THE ORGANS OF CIRCULATION AND RESPIRATION.
_SPECIAL REFERENCES._
VERLOREN. Mém. sur la Circulation dans les Insectes. Mém. cour,
par l’Acad. Roy. de Belgique, Tom. XIX. (1847). [Structure of
Circulatory Organs in a number of different Insects.]
GRABER. Ueb. den Propulsatorischen Apparat der Insekten. Arch. f.
mikr. Anat., Bd. IX. (1872). [Heart and Pericardium.]
LEYDIG. Larve von _Corethra plumicornis_. Zeits. f. wiss. Zool.,
Bd. III. (1852). [Valves in Heart.]
LANDOIS, H. Beob. üb. das Blut der Insekten. Zeits. f. wiss. Zool.,
Bd. XIV. (1864). [Blood of Insects.]
JAWOROWSKI. Entw. des Rückengefässes, &c., bei _Chironomus_. Sitzb.
der k. Akad. der Wiss. Wien., Bd. LXXX. (1879). [Minute Structure
and Development of Heart.]
LANDOIS, H., and THELEN. Der Tracheenverschluss bei den Insekten.
Zeits. f. wiss. Zool., Bd. XVII. (1867). [Stigmata.]
PALMEN. Zur Morphologie des Tracheensystems (1877). [Morphology of
Stigmata and Tracheal Gills.]
MACLEOD. La Structure des Trachées et la Circulation
Péritrachéenne. (Brussels, 1880.)
LUBBOCK. Distribution of Tracheæ in Insects. Trans. Linn. Soc.,
Vol. XXIII. (1860).
RATHKE. Untersuch. üb. den Athmungsprozess der Insekten. Schr. d.
Phys. Oek. Gesellsch. zu Königsberg. Jahrg. I. (1861). [Experiments
and Observations on Insect-respiration.]
PLATEAU. Rech. Expérimentales sur les Mouvements Respiratoires
des Insectes. Mém. de l’Acad. Roy. de Belgique, Tom. XLV. (1884).
Preliminary notice in Bull. Acad. Roy. de Belgique, 1882.
LANGENDORFF. Studien üb. die Innervation der Athembewegungen.--Das
Athmungscentrum der Insekten. Arch. f. Anat. u. Phys. (1883).
[Respiratory Centres of Insects.]
_Circulation of Insects._
A very long chapter might be written upon the views advanced by
different writers as to the circulation of Insects. Malpighi first
discovered the heart or dorsal vessel in the young Silkworm. His
account is tolerably full and remarkably free from mistakes. The heart
of the Silkworm, he tells us, extends the whole length of the body,
and its pulsations are externally visible in young larvæ. He supposed
that contraction is effected by muscular fibres, but these he could
not distinctly see. The tube, he says, has no single large chamber, but
is formed of many little hearts (_corcula_) leading one into another.
The number of these he could not certainly make out, but believed
that there was one to each segment of the body. During contraction
each chamber became more rounded, and when contraction was specially
energetic, the sides of the tube appeared to meet at the constrictions.
The flow of blood, he ascertained, was forward, the rhythm not
constant. No arteries were seen to be given off from the heart.[135]
Swammerdam thought that his injections ascertained the existence of
vessels branching out from the heart,[136] but this proved to be a
mistake. Lyonnet added many details of interest to what was previously
known. He came to the conclusion that there was no system of vessels
connected with the heart, and even doubted whether the organ so named
was in effect a heart at all. Marcel de Serres maintained that it was
merely the secreting organ of the fat-body. Cuvier and Dufour doubted
whether any circulation, except of air, existed in Insects. This was
the extreme point of scepticism, and naturalists were drawn back from
it by Herold,[137] who repeated and confirmed the views held by the
seventeenth-century anatomists, and insisted upon the demonstrable fact
that the dorsal vessel of an Insect does actually pulsate and impel a
current of fluid. Carus, in 1826, saw the blood flowing in definite
channels in the wings, antennæ, and legs. Straus-Durckheim followed up
this discovery by demonstrating the contractile and valvular structures
of the dorsal vessel. Blanchard affirmed that a complex system of
vessels accompanied the air tubes throughout the body, occupying
peritracheal spaces supposed to exist between the inner and outer
walls of the tracheæ. This peritracheal circulation has not withstood
critical inquiry,[138] and it might be pronounced wholly imaginary,
except for the fact that air tubes and nerves are found here and there
within the veins of the wings of Insects.
[135] Dissert. de Bombyce, pp. 15, 16 (1669).
[136] Biblia Naturæ, p. 410.
[137] Schrift. d. Marburg. Naturf. Gesellschaft, 1823.
[138] See, for a full account of this discussion, MacLeod sur la
Structure des Trachées, et la Circulation Péritrachéenne (1880). The
peritracheal circulation was refuted by Joly (Ann. Sci. Nat., 1849).
[Illustration: Fig. 73.--Heart, Alary Muscles, and Tracheal Arches,
seen from below; to the left is a side view of the heart. _T^2_, _T^3_,
_A^1_, alary muscles attached to the second thoracic, third thoracic,
and first abdominal terga. × 6. Fig. 35 (p. 74) is not quite correct as
to the details of the heart. The thoracic portion should be chambered,
and additional chambers and alary muscles represented at the end of the
abdomen. These omissions are rectified in the present figure.]
_Heart of the Cockroach._
[Illustration: Fig. 74.--Diagram to show the interventricular valves
and lateral inlets of the Heart. _ML_, median lobe; _V_, valve; _I_,
lateral inlet.]
The heart of the Cockroach is a long, narrow tube, lying immediately
beneath the middle line of the thorax and abdomen. It consists of
thirteen segments (fig. 73), which correspond to three thoracic and
ten abdominal somites. Each segment, as a rule, ends behind in a
conspicuous fold which projects backwards from the dorsal surface;
immediately in front of this are two lateral lobes. The median lobe
passes into the angle between two adjacent terga, and is continuous
with the dorsal wall of the segment next behind, from which it is
separated only by a deep constriction, while the lateral folds conceal
paired lateral inlets,[139] which lead from the pericardial space to
the hinder end of each chamber of the heart. Immediately in front of
each constriction is the interventricular valve, a pear-shaped mass of
nucleated cells, hanging down from the upper wall of the heart, and
inclining forward below. The position of this valve indicates that
during systole it closes upon the constricted boundary between two
chambers, thus shutting off at once the inlets and the passage into the
chambers behind. In this way the progressive and rhythmical contraction
of the chambers impels a steady forward current of blood, allowing an
intermittent stream to enter from the pericardial space, but preventing
regurgitation.
[139] It may be observed that Graber, who has paid close attention to
the heart of Insects, describes the inlets (_e. g._, in _Dytiscus_)
as situated, not at the hinder end, but in the middle of each
segment. We have not been able to discover such an arrangement in the
heart of the Cockroach.
[Illustration: Fig. 75.--Junction of two chambers of the Heart, seen
from above. _ML_, median lobe; _I_, lateral inlet.]
The wall of the heart includes several distinct layers. There are (1) a
transparent, structureless intima, only visible when thrown into folds;
(2) a partial endocardium, of scattered, nucleated cells, which passes
into the interventricular valves; (3) a muscular layer, consisting of
close-set annular, and distant longitudinal fibres. The annular muscles
are slightly interrupted at regular and frequent intervals, and are
imperfectly joined along the middle line above and below, so as to
indicate (what has been independently proved) that the heart arises
as two half-tubes, which afterwards join along the middle. Elongate
nuclei are to be seen here and there among the muscles. The adventitia
(4), or connective tissue layer, is but slightly developed in the adult
Cockroach.
Within the muscular layer is a structure which we have failed to make
out to our own satisfaction. It presents the appearance of regular but
imperfect rings, which do not extend over the upper third of the heart.
They probably meet in a ventral suture, but this and other details are
hard to make out, owing to the transparency of the parts. The rings
stain with difficulty, and we have not observed nuclei belonging to
them. Each extends over more than one bundle of annular muscles.
The difficulty of investigating a structure so minute and delicate
as the heart of an Insect may explain a good deal of the discrepancy
noted on comparing various published descriptions. Perhaps the most
obvious peculiarity which distinguishes the heart of the Cockroach, is
the subdivision of the thoracic portions into three chambers, which,
though less prominent in side-view than the abdominal chambers, are,
nevertheless, perfectly distinct. The number of abdominal chambers is
also unusually high; but it is so easy to overlook the small chambers
at the posterior end of the abdomen, that the number given in some of
the species may have been under-estimated.
_Pericardial Diaphragm and Space._
The heart lies in a pericardial chamber, which is bounded above by
the terga and the longitudinal tergal muscles; below by a fenestrated
membrane, the pericardial diaphragm. The intermediate space, which is
of inconsiderable depth, is nearly filled by a cellular mass laden with
fat, and resembling the fat-body.
The pericardial diaphragm, or floor of the pericardium, is continuous,
except for small oval openings scattered over its surface. It
consists of loosely interwoven fibres, interspersed with elongate
nuclei (connective-tissue corpuscles) and connected by a transparent
membrane. Into the diaphragm are inserted pairs of muscles, which, from
their shape and supposed continuity with the heart, have been named
_alæ cordis_, or alary muscles.[140] These are bundles of striated
muscle, about ·003 in. wide, which arise from the anterior margin of
each tergum. In the middle of the abdomen every alary muscle passes
inwards for about ·04 in., without breaking-up or widening, and then
spreads out fanwise upon the diaphragm. The fibres unite below the
heart with those of the fellow-muscle, and also join, close to the
heart, those of the muscles in front and behind. The alary muscles
are often said to distend the heart rhythmically by drawing its walls
apart, but this cannot be true. They do not pass into the heart at
all. Even if they did, a pull from opposite sides upon a flexible,
cylindrical tube, would narrow and not expand its cavity. Moreover,
direct observation[141] shows that the heart continues to beat after
all the alary muscles have been divided, and even after it has been cut
in pieces. These facts suggest that the heart of Insects is innervated
by ganglia upon or within it, and indeed transparent larvæ, such as
_Corethra_ or _Chironomus_, exhibit paired cells, very like simple
ganglia, along the sides of the heart.
[140] Lyonnet.
[141] Brandt, Ueb. d. Herz der Insekten u. Muscheln. Mél. Biol. Bull.
Acad. St. Petersb. Tom. VI. (1866).
[Illustration: Fig. 76.--Heart and Pericardial Diaphragm. On the
right, as seen from above; on the left, as seen from below; the bottom
figure represents a transverse section. _Ht_, heart; _PD_, pericardial
diaphragm; _AM_, alary muscle; _Tr_, tracheal tube; _PC_, pericardial
fat-cells; _PC_′, multinucleate fat-cells.]
Scattered over the upper surface of the pericardial diaphragm are
groups of cells, similar to the fat-masses of the perivisceral space.
Over the fan-like expansions of the alary muscles are different
fat-cells, which form branched and multinucleate lobes, and radiate in
the same direction as the underlying muscles.
Tracheal trunks, arising close to the stigmata, ascend upon the tergal
wall towards the heart. They overlie the alary muscles, and end near
the heart by bifurcation, sending one branch forward and another
backward to meet corresponding branches of adjacent trunks. A series of
arches is thus formed by the dorsal tracheæ on each side of the heart.
Occasionally an arch is subdivided into two smaller parallel tubes.
A few branches of distribution are given off to the fat-cells of the
pericardium.
Graber has explained the action of the pericardial diaphragm and
chamber in the following way.[142] When the alary muscles contract,
they depress the diaphragm, which is arched upwards when at rest.
A rush of blood towards the heart is thereby set up, and the blood
streams through the perforated diaphragm into the pericardial chamber.
Here it bathes a spongy or cavernous tissue (the fat-cells), which
is largely supplied with air tubes, and having been thus aerated,
passes immediately forwards to the heart, entering it at the moment of
diastole, which is simultaneous with the sinking of the diaphragm.
[142] Arch. f. mikr. Anat., Bd. IX. (1872); Insekten, ch. x.
In the Cockroach the facts of structure do not altogether justify this
explanation. The fenestræ of the diaphragm are mere openings without
valves. The descent of a perforated non-valvular plate can bring no
pressure to bear upon the blood, for it is not contended that the alary
muscles are powerful enough to change the figure of the abdominal
rings. Moreover, we find comparatively few tracheal tubes in the
pericardial chamber, and can discover no proof that in the Cockroach
the fat-cells adjacent to the heart have any special respiratory
character. The diaphragm appears to give mechanical support to the
heart, resisting pressure from a distended alimentary canal, while the
sheets of fat-cells, in addition to their proper physiological office,
may equalise small local pressures, and prevent displacement. The
movement of the blood towards the heart must (we think) depend, not
upon the alary muscles, but upon the far more powerful muscles of the
abdominal wall, and upon the pumping action of the heart itself.
_Circulation of the Cockroach._
The pulsations of the heart are rhythmical and usually frequent, the
number of beats in a given time varying with the species, the age, and
especially with the degree of activity or excitement of the Insect
observed.[143]
[143] Newport, in Todd’s Cyclopædia of Anatomy and Physiology, Art.
Insecta, pp. 981–2.
Cornelius[144] watched the pulsations in a white Cockroach immediately
after its change of skin, and reckoned them at eighty per minute;
but he remarks that the Insect was restless, and that the beats were
probably accelerated in consequence.
[144] Beitr. zur näheren Kenntniss von Periplaneta orientalis, p. 19.
In the living Insect a wave of contraction passes rapidly along
the heart from behind forwards; and the blood may under favourable
circumstances be seen to flow in a steady, backward stream along the
pericardial sinus, to enter the lateral aperture of the heart. The
peristaltic movement of the dorsal vessel may often be observed to set
in at the hinder end of the tube before the preceding wave has reached
the aorta.
From the heart a slender tube (the aorta) passes forward to the head.
It lies upon the dorsal surface of the œsophagus, which it accompanies
as far as the supra-œsophageal ganglia. In many Insects the thoracic
portion of the dorsal vessel is greatly narrowed and non-valvular,
forming the aorta of most writers on Insect Anatomy. The aorta often
dips downward near its origin, but in the Cockroach the thoracic
portion of the vessel keeps nearly the same level as the abdominal.
It gives off no lateral branches, but suddenly ends immediately in
front of the œsophageal ring in a trumpet-shaped orifice,[145] by
which the blood passes at once into a lacunar system which occupies
the perivisceral space. Here the blood bathes the digestive and
reproductive organs, receives the products of digestion, which are
not transmitted by lacteals, but discharged at once into the blood;
here, too, it gives up its urates to the excretory tubules, and its
superfluous fats to the finely-divided lobules of the fat-body. The
form of the various appendages of the alimentary canal (salivary
glands, cæcal tubes, and Malpighian tubules), as well as of the testes,
ovaries, and fat-body, is immediately connected with the passive
behaviour of the fluid upon which their nutrition depends. Instead
of being compact organs injected at every pulsation by blood under
pressure, they are diffuse, tubular, or branched, so as to expose as
large a surface as possible to the sluggish stream in which they float.
[145] The termination of the aorta has been described by Newport, in
_Sphinx_ (Phil. Trans., 1832, Pt. I., p. 385) _Vanessa, Meloe, Blaps
and Timarcha_. (Todd’s Cycl., Art. “Insecta,” p. 978.)
From the perivisceral space the blood enters the pericardial sinus by
the apertures in its floor, and returns thence by the lateral inlets
into the heart.
No satisfactory injections of the circulatory channels can be made in
Insects, on account of the large lacunæ, or cavities without proper
wall, which are interposed between the heart and the extremities of the
body. In the wings and other transparent organs the blood has been seen
to flow along definite channels, which form a network, and resemble
true blood vessels in their arrangement. Whether they possess a proper
wall has not been ascertained. It is observed that in such cases the
course of the blood is generally forwards along the anterior, and
backwards along the posterior, side of the appendage. The direction of
the current is not, however, quite constant, and the same cross branch
may at different times transmit blood in different directions.[146]
[146] Moseley, Q. J. Micr. Sci. (1871).
_Blood of the Cockroach._
The blood of the Cockroach may be collected for examination by cutting
off one of the legs, and wiping the cut end with a cover-slip. It
abounds in large corpuscles, each of which consists of a rounded
nucleus invested by protoplasm. Amœboid movements may often be
observed, and dividing corpuscles are occasionally seen. Crystals
may be obtained by evaporating a drop of the blood without pressure;
they form radiating clusters of pointed needles. The fresh-drawn
blood is slightly alkaline; it is colourless in the Cockroach, but
milky, greenish, or reddish in some other Insects. The quantity varies
greatly, according to the nutrition of the individual: after a few
days’ starvation, nearly all the blood is absorbed. Larvæ contain much
more blood, in proportion to their weight, than other Insects.
_Respiratory Organs of Insects._
The respiratory organs of Insects consist of ramified tracheal tubes,
which communicate with the external air by stigmata or spiracles. Of
these spiracles the Cockroach has ten pairs--eight in the abdomen and
two in the thorax. The first thoracic spiracle lies in front of the
mesothorax, beneath the edge of the tergum; the second is similarly
placed in front of the metathorax. The eight abdominal spiracles belong
to the first eight somites; each lies in the fore part of its segment,
and hence, apparently, in the interspace between two terga and two
sterna. The first abdominal spiracle is distinctly dorsal in position.
The disposition of the spiracles observed in the Cockroach is common in
Insects, and, of all the recorded arrangements, this approaches nearest
to the plan of the primitive respiratory system of Tracheata, in which
there may be supposed to be as many spiracles as somites.[147] The head
never carries spiracles except in _Smynthurus_, one of the Collembola
(Lubbock). Many larvæ possess only the first of the three possible
thoracic spiracles; in perfect Insects this is rarely or never met with
(_Pulicidæ?_), but either the second, or both the second and third, are
commonly developed. Of the abdominal somites, only the first eight ever
bear spiracles, and these may be reduced in burrowing or aquatic larvæ
to one pair (the eighth), while all disappear in the aquatic larva of
_Ephemera_.
[147] The oldest Tracheate actually known to bear spiracles is the
Silurian Scorpion of Gothland and Scotland (Scudder, in Zittel’s
Palæontologie, p. 738). We need not say that this is very far removed
from the primitive Tracheate which morphological theory requires. The
existing _Peripatus_ makes a nearer approach to the ideal ancestor
of all Tracheates, if we suppose that all Tracheates had a common
ancestor of any kind, which is not as yet beyond doubt.
From the spiracles, short, wide air-tubes pass inwards, and break up
into branches, which supply the walls of the body and all the viscera.
Dorsal branches ascend towards the heart on the upper side of the alary
muscles; each bifurcates above, and its divisions join those of the
preceding and succeeding segments, thus forming loops or arches. The
principal ventral branches take a transverse direction, and are usually
connected by large longitudinal trunks, which pass along the sides
of the body; the Cockroach, in addition to these, possesses smaller
longitudinal vessels, which lie close to the middle line, on either
side of the nerve-cord.[148] The ultimate branches form an intricate
network of extremely delicate tubes, which penetrates or overlies every
tissue.
[148] The longitudinal air-tubes are characteristic of the more
specialised Tracheata. In Araneidæ, many Julidæ, and Peripatus each
spiracle has a separate tracheal system of its own.
[Illustration: Fig. 77.--Tracheal System of Cockroach. Side view of
head seen from without, introducing the chief branches of the left
half. × 15.]
[Illustration: Fig. 78.--Tracheal System of Cockroach. Top and front of
head seen from without. × 15.]
[Illustration: Fig. 79.--Tracheal System of Cockroach. Back of head,
seen from the front, the fore half being removed. × 15. The letters A–J
indicate corresponding branches in figs. 77, 78, and 79.]
[Illustration: Fig. 80.--Tracheal System of Cockroach. The dorsal
integument removed and the viscera in place. × 5.]
[Illustration: Fig. 81.--Tracheal System of Cockroach. The viscera
removed to show the ventral tracheal communications. × 5.]
[Illustration: Fig. 82.--Tracheal System of Cockroach. The ventral
integument and viscera removed to show the dorsal tracheal
communications. × 5.]
_Tracheal Tubes._
The accompanying figures sufficiently explain the chief features of
the tracheal system of the Cockroach, so far as it can be explored by
simple dissection. Leaving them to tell their own tale, we shall pass
on to the minute structure of the air-tubes, the spiracles, and the
physiology of Insect respiration.
The tracheal wall is a folding-in of the integument, and agrees with
it in general structure. Its inner lining, the intima, is chitinous,
and continuous with the outer cuticle. It is secreted by an epithelium
of nucleated, chitinogenous cells, and outside this is a thin and
homogeneous basement membrane. The integument, the tracheal wall, and
the inner layers of nearly the whole alimentary canal are continuous
and equivalent structures. The lining of the larger tracheal tubes at
least is shed at every moult, like that of the stomodæum and proctodæum.
[Illustration: Fig. 83.--Tracheal tube with its epithelium and spiral
thread. Slightly altered from a figure given by Chun (Rectal-drüsen bei
den Insekten, pl. iv., fig. 1).]
_Tracheal Thread._
In the finest tracheal tubes (·0001 in. and under) the intima is to
all appearance homogeneous. In wider tubes it is strengthened by a
spiral thread, which is denser, more refractive, and more flexible
than the intervening membrane. The thread projects slightly into the
lumen of the tube, and is often branched. It is interrupted frequently,
each length making but a few turns round the tube, and ending in a
point. The thread of a branch is never continued into a main trunk.
Both the thread and the intervening membrane become invisible or faint
when the tissue is soaked with a transparent fluid, so as to expel
the air. Both, but especially the thread, absorb colouring matter
with difficulty. The thread, from its greater thickness, offers a
longer resistance to solvents, such as caustic alkalies, and also to
mechanical force; it can therefore be readily unrolled, and often
projects as a loose spiral from the end of a torn tube, while the
membrane breaks up or crumbles away.[149]
[149] Investigators are not yet agreed as to the minute structure
of the tracheal thread. Chun (Abh. d. Senkenberg. Naturf. Gesells.,
Bd. X., 1876) considers it an independent chitinous formation, not a
mere thickening of the intima. He describes the thread as solid. The
intima itself is, he believes, divisible in the larger tubes into
an inner and an outer layer, into both of which the thread is sunk.
Macloskie (Amer. Nat., June, 1884) describes the spiral as a fine
tubule, opening by a fissure along its length. He regards it as a
hollow crenulation of the intima, and continuous therewith. Packard
(Amer. Nat. Mag., May, 1886) endeavours to show that the thread is
not spiral, but consists of parallel thickenings of the intima. He
is unable to find proof of the tubular structure, or of the external
fissure. We have specially examined the trachea of the Cockroach, and
find that the thread can readily be unwound for several turns. It is
truly spiral.
[Illustration: Fig. 84.--Intima (chitinous lining) of a large tracheal
tube. The spiral thread divides here and there. Copied from MacLeod,
loc. cit., fig. 9.]
The large tracheal tubes close to the spiracles are without spiral
thread, and the intima is here subdivided into polygonal areas, each
of which is occupied by a reticulation of very fine threads. This
structure may be traced for a short distance between the turns of the
spiral thread.
The chitinogenous layer of the tracheal tubes is single, and consists
of polygonal, nucleated cells, forming a mosaic pattern, but becoming
irregular and even branched in the finest branches. The cell walls are
hardly to be made out without staining. Externally, the chitinogenous
cells rest upon a delicate basement membrane.
Where a number of branches are given off together, the tracheal tube
may be dilated. Fine branches, such as accompany nerves, are often
sinuous. In the very finest branches the tube loses its thread, the
chitinogenous cells become irregular, and the intima is lost in the
nucleated protoplasmic mass which replaces the regular epithelium of
the wider tubes.[150]
[150] It has been supposed that these irregular cells of the tracheal
endings pass into those of the fat-body, but the latter can always be
distinguished by their larger and more spherical nuclei.
_The Spiracles._
The spiracles of the Cockroach are by no means of complicated
structure, but their small size, and the differences between one
spiracle and another, are difficulties which cost some pains to
overcome.
The first thoracic spiracle (fig. 85) is the largest in the body. It
lies in front of the mesothorax, between the bases of the first and
second legs. It is placed obliquely, the slit being inclined downwards
and backwards, and is closed externally by a large, slightly two-lobed
valve, attached by its lower border. The aperture immediately within
the valve divides into two nearly equal cavities, each of which leads
to a separate tracheal trunk; and between these cavities is a septum,
thickened on its free edge, against which the margin of the valve
appears to close. A special occlusor muscle arises from the integument
below the spiracle, and is inserted into a chitinous process which
projects inwardly from the centre of the valve. A second muscle,
whose connections and mode of action we have not been able to make
out satisfactorily, lies beneath the first, and is inserted into the
thickened edge of the septum.
The second thoracic spiracle (fig. 86) lies in front of the metathorax,
between the bases of the second and third legs. It is much smaller and
simpler than the first. Its valve is nearly semi-circular, and the free
border is strengthened on its deep surface by a chitinous rim, which
terminates beyond the end of the hinge of the valve in a process which
gives insertion to the occlusor muscle.
[Illustration: Fig. 85.--First Thoracic Spiracle (left side), seen from
the outside. × 70. _V_, valve; _I_, setose lining of valve (mouth of
tracheal tube) × 230. The occlusor muscle is shown. The arrow indicates
the direction of air entering the spiracle. In the natural position
this spiracle is set obliquely, the slit being inclined downwards and
backwards. (_P. americana._)]
The abdominal spiracles present quite a different plan of structure.
The external orifice is permanently open, owing to the absence of
valves, but communication with the tracheal trunk may be cut off at
pleasure by an internal occluding apparatus. The external orifice leads
into a shallow oval cup, which communicates with the tracheal trunk
by a narrow slit, or internal aperture of the spiracle. The chitinous
cuticle, surrounding this internal aperture, is richly provided with
setæ, which are turned towards the opening.[151] Fig. 87_C_ represents
a spiracle seen from within, and shows that the slit divides the cup
into two unequal lips, the smaller of which inclines away from the
middle line of the body, is movable, and is strengthened on its deep
surface by a curved chitinous rod, the “bow” of Landois. From the
opposite lip, a pouch is thrown out, which serves for the attachment of
the occlusor muscle. The muscle is inserted into the extremity of the
bow, and when it contracts, the bow is pulled over into the position
shown in fig. 87_D_, and the opening is closed. The antagonist muscle,
which exists in all the abdominal spiracles, is shown in fig. 88; it
arises from the supporting plate of the spiracle, and is inserted
opposite to the occlusor, into the extremity of the bow.
[151] In the first abdominal spiracle the setæ are developed only on
that lip which carries the bow.
[Illustration: Fig. 86.--Second Thoracic Spiracle (left side), seen
from the outside. × 70. _V_, lower (movable) valve. The occlusor
muscle is shown. The arrow indicates the direction of air entering the
spiracle. (_P. americana._)]
[Illustration: Fig. 87.--Four views of the First Abdominal Spiracle
(left side). × 70. The bow is shaded in all the figures. (_P.
americana._)
_A_--The spiracle, seen from the outside; _p_, lateral pouch; _I_,
internal aperture.
_B_-- Do., side view.
_C_-- Do., seen from the inside, the aperture open. The occlusor muscle
is shown.
_D_--The spiracle, seen from the inside, the aperture shut.]
Each of the eight abdominal spiracles is constructed on this plan; the
first merely differs from the others in its larger size and dorsal
position, being carried upon the lateral margin of the first abdominal
tergum, whereas the others are placed on the side of the body, each
occupying an interspace between two terga and two sterna. The bow is of
about the same length in all; hence the apparent disproportion in the
figures of different spiracles. The external aperture of the abdominal
spiracles is oval or elliptical, placed vertically and directed
backwards.
[Illustration: Fig. 88.--Abdominal Spiracle (left side) in side view,
showing the bow: × 70; _p_, lateral pouch of spiracle, seen from
within. The tesselated structure of the spiracle and trachea is shown
at _A_ (× 230), and the margin of the external aperture at _B_ (× 230).
(_P. americana._)]
We have already pointed out that the wall of the air-tube, for a
short distance from the spiracular orifice, has a tesselated instead
of a spiral marking. In the thoracic spiracles the tesselated cells
are grouped round regularly placed setæ (fig. 85 _I_). The chitinous
cuticle within the opening is crowded with fine setæ, which are often
arranged so as to form a fringe on one or both sides of the internal
aperture. (_Supra_, p. 152.)
_Mechanism of Respiration._
In animals with a complete circulation, aërated blood is diffused
throughout the body by means of arteries and capillaries, which deliver
it under pressure at all points. Such animals usually possess a special
aërating chamber (lung or gill), where oxygen is made to combine with
the hæmoglobin of the blood. It is otherwise with Insects. Their blood
escapes into great lacunæ, where it stagnates, or flows and ebbs
sluggishly, and a diffuse form of the internal organs becomes necessary
for their free exposure to the nutritive fluid. The blood is not
injected into the tissues, but they are bathed by it, and the compact
kidney or salivary gland is represented in Insects by tubules, or a
thin sheet of finely divided lobules. By a separate mechanism, air is
carried along ramified passages to all the tissues. Every organ is its
own lung.
We must now consider in more detail how air is made to enter and leave
the body of an Insect. The spiracles and the air-tubes have been
described, but these are not furnished with any means of creating
suction or pressure; and the tubes themselves, though highly elastic,
are non-contractile, and must be distended or emptied by some external
force. Many Insects, especially such as fly rapidly, exhibit rhythmical
movements of the abdomen. There is an alternate contraction and
dilatation, which may be supposed to be as capable of setting up
expirations and inspirations as the rise and fall of the diaphragm of
a Mammal. In many Insects, two sets of muscles serve to contract the
abdomen--viz., muscles which compress or flatten, and muscles which
approximate or telescope the segments.[152] In the Cockroach the
second set is feebly developed, but the first is more powerful, and
causes the terga and sterna alternately to approach and separate with
a slow, rhythmical movement; in a Dragon-fly or Humble-bee the action
is much more conspicuous, and it is easy to see that the abdomen is
bent as well as depressed at each contraction. No special muscles exist
for dilating the abdomen, and this seems to depend entirely upon the
elasticity of the parts. It was long supposed that, when the abdomen
contracted, air was expelled from the body, and the air passages
emptied; that when the abdomen expanded again by its own elasticity,
the air passages were refilled, and that no other mechanism was needed.
Landois pointed out, however, that this was not enough. 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 a special
muscle, this can be made to compress 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. Landois describes the occluding apparatus of the Cockroach
as completely united with the spiracle. It consists, according to
him, of two curved rods, the “bow” and the “band,” one of which forms
each lip of the orifice. From the middle of the band projects a blunt
process for the attachment of the occlusor muscle, which passes thence
to the extremity of the bow. The concave side of each rod is fringed
with setæ, and turned towards the opening, which lies between the
two. Upon this description of the spiracles of the Cockroach we have
to remark that there is no occluding apparatus at all in the thoracic
spiracles, which are provided with external valves. In the abdominal
spiracles the bow is perfectly distinct, but the “band” of Landois
has no separate existence. Though the actual mechanism in this Insect
does not altogether agree with Landois’ description, it is capable of
performing the physiological office upon which he justly lays so much
stress--viz., the closing of the outlets of the tracheal system, in
order that pressure may be brought upon the contained air.
[152] This subject is treated at greater length in Prof. Plateau’s
contribution on Respiratory Movements of Insects. (_Infra_, p. 159.)
The injection of air by muscular pressure into a system of very fine
tubes may, however, appear to the reader, as it formerly did to
ourselves, extremely difficult or even impossible. Can any pressure
be applied to tubes within the body of an Insect which will force air
along the passages of (say) ·0001 in. diameter? It may well seem that
no pressure would suffice to distend these minute tubules, in which the
actual replacement of carbonic acid by oxygen takes place, but that the
air would either contract to a smaller volume or burst the tissues.
If we question the physical possibility of Landois’ explanation, an
alternative is still open to us. The late Prof. Graham has applied the
principle of Diffusion to the respiration of animals, and has shown
how by a diffusion-process the carbonic acid produced in the remote
cavities would be moved along the smaller tubes, and emptied into wider
tubes, from which it could be expelled by muscular action. The carbonic
acid is not merely exchanged for oxygen, but for a larger volume of
oxygen (O 95 : CO_{2} 81); and there is consequently a tendency to
accumulation within the tubes, which is counteracted by the elasticity
of the air vessels, as well as by special muscular contractions.[153]
[153] Phil. Mag., 1833. Reprinted in “Researches,” p. 44. Graham
expressly applies the law of diffusion of gases to explain the
respiration of Insects. Sir John Lubbock quotes and comments upon the
passage in his paper on the Distribution of the Tracheæ in Insects.
(Linn. Trans. Vol. XXIII.)
Whether diffusion or injection by muscular pressure is the chief means
of effecting the interchange of gases between the outer air and the
inner tissues of the Insect, is a question to be dealt with by physical
enquiry.
If we suppose two reservoirs of different gases at slightly different
pressures to be connected by a capillary tube of moderate dimensions,
such as one of the larger tracheæ of the Cockroach, transference by
the molecular movements of diffusion would be small compared with that
effected by the flow of the gas in mass. But if the single tube were
replaced by a number of others, of the same total area, but of the
fineness (say) of the pores in graphite, the flow of the gas would be
stopped, and the transference would be effected by diffusion only.
We may next consider tubes of intermediate fineness, say a tracheal
tubule of the Cockroach at the point where the spiral thread ceases,
and where the exchange of gases through the wall of the tubule becomes
comparatively unobstructed. Such a tubule is about ·0001 in. diameter.
If we may extend to such tubules the laws which hold good for the flow
of gases in capillary tubes of much greater diameter, the quantity of
air which might be transmitted in a given time by muscular pressure of
known amount can be determined. Suppose the difference of pressure at
the two ends of the tubule to be one-hundredth of an atmosphere, and
further, that the tubule is a quarter of an inch long and ·0001 in.
diameter. The tubule would then be cleared out every four seconds. Such
a flow of air along innumerable tubules might well suffice for the
respiratory needs of the Cockroach. Without laying too much stress upon
this calculation, for which exact data are wanting, we may 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.[154]
[154] For an explanation of the physical principles involved in this
discussion, and for the calculation (based upon our own assumptions),
we are indebted to Mr. A. W. Rücker, F.R.S.
_Respiratory Movements of Insects._
By FÉLIX PLATEAU, Professor in the University of Ghent.
The respiratory movements of large Insects are in general very
apparent, and many observers have said something about what they
have seen in various species. It is only since the publication of
Rathke’s memoir, however, that precise views have been gained as to the
mechanism of these movements. This remarkable work, treating of the
respiratory movements in Insects, the movable skeletal plates, and 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 remained unanswered,
which required more exact methods than mere observation with the naked
eye or the simple lens.
The writer, 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 selected 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 section, 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,[155] 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.
[155] J. Hutchinson, Art. Thorax, Todd’s Cycl. of Anat. and Phys.
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 results
at which he has arrived may be summarised as follows:--
1. There is no close relation between the character of the respiratory
movements of an Insect and its position in the zoological system.
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 a Cockroach are different from those of other Orthoptera,
but resemble those of Hemiptera Heteroptera.
2. The respiratory activity of resting Insects is localised in the
abdomen. V. Graber has expressed this fact in a picturesque form, by
saying that in Insects the chest is placed at the hinder end of the
body.
3. In most cases the thoracic segments do not share in the respiratory
movements of an Insect at rest. Among the singular exceptions to this
rule is the Cockroach (_P. orientalis_), in which the terga of the
meso- and meta-thoracic segments perform movements exactly opposite in
direction to those of the abdomen. (See fig. 89, _Ms. th._, _Mt. th._)
[Illustration: Fig. 89.--Profile 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.]
4. Leaving out of account all details and all exceptions, the
respiratory movements of Insects may be said to consist of alternate
contraction and recovery of the figure of the abdomen in two
dimensions--viz., vertical and transverse. During expiration the
diameters in question are reduced, while during respiration 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).
5. Three principal types of respiratory mechanism occur in Insects, and
these admit of further subdivision:--
(_a_) Sterna usually stout and very convex, yielding but little.
Terga mobile, rising and sinking appreciably. To this class belong
all Coleoptera, Hemiptera Heteroptera, and Blattina (Orthoptera).
[Illustration: Fig. 90.--Transverse section of Abdomen, 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.]
In the Cockroach (_Periplaneta_) the sterna are slightly raised during
expiration. (See figs. 89 and 91.)
[Illustration: Fig. 91.--Transverse section of Abdomen, Cockroach (_P.
orientalis_).]
(_b_) Terga well developed, overlapping the sterna on the sides of
the body, and usually concealing the pleural membrane, which forms
a sunk 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 Acridian Orthoptera.
(Fig. 92.)
(_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 the writer assigns the Locustidæ, the Lepidoptera and
the true Neuroptera (excluding Phryganidæ). (Fig. 93.)
[Illustration: Fig. 92.--Transverse section of Abdomen, Bee (_Bombus_).]
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
zoological divisions.
[Illustration: Fig. 93.--Transverse section of Abdomen, Hawk Moth
(_Sphingina_).]
7. Among Insects 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.
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 differentiation 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. (See the tergo-sternal
muscles of the Cockroach, fig. 36, p. 76.) Special inspiratory muscles
occur in Hymenoptera, Acridiidæ, and Phryganidæ.
10. The abdominal respiratory movements of Insects are wholly
reflex. Like other physiologists who have examined this side of the
question, the writer 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.
* * * * *
The writer has made similar observations upon the respiration of
Spiders and Scorpions;[156] but to his great surprise he has been
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 intra-pulmonary,
and affect only the proper respiratory organs. The fact is less
surprising because of the wide zoological separation between Arachnida
and Insects.
[156] De l’absence de mouvements respiratoires perceptibles chez les
Arachnides (Archives de Biologie de Van Beneden et Van Bambeke, 1885.)
_Respiratory Activity of Insects._
The respiratory activity of Insects varies greatly. Warmth, feeding,
and movement are found to increase the frequency of their respirations,
and also the quantity of carbonic acid exhaled. In Liebe’s[157]
experiments a Carabus produced ·24 mgr. of carbonic acid per hour
in September, but only ·09 mgr. per hour in December. A rise of
temperature raised the product temporarily to twice its previous
amount; but when the same insect was kept under experiment for several
days without food, the amount fell in spite of its increased warmth.
Treviranus[158] gives the carbonic acid exhaled by a Humble-bee as
varying from 22 to 174, according as the temperature varied from 56° to
74° F.
[157] Ueb. d. Respiration der Tracheaten. Chemnitz (1872).
[158] See table in Burmeister’s “Manual,” Eng. trans. p. 398.
Larvæ often breathe little, especially such as lie buried in wood,
earth, or the bodies of other animals. The respiration of pupæ is also
sluggish, and not a few are buried beneath the ground or shrouded in
a dense cocoon or pupa-case. Muscular activity originates the chief
demand for oxygen, and accordingly Insects of powerful flight are most
energetic in respiration.
A rise of temperature proportionate to respiratory activity has
been observed in many insects. Newport[159] tells us how the female
Humble-bee places herself on the cells of pupæ ready to emerge, and
accelerates her inspirations to 120 or 130 per minute. During these
observations he found in some instances that the temperature of a
single Bee was more than 20° above that of the outer air.
[159] Art. “Insecta,” Cyc. Anat. and Phys., p. 989.
Some Insects can remain long without breathing. They 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 revive, and
appear none the worse. H. Müller[160] says that an Insect, placed in
a small, confined space, absorbs _all_ the oxygen. In Sir Humphry
Davy’s “Consolations in Travel”[161] is a description of the Lago dei
Tartari, near Tivoli, a small lake whose waters are warm and saturated
with carbonic acid. Insects abound on its floating islands; though
water birds, attracted by the abundance of food, are obliged to confine
themselves to the banks, as the carbonic acid disengaged from the
surface would be fatal to them, if they ventured to swim upon it when
tranquil.
[160] Pogg. Ann. 1872, Hft. 3.
[161] Works, Vol. IX., p. 287. This passage has been cited by Rathke.
_Origin of Tracheal Respiration._
Kowalewsky, Bütschli, and Hatschek have described the first stages
of development of the tracheal system. Lateral pouches form in the
integument; these send out anterior and posterior extensions, which
anastomose and form the longitudinal trunks. The tracheal ramifications
are not formed by a process of direct invagination, but by the
separation of chitinogenous cells, which cohere into strings, and then
form irregular tubules. The cells secrete a chitinous lining, and
afterwards lose their distinct contours, fusing to a continuous tissue,
in which the individual cells are indicated only by their nuclei,
though by appropriate re-agents the cell boundaries can be defined.
The ingenious hypothesis propounded by Gegenbaur, that the tracheal
tubes of Insects were originally adapted to aquatic respiration, and
that the stigmata arose as the scars of disused tracheal gills, has
been discussed in chap. iv. Semper has suggested[162] that tracheæ
may be modified segmental organs, but the most probable view of
their origin is that put forth by Moseley,[163] that they arose as
ramified cutaneous glands. In _Peripatus_ the openings are distributed
irregularly over the body; the external orifices lead to pits, from
which simple tubes, with but slight spiral markings, extend into the
deeper tissues.
[162] Arbeiten a. d. Zool. Zoot. Inst. Würzburg. Bd. II., 1874.
[163] Phil. Trans., 1874, p. 757.