detected 53 of these vesicles
in the head.
In the honey-bee (Fig. 426) and humble bee (Fig. 427) as well as the
flies there are two enormous air-sacs at the base of the abdomen. In
larval and wingless insects these sacs are entirely absent.
[Illustration:
FIG. 424.—Thorax and abdomen of the cockchafer (_Melolontha
vulgaris_), showing the tracheæ and air-sacs.—This and Fig. 425
after Straus-Dürckheim.
]
=The use of the air-sacs.=—It was supposed by Hunter as well as by
Newport, and the view has been generally held, that the use of these
sacs is to lighten the weight, _i.e._ lessen the specific gravity of the
body during flight. It has, however, been suggested to us by A. A.
Packard that this view from the standpoint of physics is incorrect. It
is evident that the wings have to support just as much weight when the
insect is flying, whether the tracheæ and vesicles are filled with air
or not, the body of the insect during flight not being lightened by the
air in the sacs. The use of these numerous sacs, some of them very
spacious, is to afford a greater supply of air or oxygen than that
contained in the air-tubes alone, and thus to afford a greater breathing
capacity. The sacs are largest in dragon-flies, moths, flies, and bees,
which are swift of flight. When we compare the active movements of these
insects on the wing with those of a caterpillar or maggot, it will be
seen that the far greater muscular exertions of the volant insect create
a demand for a sudden and abundant supply of air to correspond to the
increased rapidity of respiration; and the enlargements of the
air-tubes, rapidly filled with air at each inspiration, render it
possible to supply the demand.
[Illustration:
FIG. 425.—Head of _Melolontha vulgaris_, showing the numerous
air-sacs, represented only on the left side, front view.
]
[Illustration:
FIG. 426.—Tracheal, nervous, and digestive systems of the honey-bee
(the tracheal system on the right side only partially drawn): _tb_,
the large vesicles in the abdomen; _st_, stigmata; _hm_, honey
stomach; _cm_, chyle stomach; _vm_, urinary tubes; _rd_, rectal
glands; _ed_, rectum; _a_, antenna; _an_, eye; _b_{1}_-_b_{3}_,
legs.—After Leuckart, from Lang.
]
The case is thus seen to be very different from that of those fishes
which, having a swimming-bladder, can in the water change the
specific gravity of their bodies. The case of insects is almost
exactly paralleled by that of birds, where, as stated by
Wiedersheim, the air-sacs appear to form integral parts of the
respiratory apparatus: “a greater amount of air can by their means
pass in and out during inspiration and expiration, especially
through the larger bronchi, and consequently there is less necessity
for the expansion of the lung parenchyma.” In other words, the
supply of air in these sacs, as in insects, increases the breathing
capacity of the bird during flight. Wiedersheim’s retention of the
old idea that the specific gravity of the body is lessened (p. 262)
seems, however, to be incorrect, as the weight of the bird’s body is
not diminished by the air contained in the sacs.
_h._ The closed or partly closed tracheal system
[Illustration:
FIG. 427.—The lateral and lower series of sacs of _Bombus terrestris_,
♂: _a_, _c_, longitudinal tracheæ;, connected by _b_, and dilated at
_f_, and again in the succeeding segments; _i_, _k_, funnel-shaped
dilatations passing over the dorsal surface of the abdomen and
anastomosing (_g_) with their fellows opposite; at _l_,
communicating directly by a large branch.—After Newport.
]
There are two chief morphological tracheal systems: 1. The open or
normal and primitive (_holopneustic_) type, and 2. The closed, or
secondary and adaptive, _i.e._ _apneustic_, type. The open system is
characterized by the presence of the stigmata. Through them the air
directly enters into the tracheal tubes, whose delicate walls allow the
exchange of gases in the blood. This type occurs in all sexually mature
individuals, and also in the greater number of larvæ.
The closed or apneustic tracheal system is distinguished either by the
want of stigmata, or, if present, they are not open, and do not
function, so that the tracheæ cannot communicate with the air. In such
cases the direct oxygenation of the blood is effected through the
delicate integument, especially over the surface of the body in general,
or in certain specialized places where the gill-like expansions of the
skin are rich in tracheæ; such outgrowths, generally tubular or
leaf-like, are called by Palmén _tracheal gills_.
This closed form of the tracheal system only occurs in the larval stage
of aquatic or parasitic insects, as in the Plectoptera (Ephemeridæ),
Perlidæ, Odonata, and Trichoptera, besides single genera of other
orders, _i.e._ among Coleoptera, Gyrinus, Pelobius, Cnemidotus, and the
young larva of Elmis; in the aquatic caterpillar of Paraponyx; in
certain Diptera (Corethra, Chironomus, etc.), and some of the parasitic
Hymenoptera (Microgaster).
Palmén has discovered that in the nymphs of Ephemeridæ, Perlidæ,
Odonata, and the larvæ of most Trichoptera the tracheal branch
(stigmatal branch) sent from the longitudinal trachea to where the
thoracic stigmata would be situated if present, or where their vestiges
only exist, are aborted, becoming simple solid cords not filled with air
(Fig. 436, _vf_, and 447, _f_, funiculus or stigmatic cord). In the
imago, however, they resume their function, connecting with the open
functional stigmata. In Corethra, in its earliest stages, the entire
tracheal system is, like the stigmatic branch, a system of solid cords
and empty of air. (Palmén.)
Embryology shows that these stigmatal branches are well developed,
and are formed at the same time as the stigmata. It was also shown
by Dewitz, in a posthumous paper (1890), that in the young larval
stage of the Odonata and Ephemeridæ the tracheal system is at first
an open one, and in some of the families (Libellulidæ, Agrionidæ,
and Ephemeridæ) thoracic stigmata are seen at a very early stage.
From numerous experiments Dewitz concludes that in the young stages
of Odonata and Ephemeridæ there is an open tracheal system;
certainly in very young nymphs the thoracic spiracles allow the air
to pass out. Fully grown nymphs of Æschnidæ, Libellulidæ, and
Agrionidæ are capable not only of forcing the air out, but also,
like the perfect insect, of inhaling it. Moreover, he proved that
the gills of Ephemeridæ and Agrionidæ are not indispensable for the
maintenance of life, as the insects can live without them, breathing
either through the skin or by the rectum, or in both ways. It would
seem that while in freshly hatched or very young larvæ of aquatic
insects of different orders the skin is so delicate as to allow of
dermal respiration, in after life, when the skin becomes thicker and
denser, these expansions (gills), provided with a very thin and
delicate skin, of a necessity grow out from the walls of the body.
It thus appears that the closure and total or partial abolition of
the stigmata are in adaptation to aquatic life, and that such
insects have descended from terrestrial air-breathing winged forms.
This is an important argument against the view that the wings are
modified tracheal gills.
In this connection may be noticed the closure of the 2d and 3d
thoracic stigmata in holopneustic insects. We have found on laying
open the body of a Sphinx larva that a large number of tracheal
branches are seen to arise from the prothoracic and from the first
pair of abdominal stigmata. Now between these points there are no
spiracles or any external signs of them, there being in Lepidoptera
no mesothoracic or metathoracic spiracles. Yet the main lateral
trachea between the prothoracic and first abdominal segments
deviates from its course and bends down to send off a small
shrivelled stigmatal branch or cord to a place where, did a spiracle
exist, we should look for it. In the larva of _Platysamia cecropia_,
a similar vestigial stigmata branch is present.
In the larva of Corydalus, also, a trachea as large as the main
longitudinal one takes its origin and passes directly under the main
trachea. Now both tracheæ send a stigmatal branch opposite to where
the mesothoracic stigma should be, if present, _i.e._ on the hind
edge of the segment.
Verson, moreover, has found in the freshly hatched silkworm vestiges
of meso- and metathoracic stigmata, each consisting of a circle of
high hypodermal cells radially arranged around a common centre. The
stigmatal branch is long, but shrivelled; its peritoneum is widened
out into several berry-like saccules filled with cell-elements. In
profile these rudimentary stigmata appear as a series of high
hypodermal cells, which form the basis of a short blind tube.
[Illustration:
Lydia M. Hart _del._
PLATE I.—Examples of metapneustic insects: 1, _Bittacomorpha
clavipes_, larva; 1 _a_, false foot; 1 _b_, its pupa; 2,
_Limnophila luteipennis_; 2 _a_, end of larva; 2 _b_, its pupa; 3,
end of larva of _Tipula eluta_.—After C. A. Hart.
]
After the second moult there begins a peculiar transformation of the
rudimentary stigmata. The stigmatal branch connected with them sends
off at various points thick tufts of capillary tracheæ which press
against the base of the blind tube. Gradually lengthening, they form
a fold which continues to increase in length. The numerous tufts of
tracheal capillaries extend beyond the inner surface of the two
layers of which the developing wing consists, the berry-like
saccules are drawn into the wing and converted into more or less
thick tubes, which finally form the “veins.” It is clear, therefore,
says Verson, as Landois claimed, that the wings of Lepidoptera must
be regarded as in the fullest sense organs of respiration. (Zool.
Anz., 1890, p. 116.)
The number of pairs of stigmata varies, especially in maggots or larval
Diptera, in adaptation to their varied modes of life. The larvæ of most
flies (Muscidæ) have a pair of peculiarly shaped processes on the
prothoracic segment bearing spiracular openings, and two anal spiracles,
while in _Ctenophora atrata_ L. only the anal pair are present. In the
rat-tailed maggots (Eristalis) the long caudal process ends in two
stigmata forming a respiratory tube, which can be thrust out of the
water for the reception of air. In the larval mosquito (Fig. 433) and
its ally, Mochlonyx, a short thick dorsal tube arises from the
penultimate segment of the body, in which the two main tracheæ end,
opening outward by a single spiracular aperture. Other dipterous larvæ
(Simulium, Tanypus, and Ceratopogon) possess no spiracles, the tracheal
system being a closed one.
The larvæ of most water beetles (Dyticidæ, Hydrophilidæ) possess but two
spiracles, which, as in maggots, are situated at the end of the body.
The aquatic larva of Amphizoa, according to Hubbard, breathes much as in
the Dyticidæ, by means of two large valvular spiracles placed close
together at the end of the body; “closed or rudimentary stigmata also
occur on the mesothorax and on abdominal segments one to seven
inclusive.”
Hubbard adds: “The larva of Pelobius is wholly aquatic and breathes
by branchiæ, but the obsolete stigmata are indicated precisely as in
Amphizoa, with the exception of the last pair, which in Amphizoa are
open spiracles, but in Pelobius are suppressed; the terminal eight
segments being prolonged in a swimming stylet.”
From a review of the distribution of spiracles, and their atrophy,
partial or total, it will be seen that there are intermediate stages
between the open (holopneustic) and closed (apneustic) systems. These,
following Schiner, Brauer, and Palmén, may be defined thus:
1. _Metapneustic type._—The larvæ possess only a single pair of open
stigmata situated at the end of the body. (The dipterous Eristalis,
Tipula, Culex, Ptychoptera, Bittacomorpha (Plate I.) with certain
Tachinidæ, and in Coleoptera, the larvæ of Dyticus, and allies of
Hydrophilus and Cyphon.)
2. _Propneustic type._—The pupæ of Corethra, Culex, etc., in which
only the most anterior pair of spiracles are open.
[Illustration:
FIG. 428.—Visceral tracheal system of the nymph of _Æschna
maculatissima_: _o_, œsophagus; _E_, stomach; _M_, urinary tubes;
_R_, rectum; _A_, anus; _tv_, visceral tracheal trunks; _td_,
dorsal trunks.—After Oustalet.
]
3. _Amphipneustic type._—Larvæ with a pair of open spiracles
situated at each end of the body, the intermediate spiracles being
closed. (Most dipterous larvæ, Musca, after the first moult,
Œstridæ, Asilidæ, and Syrphus.)
[Illustration:
FIG. 429.—Branchial tuft of nymph of Æschna.
]
4. _Peripneustic type_; with prothoracic and abdominal spiracles,
the mesothoracic pair atrophied or closed. (The larvæ of Neuroptera,
Mecoptera, Trichoptera, Lepidoptera, of most Coleoptera,[72] of most
Diptera, and of most of the Hymenoptera.[73])
[Illustration:
FIG. 430.—Part of three rows of respiratory folds from cuticular
living rectum of Æschna. The shaded parts are abundantly supplied
with tracheal tubes. The leaflets appear to be connected with a
central trachea, but this is not really the case.—After Miall.
]
These differences in the number of functional spiracles are in
direct relation with the surroundings of the insects, the physical
conditions of existence evidently determining the position of the
active functional open spiracles and the closure of those useless to
the organism.
_i._ The rectal tracheal gills, and rectal respiration of larval Odonata
and other insects
The remarkable mode of respiration by tracheal gills situated within the
intestine of the nymphs of dragon-flies was first described by
Swammerdam and afterwards by Réaumur. The most complete and best
illustrated modern account is that of Oustalet. In these insects the
large rectum is lined with six double longitudinal ridges, in Æschna
bearing numerous delicate tubes or papillæ, each of which contains very
numerous (by estimate 24,000) tracheal branches (Fig. 431); while in
Libellula the gills are lamellate (Fig. 432). The tracheæ arise both
from the main dorsal and visceral longitudinal trunks, which give rise
to secondary branches passing into the walls of the rectum and sending
into the branchial papillæ fine twigs, which, extending to the distal
end of the papilla or lamella, recurve and anastomose with the efferent
twigs.
[Illustration:
FIG. 431.—A small part of one leaflet, highly magnified, showing many
fine tracheal branches. The portion shown is marked by a small
circle in Fig. 430, lower left-hand corner.—After Miall.
]
[Illustration:
FIG. 432.—Leaves, _mh_, from a lamellate tracheal gill of Libellula:
_t_, trachea.—This and Fig. 429, after Oustalet.
]
The anal opening is externally protected by the suranal and lateral
triangular chitinous plates, three to five in all. When open, the water
passes into the rectum and bathes the rectal gills, where it may be
forcibly expelled as if shot out from a syringe, thus propelling the
insect forward. In Libellula the anus affords direct access to the
intestinal cavity, but in Æschna Oustalet describes “a sort of vestibule
separated from the rectum by a circular valvule.” He also states that
the inspiration and the repulsion of water is produced at irregular
intervals, and rather by the movements of the dorsal and sternal arches
of the abdomen than by the contractions of the rectum, since the walls
of this organ are less muscular than is supposed.
[Illustration:
FIG. 433.—Larva of a mosquito (_Culex nemorosus_) of middle age, seen
from above, the tracheal system omitted: _at_, antennæ; _ab_, their
middle joint; _eg_, elastic articular membrane; _atm_, antennal
muscle; _atn_, antennal nerve; _zau_, compound; _eau_, simple eye;
_os_, brain; _oex_, extensor; _ofl_, flexor of labrum; _ha_, neck;
_œ_, œsophagus; _spd_, salivary gland; _mau_, cœca; _ch_, chyle
stomach; _di_, contents of intestine; _mg_, urinary tubes; _dd_,
ileum; _ed_, rectum; _a_, anus; _s_, sipho; _z″_, its bristles;
_kb_, tracheal gills: _k_{1}_, _k_{2}_, _k_{3}_, closing lobes of
the sipho; _kn_, basal tubercle of tactile hair; _g_, its ganglion
cell; _th_, tactile hair of the siphon valve.
]
The nymph of Calopteryx (and probably of all the group
Calopteryginæ) possesses rectal gills besides external caudal
tracheal gills. There are three double rectal longitudinal folds or
ridges, interpenetrated by tracheal twigs. (Dufour, denied by
Poletaiew, but confirmed by Hagen.)
Dewitz claims that the caudal gills of the Agrionidæ are not their
sole means of respiration, since he cut off the caudal tracheal
gills of an Agrionid nymph, which continued to live for a week.
Hence he thinks that there may be a rectal respiration, since under
the microscope he saw a stream of water pass in and out of the end
of the intestine.
Dewitz’ experiments prove that in young Ephemerids there may be
besides branchial, both rectal and skin respiration. He saw under
the microscope the anus for a while opened and then closed, causing
the rectum to move; powdered carmine mixed with water was drawn into
and then expelled from the rectum. There was, however, no
enlargement and contraction of the abdomen as in the rectal
respiration of Æschna. (Zool. Anz. 1890, p. 500.)
[Illustration:
FIG. 434.—End of the body of the same larva as in Fig. 431, seen from
the side, the branches of the main tracheæ (_htr_) omitted: _kbl_,
excrementitial pellet in rectum; _kb_, tracheal gills; _b_, funnel
of the closing apparatus; _hz_, hollow tooth of the closing
apparatus; _k_{1}_, _k_{2}_, _k_{3}_, siphonal lobes; _th_, tactile
hair; _as_, chitinous plate; _str_, rudder; _l_, its thickened edge;
_sch_, its shank; _z′_, _z″_, bristles.—This and Fig. 433, after
Raschke.
]
Eaton states that there is a rectal respiration in the nymphs of
may-flies, and Palmén observed in young larvæ of Bætis and Cloëon that
the rectum took in “by gulps” water colored by carmine and expelled the
whole of it at once, in order to fill it again in the same way. “This
rectal respiration therefore corresponds to that of Libellulid larvæ.”
[Illustration:
FIG. 435.—Thorax and anterior abdominal segments of the nymph of a
may-fly (_Cloëon dimidiatum_) with tracheal gills (_tk_{1}_,
_tk_{2}_, _tk_{3}_) and the rudiments of the fore wings (_VF_) and
hind wing (_HF_): _tl_, tracheal longitudinal trunks.—After Graber,
from Lang.
]
[Illustration:
FIG. 436.—Gills on the middle abdominal segments of larva of _Bætis
binoculatus_: _trl_, longitudinal tracheal trunks; _vf_, stigmatic
cord; _ktr_, gill-tracheæ; _trk_, tracheal gills.—After Palmén, from
Lang.
]
Besides breathing by spiracles, by tracheal gills, as well as through
the integument, the larva of Culex has been observed by Raschke to have
a rectal respiration. At the anterior end of the rectum arises a
countless number of fine tracheæ, which pass through the walls and,
subdividing, end in numberless very fine twigs in the papilla-like folds
situated within the rectum. The supply of tracheal twigs is greatest
where the papillæ are largest. (Figs. 433, 434.)
_j._ Tracheal gills of the larvæ of insects
In many aquatic insects respiration is carried on by tracheal gills.
These are delicate, hollow, leaf-like or tubular outgrowths of the
integument usually attached to the sides or end of the hind-body, and
containing a trachea which usually sends off numerous minute branches,
so that the exchange of gases readily takes place in them.
[Illustration:
FIG. 437.—A, nymph of _Ephemerella ignita_, with gills of left side
removed; _g_, gills. _B_, nymph of Tricorythrus (_sp_), with
gill-cover of right side removed; _gc_, gill-cover; _g_, _g′_,
gills.—After Vayssière.
]
[Illustration:
FIG. 438.—Left maxilla of _Jolia weselii_, with the cephalic tracheal
gill (_h_) inserted at the base on the under side.—After Vayssière.
]
Palmén has shown that these tracheal gills, as he calls them, are not
developed on the same segments as the stigmata, and that the two
structures have no genetic connection with each other. It is evident
that these gills are secondary, adaptive organs.
In some cases (see p. 475) the tracheæ are wanting, but as such gills
are filled with blood, the air contained in the water must pass in
through their delicate walls.
In the Plectoptera (Ephemeridæ) the tracheal gills are either foliaceous
or filamentous; when foliaceous they form simple or double leaves, with
or without branches, or with a fringe of tubules, or under the leaf-like
cover-bearing tufts of filaments. They are situated on the (usually)
basal seven abdominal segments, at their hinder edge (Figs. 435, 436).
In Oligoneuria and Jolia a pair occurs on the under side of the head,
attached to the maxillæ, while in Jolia there is a pair on the under
side of the first thoracic segment at the insertion of each of the legs.
In certain genera (Heptagenia, Oligoneuria, and Jolia), they are in the
form of a flat cover, under which lies a tuft of respiratory tubes, or
(Ephemerella) a small bifid cluster of very delicate leaves (Fig. 437,
_A_). In Cœnis and Tricorythus the tracheal gills of the second pair are
modified to form plates covering all the succeeding pairs, those of the
first pair being nearly atrophied and well-nigh functionless. (Fig. 437,
_B_.)
[Illustration:
FIG. 439.—Inner side of a gill-cover of the first pair, of
Ephemerella, with the tracheal gills.—After Vayssière.
]
[Illustration:
FIG. 440.—Nymph of Bætisca: III, section of abdomen; _a_, gills; _b_,
flap; 1–9, abdominal segments.—After Walsh.
]
[Illustration:
FIG. 441.—Nymph of _Prosopistoma punctifrons_: _o_, upper orifice of
the respiratory chamber.—After Vayssière.
]
[Illustration:
FIG. 442.—Filamentous tracheal gill and part of a trachea of
Pteronarcys.—After Newport from Sharp.
]
Finally, in the highly modified forms Bætisca and Prosopistoma the
tracheal gills are entirely concealed and protected by mesothoracic
projections so as to form a true respiratory chamber, to which the water
has access either by an opening behind, as in Bætisca, or by three
openings, two ventral and one dorsal (Fig. 441), as in Prosopistoma.
The slender cylindrical tracheal gills of Heptagenia in the third or
fourth nymphal stage are 2–jointed, and the first abdominal pair in
Cænis are said by Palmén to be finger-shaped and 2–jointed. In
_Polymitarcys virgo_ the gills do not appear until the eighth or tenth
day after hatching.
Dewitz found that young nymphs of Ephemerids will well endure the
amputation of their gills, while fully grown ones die. Amputation of
the lateral gills hastens ecdysis. After the change of skin, the
gills are smaller than before, and at first contain no tracheæ, but
in a few weeks they develop as completely as in normal individuals.
The caudal gills were also renewed.
[Illustration:
FIG. 443.—_A_, larva of Sisyra, enlarged. _B_, one of the hinder
gills, with its tracheæ.—After Westwood, from Sharp. _C_, a gill,
showing the branched tracheæ.—After Grube.
]
In the nymphs of Perlidæ the tracheal gills are usually present, and are
either foliaceous (Nemoura) or more commonly filamentous in shape (Fig.
442). They are situated either on the prosternum (Nemoura and
Pteronarcys), or on each side of the thorax, or on the sides of the
abdomen, or are restricted to a tuft on each side of the anus at the
base of the caudal stylets (Pteronarcys and Perla). Unlike the
Ephemeridæ the gills persist in certain genera throughout life.
The larvæ of the aquatic Neuroptera, Sisyra, Sialis, and Corydalus
possess lateral pointed bristle-like tracheal gills, which in Sisyra are
2–jointed; those of Sialis are, in the living larva, curved upwards and
backwards (Fig. 444). Corydalus is also provided with a ventral tuft of
delicate filamentous gills, which, however, according to Riley, do not
appear until after the first moult.
While the nymphs of Agrionidæ (which have rectal gills) respire chiefly
by the large caudal foliaceous gills (Fig. 445), there are, according to
Hagen, two genera of the Calopteryginæ (Euphæa, Fig. 445, and
Anisopleura) whose nymphs possess seven pairs of external lateral
tracheal gills, in shape like those of Sialis, besides three caudal and
three rectal tracheal gills.[74]
[Illustration:
FIG. 444.—Larva of _Sialis lutarius_.—After Miall.
]
[Illustration:
FIG. 445.—Caudal tracheal gill of nymph of Agrion.
]
Hagen has also detected in the under side of the 5th abdominal
segment of Epitheca and Libellula a pair of sacs of the shape of a
Phrygian bonnet, each of which contains a smaller sac lined with
epithelium,—as in Æschna they occur in the 5th and 6th, and in
Gomphus in the 4th, 5th, and 6th segments. This serial arrangement
appears to confirm Hagen’s suggestion that they are survivals of
abdominal gills, which in Euphæa are completely evaginated.
[Illustration:
FIG. 446.—Nymph of Euphæa, showing the lateral gills: _a_, one
enlarged.—Folsom _del._
]
In the Trichoptera, all of which, except Enoicyla, are apneustic, and
most of which have tracheal gills, the latter are filamentous, and arise
either from the dorsal and ventral sides of the abdominal segment, or
they grow out from the sides; while in certain genera (Neuronia,
Phryganea, etc.) the gills are represented by conical hooks on the sides
of the 1st abdominal segment, which are evidently respiratory, as they
contain numerous tracheæ. The tracheal gills are either single or more
rarely form tufts (Figs. 447, 448).
In Hydropsyche (Fig. 448) the tracheal gills persist throughout life,
while in other genera they only last through the pupal stage. When first
hatched, the larva of Phryganea lacks gills. The larvæ of most of the
Hydropsychidæ, Rhyacophilidæ, and Hydroptilidæ have no gills, though
they appear well developed in the pupal stage. (Klapálek.)
[Illustration:
FIG. 447.—_A_, an abdominal segment of Hydropsyche, with the tracheal
gills (_lbr_): _trl_, longitudinal tracheal trunk; _f_, stigmatal
branch. _B_, 5th abdominal segment of pupa of the same; _l_, the
three lateral flaps of the tergite; _br^1_, _br^2_, branchiæ.
]
[Illustration:
FIG. 448.—Imago, abdominal segments IV to VI, with the gills at _a_
concealed in their natural condition; at _b_, drawn out with the
needle; at _c_, projecting abnormally and dried.—This and Fig. 447
after Palmén.
]
[Illustration:
FIG. 449.—Larva and pupa of _Paraponyx stratiolata_, enlarged; _s_,
spiracle.—After De Geer (compare Hart’s figure of _P. obscularis_,
living in the Illinois River).
]
The only lepidopterous larva known to be provided with tracheal gills is
that of the pyralid genus Paraponyx. Its thread-like gills, arranged in
tufts of three or four, arise from a common tubercle situated on the
sides of nearly all the segments. Wood-Mason describes the East Indian
_P. oryzalis_ as “covered with a perfect forest of soft and delicate
white filaments,” arranged in tufts disposed in four longitudinal rows.
“The stigmata of the 2d, 3d, and 4th abdominal somites only are clearly
discernible.” The caterpillar crawls “free and uncovered” over the
submerged leaves of the rice plant “in the very midst of the water.” In
a Brazilian species of Paraponyx described as _Cataclysta pyropalis_, by
W. Müller, the tufts are reduced to simple unbranched filaments, and the
case is more complex than in the European species (Fig. 449).
[Illustration:
FIG. 450.—Anterior end of larva of _P. stratiolata_, showing the head
and first two thoracic segments, with their gills: _A_, a tuft of
gills, much enlarged.—After De Geer.
]
[Illustration:
FIG. 451.—Larva (1) and pupa (2_a_) of _Paraponyx pyropalis_ enlarged:
_st_, stigmata.—After W. Müller.
]
Of coleopterous larvæ breathing by tracheal gills there are but few. The
larva of Gyrinus (Fig. 454) respires by 10 pairs of slender, hairy
abdominal gills similar to those of Corydalus, and the stigmata are
entirely wanting. Somewhat similar are the tracheal gills of
_Hydrocharis caraboides_. Hydrobius has shorter setose gills, our
American species having seven pairs of short setose gills. It has two
spiracles at the end of the body, through which the air is taken by
thrusting the body out of the water. The larvæ of two other aquatic
coleopterous genera, Pelobius and Cnemidotus, also have gills; those of
the former situated at the base of the coxæ, and brush-like, but
containing no tracheæ, though filled with blood, while those of
Cnemidotus are very long, bristle-like, jointed, and arising from the
dorsal side of the thoracic and abdominal segments. The stigmata are
wanting. (Schiödte.)
The larva of the dipterous genus Tanypus respires by two caudal
papilliform processes, in each of which a trachea ramifies.
[Illustration:
FIG. 452.—Freshly hatched larva of Hydrobius: _t_, enlarged tracheæ,
the heart between them; _g^1_-_g^7_, the seven pairs of gills. _A_,
end of body, enlarged, showing the two terminal stigmata.—Emerton
_del._
]
Certain larvæ with both stigmata and tracheal gills are enabled either
to live in or out of water or on the surface, as in the case of certain
beetles (Cyphonidæ, Elmidæ, Hydrophilidæ, Fig. 452), or the larval
mosquito and Psychodes (Fig. 455); also the nymphs of dragon-flies.
The larvæ of the Cyphonidæ (Helodes, Cyphon, Hydrocyphon) possess but a
single pair of stigmata, situated in the penultimate abdominal segment,
while at the end of the abdomen are delicate tracheal gills. The two
main tracheal trunks are much swollen. When on the surface of the water
the larva breathes through the stigmata situated near the end of the
abdomen; when floating in the water, the larva, like that of Gyrinus,
carries along at the end of its body a bubble of air. The gills are only
of use, as Rolph thinks, when the insect is compelled to remain a long
time under water.
The larva of our native _Prionocyphon discoideus_ (Say) is described by
Walsh as “vibrating vigorously up and down a pencil of hairs proceeding
from a horizontal slit in the tail”; this pencil is composed “of three
pairs of filaments, each beautifully bipectinate. I presume it is used
to extract air from the water.” When the larva is at the surface the
pencil of hairs touches the surface of the water, and occasionally a
bubble of air is discharged from the tail. “The general habit is to
crawl on decayed wood beneath the surface, occasionally swimming to the
surface, probably for a fresh supply of air.” (Proc. Ent. Soc. Phil., i,
p. 117.)
[Illustration:
FIG. 453.—Larva of Psephenus, enlarged.
]
[Illustration:
FIG. 454.—Larva of Gyrinus.—After Westwood.
]
The larvæ of the small water beetles of the family Elmidæ (Elmis,
Potamophilus, Macronychus, and Psephenus) have similar habits. That of
Elmis has ten dorsally situated pairs of spiracles, and on the end of
the body bushy gills which are protruded at pleasure. The young larva is
without spiracles, its tracheal system being closed. Macronychus and
Potamophilus have similar habits. In the larva of the latter genus,
which has nine pairs of spiracles, there are at the end of the body on
each side three tufts of thread-like gills which are connected with the
two main horizontal tracheæ, while the branches of the abdominal tracheæ
are dilated into numerous (64) bladder-like sacs. The larva usually
breathes through the caudal gills. When the water is low or dried up,
the air is inhaled directly through the spiracles. (Kolbe.)
The larva of _Psephenus lecontei_, by its broad hemispherical body, is
adapted to adhere to the smooth surface of rounded stones, in which
situation we have found it. Although it is said by Rolph to have two
pairs of spiracles, one pair on the mesothoracic and the other on the
1st abdominal segment, it probably rarely rises to the surface to
breathe the air direct.
[Illustration:
FIG. 455.—End of body of a Psychodes larva: _A_, end of body of a
young, freshly moulted larva, side view: _a_, the three anal gills;
_b_, the left air-cavity. _B_, older larva of the same species, with
the open air-cavity seen from above. _C_, end of larva of another
species as it goes down into the water with a bubble of air, _b_,
between the crown of hairs of the air-cavity or tube: _a_, the two
pairs of anal gills; _b_, the two main tracheæ.—After F. Müller.
]
It possesses five pairs of gills on the under side of the 2d to the 6th
abdominal segments. Each gill has finger-shaped processes on its hinder
edge, which are “from their constant motion evidently connected with
respiration.” Tracheæ may be seen, according to H. J. Clark, entering
the gills, and “the circulation of water among the branchiæ is kept up
by the flapping of the tail-pieces.” The larva of _Helichus fastigiatus_
is said by Leconte to be “very nearly allied, while the remotely allied
_Stenelmis crenatus_ has no external branchiæ.”[75]
The larva of the mosquito also has two modes of respiration,
breathing either at the surface of the water through the two
spiracles situated on the projection (siphon) at the hinder end of
the body which is thrust out into the air; or when at the bottom
respiring by tracheal gills. The pupa also has a double mode of
respiration, either taking in air at the surface by the two thoracic
horns with stigmatic openings, or when submerged using its tracheal
gills.
Besides its long caudal tracheal air-tubes, the larval Eristalis is
said by Chun to thrust out from the anus a number (20) of short
tracheal filaments which float about in the water and serve to
absorb the air.
An aquatic Brazilian larva of the family Psychodidæ has been found by
Fritz Müller to take down under the water a large bubble of air (Fig.
455, _C_), the main tracheal trunk ending each in an opening at the end
of the body (_A_, _B_); besides this, while at the bottom it breathes by
three digitiform tracheal gills; another species having two pairs (_C_,
_a_).
[Illustration:
FIG. 456.—Under side of body of larva of Blepharocera. showing the
position of the tracheal gills: _A_, section of the body through a
sucker, showing position of the gills. _B_, section of a sucker:
_br_, gill with numerous tracheæ; _gl_, outlet of excretory gland;
_M_, _m_, muscles.—After F. Müller.
]
The remarkable larvæ of the Blepharoceridæ (represented in the United
States by _Blepharocera capitata_), which live permanently in swift
streams, attached by median suckers to stones, are apneustic, and
breathe solely by leaf-like tracheal gills (Fig. 456, _br_) attached to
the under side of the second to sixth abdominal segments. Those of the
European Liponeura are said by Wierzejski to be branched, tree-like.
Also immediately in front of the anus and behind the last sucker are
four membranous sacs provided with tracheæ, but which are not capable of
being withdrawn. These are said by Müller to be the same as what Dewitz
states to serve as gills, and by Wierzejski they are homologized with
the four anal gills of Chironomus.
The double mode of respiration in the larva of the horse bot-fly has
been described by Scheiber. On the hinder end of the body are the
stigmatic plates, which contain two lateral gill-plates and the
middle stigmatal leaf. Besides this there is a pair of slightly
developed prothoracic spiracles. The embryo and also freshly hatched
larva of _Gastrophilus equi_ do not possess these gill-plates, but
on the end of the body are, according to Joli, two long thread-like
gills. The freshly hatched larva of the allied _Cephenomyia
rufibarbis_ bears two caudal projections. (Kolbe.) As in shrimps and
other Crustacea the gills are kept in constant motion, the water
being driven over them by the rapid movements of the telson, so in
the larval may-flies, and in the case-worm (Macronema), the gills
move more or less rapidly. In case-worms as well as larval Perlidæ,
Sialidæ, Paraponyx, and Hydrophilidæ the abdominal region is
constantly moved to promote respiration. (Kolbe.)
=Blood-gills.=—Fritz Müller describes in trichopterous larvæ certain
delicate anal tubular processes into which the blood flows, and which do
not as a rule contain tracheæ, though occasionally very fine tracheal
branches. Müller compares them with the gills of crabs and of shrimps.
They are eversible finger-like tubules. They are used when the tracheal
gills are temporarily not available. Their number varies even in the
same genus. There are six in certain Rhyacophilidæ; five in different
Hydropsychidæ; in Macronema there are four, and they are green when
filled with the green blood of that insect, the tracheal gills being
whitish. In the freshly hatched larva, while the tracheal gills are
present, no anal blood-gills are visible. Similar blood-gills also occur
in the pupæ of certain caddis-flies. (Pictet.)
Similar anal gills filled with blood occur in the larvæ of the fireflies
(Lampyris, etc.), and perhaps, Kolbe thinks, serve for respiration,
though other authors believe them to be adhesive organs.
The larva of Pelobius has true blood-gills. (Schiödte. See p. 461.)
The eversible ventral segmental sacs of Scolopendrella, Campodea,
and Machilis, as well as the ventral tube (collophore) of Podura,
Smynthurus, etc., may, as Oudemans and Haase have suggested, serve a
respiratory purpose, though they lack tracheæ, and differ from
blood-gills in containing no gases; yet the blood is forced into
them, causing their eversion. Oudemans observed that Machilis
everted its sacs when the vessel in which it was put was filled with
warm, damp air. The sacs are only thrust out when the creature is
completely at rest.
Structures referable to blood-gills also occur temporarily in the embryo
of Orthoptera; Rathke observed them in the mole-cricket; Ayres observed
them in _Œcanthus niveus_, where they form two stalked broad oval
appendages on the first abdominal appendages, which he regarded as
gills. Patten observed them in _Phyllodromia germanica_, as pear-shaped
structures occurring in the same situation, but regarded them as
sense-organs, as did Cholodkowsky. Graber found these structures in the
embryo of the May-beetle, which looked like the other embryonic limbs,
but survived after the disappearance of the latter, being longer and
broader and unjointed. These disappeared shortly before birth. In
Hydrophilus they remain, Graber states, after birth. Nussbaum has seen
them in Meloë.
Finally, Wheeler has discussed at length these embryonic organs, which
he regards as glandular structures, and calls _pleuropodia_, their
primitive function having been that of limbs. He has detected them in
the embryo of _Periplaneta orientalis_, _Mantis carolina_, _Xiphidium
ensiferum_ (Fig. 387); also in the Hemiptera (_Cicada septemdecim_,
_Zaitha fluminea_), and in _Sialis infumata_. He discards the view that
they were once gills or sense-organs, and concludes that they were
glands. But, as we have suggested, their function once that of gills,
and still respiratory in Synaptera, has perhaps become in the winged
insects glandular and repugnatorial. Instead, then, of being modified
abdominal limbs afterwards serving as glands, as Wheeler claims, we are
inclined to believe that they functioned as blood-gills.
_k._ Tracheal gills of adult insects
Tracheal gills are known to be retained by a few insects in the imago
stage, the nymphs in all stages breathing by them. The most notable
example is the perlid genus Pteronarcys, in which, as Newport states,
there are eight sets, comprising 13 pairs of branchial tufts distributed
over the under surface of the thoracic and first two abdominal segments.
The first set, consisting of three pairs of tufts, partly encircling
the neck like a ruff, arises from the soft membrane connecting the
head and prosternum. The thoracic tufts originate between and behind
the coxæ, as well as on the front margin of the meso- and
metathoracic segments. The number of filaments in each tuft varies
from about 20 to 50 or more, the densest tufts being those of the
two hinder thoracic segments. Each filament is usually simple,
though in a few cases they are branched (Fig. 457, _A_).