THE ORGANS OF CIRCULATION
Although Malpighi was the first to discover the heart in the young silkworm, it was not until 1826 that Carus proved that there was a circulation of blood in insects, which he saw flowing along each side of the body, and coursing through the wings, antennæ, and legs of the transparent larva of Ephemera, though three years earlier Herold demonstrated that the dorsal vessel of an insect is a true heart, pulsating and impelling a current of blood towards the head. This discovery was extended by Straus-Dürckheim, who discovered the contractile and valvular structures of the heart. It is noteworthy that both Cuvier and Dufour denied that any circulation, except of air, existed in insects; and so great an anatomist as Lyonet doubted whether the dorsal vessel was a genuine heart, though he pointed out the fact that there are no arteries and veins connected with this vessel. Another French anatomist, Marcel de Serres, thought that the dorsal vessel was merely the secreting organ of the fat-body.
The so-called peritracheal circulation claimed by Blanchard and by Agassiz has been shown by McLeod to be an anatomical impossibility, the view having first been refuted by Joly in 1849.
Except the aorta-like continuation in the thorax and head which divides into two short branches, there are, with slight exceptions (p. 405), no distinct arteries, such as are to be found in the lobster and other Crustacea, and no great collective veins, such as exist in Crustacea and in Limulus. This is probably the result of a reduction by disuse in the circulatory system, since in myriopods (Julidæ and Scolopendridæ) lateral arteries are said to diverge near the ostia.
a. The heart
The heart or “dorsal vessel” is a delicate, pulsating tube, situated just under the integument of the back, in the median line of the body, and above the digestive canal. It can be partially seen without dissection in caterpillars. It is covered externally and lined within by membranes which are probably elastic; and between these two membranes extends a system of delicate muscular fibres, which generally have a circular course, but sometimes cross each other. The heart is divided by constrictions into chambers, separated by valvular folds. The internal lining membrane referred to forms the valvular folds separating the chambers. Each of these chambers has, at the anterior end, on each side, a valvular orifice (Fig. 370, ostium, i) which can be inwardly closed.
Miall and Denny thus describe the different layers of the wall of the heart of the cockroach:
“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.”
Fig. 370.—Part of the heart of Lucanus cervus: a, the posterior chambers (the anterior ones are covered by a part of the ligaments which hold the heart in place); i, auriculo-ventricular openings; g, g, the lateral muscles fixed by the prolongations h, h, to the upper side of the abdomen.—After Straus-Dürckheim.
Graber says that the heart of insects may be regarded not as an organ de novo, but only as the somewhat modified contractile dorsal vessel of the annelids, in which, however, the transverse arteries arising on each side became, with the gradual development of the tracheæ, superfluous and finally abortive. He describes it as a muscular tube composed of very delicate annular fibres, which within and without is covered by a relatively homogeneous, strong, elastic membrane.
The division into separate chambers is effected by means of a folding inwards and forwards of the entire muscular wall. “A portion of each side of the heart is first extended inwards so as very nearly to meet a corresponding portion from the opposite side, and then, being reflected backwards, forms, according to Straus (Consid., etc., p. 356), the interventricular valve which separates each chamber from that which follows it. Posteriorly to this valve, at the anterior part of each chamber, is a transverse opening or slit (Fig. 371, b), the auriculo-ventricular orifice, through which the blood passes into each chamber, and immediately behind it is a second, but much smaller, semilunar valve (c), which, like the first, is directed forwards into the chamber. It is between these two valves on each side that the blood passes into the heart, and is prevented from returning by the closing of the semilunar valve. When the blood is passing into the chamber, the interventricular valve is thrown back against the side of the cavity, but is closed when, by the contraction of the transverse fibres, the diameter of each chamber is narrowed, and the blood is forced along into the next chamber.” (Newport.)
Fig. 371.—A, heart of Lucanus cervus: a, valves or chambers; bb, alary muscles; c, supposed auricular space around the heart. B, division into arteries of the end of the aorta in larva of Vanessa urticæ. C, interior of the chamber, showing the transverse fibres; b, auriculo-ventricular opening and valve into the chambers; c, semilunar valve; d, interventricular valve.—After Straus-Dürckheim, from Newport.
Fig. 372.—Heart of Belostoma.—After Locy.
According to Müller, there is but a single pair of ostia in Phasma, and, in the larva of Corethra, the heart is a simple, unjointed tube, not divided into chambers, and Viallanes states that, in the very young larva of Musca, there are no ostia (Kolbe). In the larva of Ptychoptera, Grobben found a short oval heart, with one pair of ostia situated in the 6th abdominal segment; a long aorta proceeds from it, the thoracic portion of which pulsates; from behind the heart arises a pulsating pouch, which connects with the hinder aorta, which does not pulsate, and ends at the base of two tracheal gills. Burmeister was able to find only four pairs of openings in the larva of Calosoma. Newport states that, while Straus figures nine chambers in Melolontha, and, consequently, eight pairs of openings, he has not been able to observe more than seven pairs of openings in Lucanus cervus. He has invariably found eight pairs of openings both in the larva and imago of Sphinx ligustri, as well as in other Lepidoptera. According to Béla-Dezso, the number of pairs of ostia corresponds to that of the pairs of stigmata.
There also occur, on each side of each chamber, two so-called pear-shaped bodies which are separated from the tubular portion of the heart itself, but, by means of muscular fibres, are united with the chamber and with their valves. These pyriform bodies appear as vesicles or cells with granular contents, besides some nuclei with nucleoli. They are of very small size. According to the measurements of Dogiel, in the larva of Corethra plumicornis, they are 0.02 to 0.1 mm. long, and 0.06 to 0.08 mm. broad. He regards these peculiar bodies as apolar nerve-cells of the heart. (Kolbe.)
Fig. 373.—A, part of the heart of Dyticus marginalis, showing the spiral arrangement of the muscular fibres; c, closed, e, open, valve; a, dorsal diaphragm with interwoven muscular fibres; b, arrangement of fibres, recalling the screw-like features of the fibres of the human heart; d, narrow end. B, diagrammatic figure of the valvular openings, with the terminal flap (e), and the cellular valve, of a May beetle; a, valvular opening of a dipterous larva, with the interventricular valve (b). C, abdomen of a mole-cricket, ventral view; c, the segmented heart; a, aorta; b, segmented diaphragm under it.—After Graber.
Besides the venous openings of the heart which open into the pericardial region, Kowalevsky has discovered, in the heart of some Orthoptera (Caloptenus, Locusta, etc.), five pairs of openings by which the cardiac chambers receive the blood of the peri-intestinal region. Graber had divided the cœlom of insects into three regions (pericardial, peri-intestinal, and perineural regions), and hitherto only a union of the heart with the pericardial region by slit-like openings was known. These openings are symmetrically distributed on five abdominal segments; each section of the heart in this region has, therefore, four openings, which are all of a truly venous nature. These openings, called cardio-cœlomic apertures, are visible to the naked eye, being situated on conical papillæ of the walls of the heart. These papillæ pass through the outer diaphragm, and open into the peri-intestinal part of the cœlom, in the Acrydiidæ directly, in the Locustidæ through special canals. The cells of the papillæ are spongy, possessing large nuclei, and similar, as a whole, to glandular cells. (Comptes rendus, cxix, 1894.)
The mechanism by which the ostia are closed consists, according to Graber, of an ∞-shaped muscle passing around the two openings, and which, being interlaced, is sufficient to close the openings. But this is not all. The fore and hinder edge of the ostia project, leaf-like, into the cavity of the heart, and thus form, with the outer walls, two valves which, during the systole, filled with the blood rushing in, not only hermetically close the lateral openings, but also, by the simultaneous closure of the entire chamber by the circular muscles in the middle of the same, the two valves, simultaneously approaching each other, so nearly touch that they form a transverse partition wall in the chamber. But, for the last purpose, i.e. for the separation of the chambers from one another, there is a very special contrivance. In the May beetle, we find, besides a valve (Fig. 373, B, e), opening into the middle of the chambers, a large, stalked cell (d), which, in the diastole, i.e. in the expansion of the heart, hangs down free on the walls of the heart; but, in the systole or contraction, like a cork, closes the middle of the valve, but does not wholly close the cavity. He has observed, in the larva of Corethra, formal, interventricular valves, which also are not in the middle, but are separated from one another in the interlaced ends. They consist of two longitudinally membranous flaps which move against each other like two valves (Fig. 373, B, b).
“But what is the necessity for such a complicated mechanism? All the blood from behind passes into the heart, and, for its propulsion a simple muscular tube, whose circular fibres would draw together and contract it, would be thought to be sufficient. But the heart, except in some larvæ, ends posteriorly in a blind sac, and the blood can only pass into it by a series of pairs of lateral openings. Now, as regards the reception and the propulsion of the blood forwards, two modes are conceivable. The simplest way would be that the tubular heart should, along its whole length, contract or expand; that, moreover, the blood should be simultaneously sucked in through all the openings, and that then, also, the contraction, or systole, should take place in every part of the heart at the same moment. But this would, plainly, in so long and thin-walled a vessel, be highly impracticable, since, through such a manipulation, the mass of blood enclosed in the heart would be crowded together rather than really impelled forwards. Only the second case could be admissible, and that is this, that each chamber pulsates, one after another, from behind forwards. But, then, each segmental heart must be separated from the others by a valve. To make the matter wholly clear, we may observe an insect heart pulsating, and this is best seen in one of its middle chambers. This chamber expands (simply by the relaxation of its circular muscles), the ostia, also, consequently open, and a given quantity of blood is drawn in from the pericardial cavity. What now would happen after the succeeding contraction if there were no valves between? The blood would not flow forwards, but seek a way out backwards.
“But, in fact, the valve of the hinder chamber, at this time, closes itself, while, by the simultaneous expansion of the anterior ones, their door opens, and this section of the heart, at the same time, causes a sucking in of the contents of the posterior chamber. This phenomenon is repeated, in the same way, from chamber to chamber, which also acts alternately as ventricle and auricle, or by a sucking and pumping action. One is involuntarily reminded of the ingenious manipulation by which, by the alternate opening and shutting of the flood-gates, a vessel is carried along a canal.
“This wave-like motion of an insect’s heart also has the advantage that, just before a pulse-wave has reached the chambers farthest in front, the hinder ones are already prepared for the production of a second, for, as a matter of fact, often 60, and even 100, and, in very agile insects, 150, waves pass, in a single minute, through the series of chambers, which make it very difficult to follow the flowing of their waves.” (Graber.)
The propulsatory apparatus.—But the heart itself is only a part of the entire propulsatorial apparatus to which belongs the following contrivance, the nature of which has been worked out by Graber.
Under the dorsal vessel is stretched a sort of roof-like diaphragm, i.e. a membrane, arched like the dorsal wall of the hind-body which is attached, in a peculiar way, to the sides of the body. The best idea can be gained by a cross-section through the entire body (Fig. 374): H is the true dorsal vessel; S, the diaphragm. A surface view is seen at 373, C, b, where it appears as a plate with the edge regularly curved outwards on each side. Its precise mode of working is thus: from each dorsal band of the sides of the abdomen arises a pair of muscles spreading out fan-like, and extending to the heart, so that the fibres of one side pass directly over to those of the other, often splitting apart, or, between the two, extends outwards a perforated, thin web, like an elastic, fibrous sheet (Fig. 373, A, a), with numerous perforations, forming a diaphragm.
Fig. 374.—Diagram of transverse section of pericardial sinus of Ædipoda cœrulescens: H, heart; s, septum; m, muscles,—the upper suspensory, the lower alary.—After Graber, from Sharp. (See also Fig. 377.)
Graber has thus explained the action of the pericardial diaphragm and chamber, as freely translated by Miall and Denny: “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.”
In the cockroach, however, Miall and Denny think that 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.... 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 equalize 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.”
“The peculiar office,” says Graber, “performed by the heart has already been stated. It is nothing more than a regulator; than an organ for directing the blood in a determinate course in order that this may not wholly stagnate, or only be the plaything of a force acting in another way, as, for example, through that afforded by the body-cavity and the inner digestive canal. At regular intervals a portion of the blood is sucked through the same, and then by means of the anterior supply tube it is pushed onward into the head, whence it passes into the cavities of the tissues. The different conditions of tension under which the mass of blood stands in the different regions of the body then causes a farther circulation. Besides this, the blood passes through separate smaller pumping apparatuses, and through vessel-like modifications of cavities, also through hollow spaces between the muscles, as, for example, in the appendages where a regular backward and forward flow of the blood, especially in the limbs, wings, antennæ, and certain abdominal appendages takes place. Here and there may occasionally occur a narrow place where the flow of blood is obstructed by the accumulation of the blood corpuscles, causing a considerable stagnation.” (Graber.)
Fig. 375.—Libellula depressa, opened from the back, showing the nervous cord (b1-b3, thoracic, h1-h7, abdominal, ganglia), also the furrow-like ventral sinus closed by a muscular diaphragm.
Fig. 376.—A, part of the ventral furrow of Libellula depressa more highly magnified: a, a sternal plate (urite); c, the septum stretched over it, at s in a relaxed or collapsed state; b and d, the wing-like, sternal processes from which the muscular bundles of the diaphragm arise. B, same in Acridium.
Fig. 377.—Diagrammatic section of the abdomen of Acridium tartaricum, showing the ventral septum (i, p, l) contracted, and (i, k, l) stretched out; oh, rib-like lateral processes of the urite; f, ganglia; b, heart, with its suspensorium (a); c, fat tissue in the pericardial tissue sinus; d, dorsal septum or diaphragm contracted. q, extended; g, fat-body; e, muscular part of diaphragm; no, expiration, hm, inspiration, muscle.—This and Figs. 375, 376, after Graber.
The supraspinal vessel.—In many insects there is a ventral heart acting on the heart’s blood as an aspirator, or more correctly a ventral sinus lying on the nervous cord, and closed by a pulsating diaphragm. This was discovered by Réaumur in the larva of a fly, and by Graber in the dragon-fly and locusts (Acrydiidæ). A glance at Figs. 375 and 376 will save a long description. The ventral wall forms a furrow, and between its borders (Fig. 377, e) extends the diaphragm. During the contraction of the muscles—and this, here, acts from before backwards—the membrane rises up and makes a cavity for the blood, which passes backwards over the nervous cord. The dorsal and ventral sinuses together thus bring about a closed circulation.
It thus appears that the insects are well provided with the means of distribution of their nutritive fluid, and that the blood is kept continually fresh and rich in oxygen. (Graber.)
The aorta.—While the heart is mostly situated within the abdomen, it is continued into the thorax and the head as a simple, non-pulsating tube, called the aorta. In Sphinx the aorta, as described by Newport, begins at the anterior part of the 1st abdominal segment, where it bends downwards to pass under the metaphragma and enter the thorax; it then ascends again between the great longitudinal dorsal muscles of the wings, and passes onwards until it arrives at the posterior margin of the pronotum; it then again descends and continues its course along the upper surface of the œsophagus, with which it passes beneath the brain, in front of which and immediately above the pharynx, it divides into two branches, each of which subdivides. Newport, however, overlooked a thoracic enlargement of the aorta called by Burgess the “aortal chamber” (Fig. 310, a, c).
Fig. 378.—A, last three abdominal segments and bases of the three caudal processes of Cloëon dipterum: r, dorsal vessel; kl, ostia; k, special terminal chamber of the dorsal vessel with its entrance a; b, blood-vessel of the left caudal process. B, 26th joint of the left caudal appendage from below: b, a portion of the blood-vessel; o, orifice in the latter.—After Zimmermann, from Sharp.
“In Sphinx and Vanessa urticæ, immediately after the aorta has passed beneath the cerebrum, it gives off laterally two large trunks, which are each equal in capacity to about one-third of the main vessel. These pass one on each side of the head, and are divided into three branches which are directed backwards, but have not been traced farther in consequence of their extreme delicacy. Anterior to these trunks are two smaller ones which appear to be given to the parts of the mouth and antennæ, and nearer the median line are two others which are the continuations of the aorta. These pass upwards, and are lost in the integument. The whole of these parts are so exceedingly delicate that we have not, as yet, been able to follow them beyond their origin at the termination of the aorta, but believe them to be continuous, with very delicate, circulatory passages along the course of the tracheal vessels. It is in the head alone that the aorta is divided into branches, since, throughout its whole course from the abdomen, it is one continuous vessel, neither giving off branches, nor possessing lateral muscles, auricular orifices, or separate chambers.” (Newport, art. Insecta, p. 978.)
Dogiel observed in the transparent larva of Corethra plumicornis that the aorta extends only to the hinder border of the brain. Here it divides into two lamellæ, each of which independently extends farther on. One lamella is seen under the brain and under the eye, the other reaches near the eye. The lamellæ are tied to the integument by threads. At the point of division of the aorta is an opening. (Kolbe.)
True blood-vessels appear to exist in the caudal appendages of the May-flies, as the heart appears to divide and pass directly into them (Fig. 378). The last chamber of the heart diminishes in size at the end of the body, and then divides into three delicate tubular vessels which pass into the three caudal appendages, and extend to the end of each one, along the upper side. While the valves of the heart, in all insects, are directed anteriorly because the blood flows from behind, in the larva of the Ephemeridæ the valves of the last chamber of the heart are directed backwards, because from this chamber the blood flows in the opposite direction, i.e. into the caudal appendages. During the contraction of the heart, the elongated section of the same in the last abdominal segment receives a part of the mass of blood contained in the last chamber, which is driven by independent contractions into the caudal appendages. These vessels have openings before the end through which the blood enters into the cavity of the appendages, and can also pass back, in order to be taken up by the body cavity. It is possible that these blood-vessels stand in direct relation to respiration. (Zimmermann, Creutzburg, in Kolbe, p. 544.)
The pericardial cells.—Along the heart, on both sides, occur the so-called pericardial cells, which differ from the fat-cells, and also the peritracheal cells of Frenzel, and are mostly arranged in linear series, which have a close relation to the circulation of the blood. In the larva of Chironomus, they lie in groups; in that of Culex, they are arranged segmentally. In caterpillars, these pericardial cells are not situated in the region of the heart, but are arranged linearly on the side, and form a network of granulated cells situated between the fat-bodies. Other rows of these cells are situated near the stigmata and the main lateral tracheæ. (Kolbe.)
According to Kowalevsky, the pericardial cells, and the garland-shaped, cellular cord consist of cells, whose function it is to purify the blood, and to remove the foreign or injurious matters mingled with the blood.
Fig. 379.—Diagram of the circulatory organs in the head of the cockroach, seen from above: A, ampulla; V, antennal vessel; M, chief muscular cord; m, muscular band; Bs, wall of the blood sinus; am, opening of the aorta (a); rg, anterior sympathetic or visceral ganglion; hg, hinder visceral ganglion; F, F, facetted eyes; o, vestigial ocellus; G, G, brain; S, œsophagus.—After Pawlowa.
Ampulla-like blood circulation in the head.—In the head of the cockroach occurs, according to Pawlowa, a contractile vascular sac at the base of each antenna. The cavity has a valvular communication with the blood space below and in front of the brain, and muscle-fibres effect systole and diastole. Each sac is beyond doubt an independently active part of the circulatory system. These organs also occur in Locusta and other Acrydiidæ, and Selvatico has described similar structures in Bombyx mori and certain other Lepidoptera.
Pulsatile organs of the legs.—Accessory to the circulation is a special system of pulsatile organs in the three pairs of legs of Nepidæ, generally situated in the tibia just below its articulation with the femur, but in the fore legs of Ranatra, in the clasp-joint or tarsus, just below its articulation with the tibia. First observed by Behn (1835), Locy has studied the organ (Fig. 380) in Corixa, Notonecta, Gerris, besides the Nepidæ. It is a whip-like structure attached at both ends, with fibres extending upward and backward to the integument of the leg, separate from the muscular fibres and does not involve them in its motions, and is not affected by the muscles themselves. “As the blood-corpuscles flow near the pulsating body they move faster, and around the organ itself there is a whirlpool of motion.” The beating of these organs aids the circulation in both directions, and when the motion ceases, the blood-currents in the legs stop; the rate of the pulsating organ is always faster than that of the heart, and the action is automatic.
Fig. 380.—Pulsating organs in Hemiptera: A, Belostoma nymph, B, legs of Corixa. C, Ranatra, adult, to show the exceptional position of the pulsating organ in the fore legs. D, pulsatile organ in tibia of Ranatra.—After Locy.
b. The blood
The blood of insects, as in other invertebrates, differs from that of the higher animals in having no red corpuscles. It is a thin fluid, a mixture of blood (serum) and chyle, usually colorless, but sometimes yellowish or reddish, which contains pale amœboid corpuscles corresponding to the white corpuscles (leucocytes) of the vertebrates, though they are relatively less numerous in the blood of insects. The yellow fluid expelled from the joints of certain beetles (Coccinella, Timarcha, and the Meloidæ) is, according to Leydig, only the serum of the blood. In phytophagous insects the blood is colored greenish by the chlorophyll set free during digestion. The blood of Deilephila euphorbia is colored an intense olive-green, and that of Cossus ligniperda is pale yellow. (Urech.) The blood of case-worms (Trichoptera) is greenish. In some insects it is brownish or violet. The serum is the principal bearer of the coloring material, yet Graber has shown that in certain insects the corpuscles are more or less beset with bright yellow or red fat-globules, so as to give the same hue to the blood.
The leucocytes.—The corpuscles are usually elongated, oval, or flattened oat-shaped, with a rounded nucleus, or are often amœbiform; and they are occasionally seen undergoing self-division. When about to die the corpuscles become amœbiform or star-shaped. (Cattaneo.) Their number varies with the developmental stage of the insect, and in larvæ increases as they grow, becoming most abundant shortly before pupation. The blood diminishes in quantity in the pupal stage, and becomes still less abundant in the imago. (Landois.) The quantity also varies with the nutrition of the insect, and after a few days’ starvation nearly all the blood is absorbed. Crystals may be obtained by evaporating a drop of the blood without pressure; they form radiating clusters of pointed needles. The freshly drawn blood is slightly alkaline. (Miall and Denny.)
The size of the corpuscles has been ascertained by Graber, who found that the diameter of the circular blood-disks of the leaf-beetle, Lina populi, is 0.006 mm.; of Cetonia aurata and Zabrus gibbus, 0.008 to 0.01 mm.; and those of certain Orthoptera (Decticus verrucivorus, Ephippiger vitium and Œdipoda cœrulescens), 0.011 to 0.014 mm. The longest diameter of the elongated corpuscles of Carabus cancellatus is 0.008 mm.; of Gryllus campestris, Locusta viridissima, Cossus ligniperda, Sphinx ligustri (pupa), and others, 0.008 to 0.01 mm.; of Caloptenus italicus, Saturnia pyri, Anax formosus, and others, 0.011 to 0.014 mm.; of Ephippiger vitium, Œdipoda cœrulescens, Pezotettix mendax, Zabrus gibbus, Phryganea, and others, 0.012 to 0.022 mm.; in Stenobothrus donatus and variabilis, 0.012 to 0.035 mm. The largest known are those of Melolontha vulgaris, which measure from 0.027 to 0.03 mm.
Fig. 381.—Blood corpuscles, or leucocytes, of insects: A, a-g, of Stenobothrus dorsatus (the same forms occur in most Orthoptera and in other insects). B, a, leucocyte of the same insect with the nucleus brought out by ether; b, another of serpentine shape. C, leucocytes of the same insect after a longer stay in ether. D, leucocytes of the same after being in glycerine 14 days.—After Graber.
As regards the nature of the corpuscles, Graber concludes that they are more like the cells of the fat-bodies than genuine cells. That they are not true cells is shown by the fact that after remaining in their normal condition a long time they finally coalesce and form cords. After shrivelling, or after the blood has been subjected to different kinds of treatment, the nucleus is clearly brought out (Fig. 381).
Besides the blood corpuscles there have been detected in the blood round bodies which are regarded as fat-cells. They are circular, and for the most part larger than the blood corpuscles, have a sharp, even, dark outline, and an invariably circular nucleus. (Kolbe.)
The blood of Meloe, besides the amœboid corpuscles, according to Cuénot, contains abundant fibrinogen, which forms a clot; a pigment (uranidine), which is oxidized and precipitated when exposed to the air; a dissolved albuminoid (hæmoxanthine), which has both a respiratory and nutritive function; and, finally, dissolved cantharidine.
The corpuscles arise from tissues which are very similar to the fat-bodies, and which, at given times, separate into cells. The position of these tissues is not always the same in different insects. In caterpillars, they occur in the thorax, near the germs of the wings; in the saw-flies (Lyda), in all parts of the thorax and abdomen; in larval flies (Musca), in the end of the abdomen, just in front of the large terminal stigmata. The place where the blood corpuscles are formed is usually near, or in relation with, the fat-bodies. But while the fat-bodies mostly serve as the material for the formation of the blood-building tissues, in caterpillars the tracheal matrix also, and, in dipterous larvæ, the hypodermis serve this purpose. (Cæsar Schaeffer in Kolbe. See also Wielowiejski, Ueber das Blutgewebe.)
Other substances occur in the blood of insects. Landois (1864) demonstrated the existence of egg albumen, globulin, fibrin, and iron in the blood of caterpillars. Poulton found that the blood of caterpillars often contained chlorophyll and xanthophyll derived from their food plants. A. G. Mayer has recently found that the blood (hæmolymph) of the pupæ of Saturniidæ (Callosamia promethea) contains egg albumin, globulin, fibrin, xanthophyll, and orthophosphoric acid, and Oenslager has determined that iron, potassium, and sodium are also present. (Mayer.)
c. The circulation of the blood
Every part of the body and its appendages is bathed by the blood, which circulates in the wings of insects freshly emerged from the nymph or pupal state, and even courses through the scales of Lepidoptera, as discovered by Jaeger (Isis, 1837).
In describing the mechanism of the heart we have already considered in a general way the mode of circulation of the blood.
The heart pumping the blood into the aorta, the nutritive fluid passes out and returns along each side of the body; distinct, smaller streams passing into the antennæ, the legs, wings (of certain insects), and into the abdominal appendages when they are present. All this may readily be observed in transparent aquatic insects, such as larval Ephemeræ, dragon-flies, etc., kept alive for the purpose under the microscope in the animalcule box.
Carus, in 1827, first discovered the fact of a complete circulation of the blood, in the larva of Ephemera. He saw the blood issuing in several streams from the end of the aorta in the head and returning in currents which entered the base of the antennæ and limbs in which it formed loops, and then flowing into the abdomen, entered the posterior end of the heart. Wagner (Isis, 1832) confirmed these observations, adding one of his own, that the blood flows backward in two venous currents, one at the sides of the body and intestine, and the other alongside of the heart itself, and that the blood not only entered at the end of the heart, but also at the sides of each segment, at the position of the valves discovered by Straus-Dürckheim.
Newport maintains that the course of the blood is in any part of the body, as well as in the wings, almost invariably in immediate connection with the course of the tracheæ, for the reason that “the currents of blood in the body of an insect are often in the vicinity of the great tracheal vessels, both in their longitudinal and transverse direction across the segments.”
The circulation of the blood in the wings directly after the exuviation of the nymph or pupa skin, and before they become dry, has been proved by several observers. As stated by Newport, the so-called “veins” or “nervures” of the wings consist of tracheæ lying in a hollow cavity, the peritracheal space being situated chiefly under and on each side of the trachea.
Fig. 382.—Circulation of the blood in hind wing of Periplaneta orientalis: the arrows indicate the usual direction of the blood currents.—After Moseley.
Newport gives the following summary of the observations of the early observers, to which we add the observations of Moseley. “A motion of the fluids has been seen by Carus in wings of recently developed Libellulidæ, Ephemera lutea and E. marginata, and Chrysopa perla; among the Coleoptera, in the elytra and wings of Lampyris italica and L. splendidula, Melolontha solstitialis and Dytiscus.” Ehrenberg saw it in Mantis, and Wagner in the young of Nepa cinerea and Cimex lectularius. Carus detected a circulation in the pupal wings of some Lepidoptera, and Bowerbank witnessed it in a Noctuid (Phlogophora meticulosa); Burmeister observed it in Eristalis tenax and E. nemorum, and Mr. Tyrrel in Musca domestica, but it has not been observed in the wings of Hymenoptera.
Bowerbank observed that in the lower wing of Chrysopa perla the blood passes from the base of the wing along the costal, post-costal, and externo-medial veins, outwards to the apex of the wing, giving off smaller currents in its course, and that it returns along the anal vein to the thorax. He found that the larger veins, 1
408 in. in diameter, contained tracheæ which only measured 1
2222 of an inch in diameter; but in others the tracheæ measured 1
1340, while the cavity measured only 1
500 of an inch. He states, also, that the tracheæ very rarely give off branches while passing along the main veins, and that they lie along the canals in a tortuous course. (Newport, art. Insecta, p. 980.)
Bowerbank, also, in his observations on the circulation in the wings of Chrysopa, “used every endeavor to discover, if possible, whether the blood has proper vessels, or only occupied the internal cavities of the canals; and that he is convinced that the latter is the case, as he could frequently perceive the particles not only surrounding all parts of the tracheæ, and occupying the whole of the internal diameter of the canals, but that it frequently happens that globules experienced a momentary stoppage in their progress, occasioned by their friction against the curved surface of the tracheæ, which sometimes gave them a rotatory motion.”
Fig. 383.—Parts of a vein of the cockroach, showing the nerve (n) by the side of the trachea (tr); c, blood corpuscles.—After Moseley.
Moseley found, owing to the large size and number of the corpuscles, that the circulation of the blood in the wings of insects is most easily observed in the cockroach, especially the hind wings. As seen in Moseley’s figure, the blood flows outward from the body through the larger veins (I and II) of the front edge of the wings, which he calls the main arteries of the wings, and more generally returns to the body through the veins in the middle of the wing; the blood also flows out from the body through the inner longitudinal veins (those behind vein IV), and the blood is also seen to flow through some of the small cross-veins. Fig. 383 shows one of the main trunks during active circulation. The corpuscles change their form readily, “the spindle-shaped ones doubling up in order to pass crossways through a narrow aperture.... In the irregularly formed corpuscles, which seem to represent leucocytes amœboid movements were observed.... Corpuscles pass freely above and under the tracheæ, showing that these latter lie free in the vessels.” The hypodermis lining the vessels is best seen in the small transverse veins.
The pulse or heart-beat of insects varies in rapidity in different insects, rising at times of excitement, as Newport noticed in Anthophora retusa, to 142 beats in a minute.
When an insect, as, for example, a tineid caterpillar, has been enclosed in a tight box for a day or more, the pulsations of the heart are very languid and slow, but soon, on giving it air, the pulsations will, as we have observed, rise in frequency to about 60 a minute, Herold observed 30 to 40 in a minute in a fully-grown silkworm, and from 46 to 48 in a much younger one. Suckow observed but 30 a minute in a full-grown caterpillar of Gastropacha pini, and 18 only in its pupa.
In a series of observations made by Newport on Sphinx ligustri from the fourth day after hatching from the egg until the perfect insect was developed, he found that before the larva cast its first skin the mean number of pulsations, in a state of moderate activity and quietude, was about 82 or 83 a minute; before the second moult 89, while before the third casting it had sunk down to 63; and before its fourth to 45, while, before leaving its fourth stage, and before it had ceased to feed, preparatory to pupating, the pulse was not more than 39. “Thus the number gradually decreases during the growing larva state, but the force of the circulation is very much augmented. Now when the insect is in a state of perfect rest, previously to changing its skin, the number is pretty nearly equal at each period, being about 30. When the insect has passed into the pupa state it sinks down to 22, and subsequently to 10 or 12, and after that, during the period of hibernation, it almost entirely ceases. But when the same insect which we had watched from its earliest condition was developed into the perfect state in May of the following spring, the number of pulsations, after the insect had been for some time excited in flight around the room, amounted to from 110 to 139; and when the same insect was in a state of repose, to from 41 to 50. When, however, the great business of life, the continuation of the species, has been accomplished, or when the insect is exhausted, and perishing through want of food or other causes, the number of pulsations gradually diminishes, until the motions of the heart are almost imperceptible.” Insects, then, he remarks, do not deviate from other animals in regard to their vital phenomena, though it has been wrongly imagined that the nutrient and circulatory functions are less active in the perfect than in the larval condition.
The heart of a larval Gastrus equi taken the day previous from a horse’s stomach beat from 40 to 44 times a minute (Scheiber); while Schröder van der Kolk observed only 30 beats in the same kind of maggot.
In the larva of Corethra, while at rest, the heart contracts from 12 to 16 or 18 times a minute, but when active the number rises to 22. The systole and diastole last from 5 to 6 minutes. (Dogiel.)
Temperature also affects the pulsations, as they increase in frequency with a rise and decrease with a fall in temperature.
Influence of electricity.—The influence of electricity on the action of the insect’s heart, from Dogiel’s experiments, is such as to cause an acceleration in the frequency of the beats, while an increase in the strength of the electric currents either diminishes the frequency of the beats or entirely stops the heart’s action. A violent excitation with the induction current causes a systole when the heart’s action has stopped for a long time; and if the excitation lasts uninterruptedly, then the contractions after a while become noticeable, according to the strength of the current. In such a case there are, however, interruptions in the regularity, strength, and order of the contractions. (Kolbe.)
Effects of poisons on the pulsations.—Dogiel has also experimented on the influence of poisons in the form of vapor or as liquid solutions on the pulsations of insects, which is much as in vertebrates. The application of carbonic oxide to the larva of Corethra, whose heart one minute previous to the poisoning beat 15 times a minute, accelerated the heart-beats in about 55 minutes to 25 pulsations in a minute. Afterwards there was a retardation in the pulse to the normal beat. Carbonic acid had a similar effect.
The following results obtained by Dogiel are somewhat as tabulated by Kolbe:—
I. Substances which cause the pulsations of the heart to accelerate.
a. An induction current of electricity, acting feebly.
b. Ammonia, acting feebly.
c. Ethyl ether, acting feebly.
d. Oxalic acid, acting feebly.
e. Carbolic acid, acting feebly.
f. Potassium nitrate, acting feebly.
g. Aconite, acting feebly.
II. Substances retarding the heart’s action.
a. An induction current of electricity, acting energetically.
b. Ammonia, acting energetically.
c. Ethyl ether, acting energetically.
d. Oxalic acid, acting energetically.
e. Carbolic acid, acting energetically.
f. Veratrine, acting energetically.
g. Atropine, acting energetically.
h. Aconitine, acting energetically.
i. Potassium nitrate, acting energetically.
g. Ethyl alcohol.
h. Chloroform.
i. Carbonic oxide.
j. Carbonic acid.
k. Sulphuretted hydrogen.
III. Substances whose action is indifferent.
1. Muscarine.
2. Curare.
3. Atropine, acting slowly.
4. Strychnine.
The above-named substances comprise those which in the vertebrates effect a change in the activity of the motor nerve-ganglia of the heart and the muscular fibres. Hence it follows that the heart of the larval Corethra consists of muscular fibres provided with ganglia, and that the contractions of the muscular fibres are provoked through the agency of the ganglia. But since muscarine, atropine, and curare, whose influence in stopping the heart’s action of vertebrates is known, in insects either have no action or only make the pulsations slower; it seems to follow that the heart of the larval Corethra possesses no similar apparatus for lessening the heart’s action, and this is also confirmed by anatomical studies. On the contrary, aconite acts, as we must from observations conclude, exclusively on the motor centres and the muscles, but not on the apparatus for lessening the heart’s action, which, as has been remarked, is not present in the larval Corethra. (Kolbe ex Dogiel.)
Dewitz has discovered an onward movement of the blood corpuscles, somewhat independent of the general circulation. This independent motion of the blood corpuscles is not only a creeping one like the amœboid motion of the white corpuscles of vertebrates, but they have besides a peculiar swimming movement. Dewitz noticed this in the hind wings of a recently emerged meal-worm beetle (Tenebrio molitor), still white and soft, after they had been cut off. The tissues forming the matrix within the wings constitute a network filled with blood. The current of blood within the wing thus cut off may be stopped flowing by a tap on the firmly clamped object-bearer on which the wing is placed, or by drawing it by an apparatus described by the same author, to incite in one way or another the blood corpuscles to swim forwards. When a corpuscle is disposed to move, we see it first stirring restlessly, or wabbling about, in this way changing its form; then it moves forwards, and does not come to a standstill. If it remains still there, after a while, by tapping, it begins again its movements.
“Should one yet doubt the fact of this spontaneous movement of the blood corpuscles, he will surely be convinced of its correctness by observing the so-to-speak reluctantly springing motion of a blood corpuscle in the wing of Tenebrio molitor with the simultaneous change of appearance and shape of the corpuscle.”
This spontaneous or independent motion of the blood corpuscles is also produced by the heating apparatus. As soon as the corpuscles lie still in the severed wing and they are warmed, the corpuscles begin to pass through the meshes of the tissue. When cooled, the motion ceases, but as soon as the temperature rises to a certain grade, the corpuscles again move onwards.
To explain this independent motion Dewitz thinks that they take up and then expel the blood-fluid, and in this way cause their motion. This independent motion is necessitated, in order that the stream of blood may become so regulated, that the blood corpuscles shall not be arrested in their course, but even turn back again out of the farther end of the antennæ and limbs. The chief mechanical power for the blood circulation must go on independently of the propulsatorial apparatus and of the heart. (Kolbe.)