THE BLOOD TISSUE
Under this name Wielowiejski has included several important tissues or cellular bodies intimately concerned with the nutrition of the insect. These are:—
1. The blood corpuscles. (See p. 407, leucocytes and phagocytes.)
2. The fat-body proper (Corpus adiposum).
3. The pericardial fat-body (pericardial cells).
4. The œnocytes.
5. The garland-shaped cord of muscid larvæ.
6. The subœsophageal body, a peculiar organ found by Wheeler in the embryos and young larvæ of Blatta and Xiphidium.
7. The phosphorescent organs.
a. The fat-body
In the body cavity of winged insects and of their larvæ occur yellowish masses of large cells filled with small drops of fat, and forming the “fat-body.” It is of various shapes, more or less lobulated or net-like, and covers or envelops parts of the viscera, also forming a layer under the integument (Fig. 143). The tracheal endings are usually enveloped by the fat-body. It is larger in the larvæ than in the adults, especially in Lepidoptera, in them forming a reserve of nutrition, used during metamorphosis and during the formation and ripening of the eggs and male cells.
Wielowiejski has shown that there is a regular arrangement of the fat-body in the general cavity of the body. For example, in the larva of Chironomus occur the following forms of this tissue. Around the periphery, on each side of the body cavity, is a loose network of lobes with large meshes constituting the peripheral layer or external lobular fat-body; these lobular masses are segmentally arranged.
Within these segmental lobes, on each side of and along the digestive tract, extending along through almost the entire body, is an unbroken strand of this tissue, forming the internal fat-body cords. From the first larval stage, and even before hatching, its cells are so unusually large, being filled with large, clear, mostly colorless fat-drops, that their limits cannot be defined, and their nuclei can only with great difficulty be detected. Only in some large larvæ of Chironomus has Wielowiejski found clearly defined cells; the protoplasm of these cells contain almost no fat-drops.
The fat-body is of mesodermal origin, and as Wheeler insists, is not derived from the œnocytes, as supposed by Graber. Formed from the mesoderm, it is a differentiation of portions of the cœlomic walls, and therefore metameric in origin. That the fat-body gives origin to the blood corpuscles Wheeler is doubtful.
The fat-cells are distinct, spherical, and as a rule possess only one nucleus, though in those of Apis and Melophagus there are two nuclei, and in Musca several. Sometimes the cells contain a substance like the white of an egg, and concretions of uric acid, or these take the place of the fat-drops. The presence of uric acid shows that a very active metabolism goes on in the fat-body. “In some cases it has been proved that the fat-body in the larva is rich in fat and poor in concretions of uric acid, while in the imago it is poor in fat and rich in concretions of uric acid” (Lang).
Leydig, in 1857 (Lehrbuch der Histiologie), spoke of the presence of dark concretions in the fat-body, and afterwards (1864) showed that there was a wide distribution of uric acid salts and concretions. Witlaczil, also, has detected concretions in the fat-body of the Psyllidæ, in larval Cecidomyiidæ, in the larvæ and pupæ of ants, and in the pupa of Musca.
The physiological processes which take place in the fat-bodies are obscure. Graber regarded the whole system of the fat-bodies as “a single, many-lobed lung,” while before him Landois, taking into account the intimate relation existing between the finer tracheal branches and the fat-body, considered that the latter was concerned in respiration. Marchal thinks that the fat-body is a urinary organ, as the urates are formed within the cells of this body.
Moreover, Schäffer maintains that a special kind of fat-body cell has the important function of taking up and giving out nutritious matters during the internal processes of metamorphosis, while he also believes that there is a genetic connection between the fat-body and the blood corpuscles—a view combated by Wheeler.
Kowalevsky finds that the fat-body remains absolutely insensible to the action of the substances which stained the Malpighian tubes (p. 352). So long as the cells are healthy and living they are not stained and do not absorb the colors in question; and this insensibility persists, even when the cells are of a different nature, as those of the fly (adipose and “intercalary” cells).
b. The pericardial fat-body or pericardial cells
We have already, on p. 405, called attention to these organs, but they also have an intimate relation to the fat-body.
Kowalevsky (1892) remarks that the disposition of these cells varies much in different insects and even in the same animal. Thus, in the Diptera and the ordinary flies there are found around the lower part of the dorsal vessel 13 pairs of large pericardial cells which lie next to a crowded bed of small cells forming a compact mass around the anterior part of the dorsal vessel. In caterpillars, notably silkworms, from the compact layer of pericardial cells which surround the heart, pass off trunks which are directed towards the lateral walls of the body, also forming close networks around the tracheæ and then passing down into the abdominal cavity of the body of the larva.
In the larvæ of certain Hymenoptera, the trunks which pass off from the pericardial region form a loose cord, a sort of fatty tissue covering the entire body cavity.
This tissue, adds Kowalevsky, entirely differs from œnocytes, or from the so-called glandular body whose formation in Gryllotalpa has been described by Korotaiev, and in Bombyx mori by Tichomiroff. In a recent work wherein has been collected everything known regarding these last-named cells, Pékarsky proves that they are unique in nature and cannot be regarded either as fat-cells, or as pericardial cells, or even as formative leucocytes.
As to the structure of the pericardial cells, Kowalevsky adds that they are always attached to muscular fibres passing off from the heart, and that they lie, so to speak, upon them. In the locusts the muscular fibres supporting the pericardial cells appear distinctly like little staves or sticks. The attachment of the pericardial cells to the muscular fibres has been observed by Cuénot and reproduced by him in his work, but his description somewhat differs from that observed by Kowalevsky in the locust (Acrydium migratorium).
As to the nature of the acid excretions which are formed in the pericardial cells, in spite of his attempts to solve the problem, Kowalevsky has been unsuccessful. The only observations in this direction are those of Letellier on the pericardial glands of lamellibranch molluscs, which he found to contain hypouric acid, and it is probable, says Kowalevsky, that the acidity of the pericardial cells in insects is due to the presence of the same acid.
Leucocytes or phagocytes in connection with the pericardial cells.—It is thought by Schäffer that the leucocytes or phagocytes may be free or wandering fat-body cells. They play an important part in metamorphosis, while they absorb or feed upon the remains of the larval organs, and thus prove of use in the building up of the organs of the adult insects.
While the faculty of phagocytosis is wanting in the urinary tubes, Balbiani and more recently Cuénot have expressed the opinion that the pericardial cells of insects may have the power of absorbing hard bodies, “acting as a phagocytic gland.” This, however, is called in question by Kowalevsky, from studies made on different insects. On introducing powdered carmine into the body of an insect it has not been absorbed by the pericardial cells, as they have not been colored red. It is the leucocytes which absorb the grains of carmine, and which, after having dissolved them, transmit them to the pericardial cells. Hence, then, the pericardial cells have not the phagocytic power of which Cuénot speaks.
Returning to his own observations on hard bodies introduced into insects, or large globules introduced under the form of a milk emulsion, Kowalevsky has found that these bodies were absorbed in the first place by the free-swimming leucocytes, and in the second place by whole groups or nests of leucocytes situated in different parts of the body, principally on the threads of the adipose body. In the Orthoptera the absorption is immediately effected by means of the cells of the membrane which separates the pericardium from the cavity of the body underneath the heart. The regions where the hard bodies are absorbed in great number coincide with the regions of formation of the blood corpuscles. In his researches on the larvæ of Hyponomeuta and other Lepidoptera, Schäffer describes these regions as forming a sort of island. The nests where the blood globules are formed are the most active centres of phagocytosis.
Fig. 384.-Section of the heart (c) and pericardial cells (pc, pc) from the posterior part of the heart of a fly: l, l, nests of leucocytes situated between the heart and pericardial cells.—From a microphotograph, after Kowalevsky.
Fig. 385.—Cross-section of the heart of Truxalis nasata and of the structures around it: c, heart: ep, epithelium under the cuticula (hvpodermis); or, ovarian tubes; pc, pericardial cells, with one or two nuclei containing a deposit of carmine; l and l′, group of leucocytes, which have absorbed granules of India ink.—After Kowalevsky.
Balbiani, and also Cuénot, have supposed that the formation of the blood corpuscles takes place in the pericardial cells, but Kowalevsky insists that these cells cannot form the leucocytes, which “are probably formed in different parts of the body, notably in the special nests [Herde of Jäger] situated near the heart, but outside of the pericardial cells.”
In Fig. 384, where the nests of leucocytes (l) are shown, it is evident that they are formed where observed, and “could not have come from the pericardial cells, which have their own structure and their special function,” these cells being very large and characteristic.
In Kowalevsky’s preparations of Truxalis, the pericardial cells with deposits of carmine and the groups of leucocytes (Fig. 385, l and l′) stained with India ink, we have to deal with elements absolutely different. If the formation of leucocytes was caused by the pericardial cells, these last would be obliged to free themselves from their contents and to modify their essential nature.
c. The œnocytes
Fig. 386.—Cluster of œnocytes from a nearly mature Phryganeid larva: o, œnocytes; t, large tracheal branch; tt, smaller tracheal ramifications; h, tracheal hypodermis.
Fig. 387.—A nearly mature embryo of Xiphidium ensiferum: o, o, œnocyte clusters seen from the surface through the integument; a, pleuropodium of the right side (appendage of the first abdominal segment); s, styli; c, cercopods.—This and Fig. 386 after Wheeler.
These cells (Fig. 386), with the exception of the eggs, are the largest in the body, and occur in most if not all winged insects. They were called œnocytes (oinos, wine; kustis, cyst), by Wielowiejski in allusion to their wine-yellow color. These cells are arranged segmentally (Fig. 387) in clusters, held in place by tracheæ, and are situated mostly on each side of the abdomen, rarely being found in the adjoining parts of the thorax. They are more or less intimately associated with the blood and fat-body. Unlike the fat-body, however, they arise in embryonic life from the ectoderm, either by delamination or by immigration, just behind the tracheal involutions.
The separate cells of each cluster are usually separate, but in rare cases may fuse in pairs or form smaller clusters. In shape they are round or oval, often sending out pseudopodia-like processes, by which they are attached to the tracheal twigs or to each other. “The cytoplasm, which is very abundant, is full of yellowish granules and is sometimes radially situated towards its periphery. The large spherical or oval nucleus contains a densely wound and delicate chromatic filament.” (Wheeler.)
Graber first pointed out the identity of these clusters of cells with certain metameric cell-masses in insect embryos, observed by Tichomiroff in those of the silkworm, and by Korotneff in the embryo mole-cricket.
Although they resemble the blood corpuscles in some insects, they are always much larger, and do not seem to be amœboid, while they are never seen to undergo self-division, or to exhibit any appearance of giving rise to the blood-cells (Wheeler). They have not yet been detected in Thysanura (Synaptera) or in Myriopoda.
d. The phosphorescent organs
Phosphorescence is not infrequent in the Protozoa, cœlenterates, worms, and has been observed in the bivalve Pholas, in a few abyssal Crustacea, in myriopods (Geophilus), in an ascidian, Pyrosoma, and in certain deep-sea fishes.
Fig. 388.—A, sagittal section through the hinder end of a male Luciola, the organs above the phosphorescent plate only drawn in outline: s, integument of the last segment, somewhat removed by the section-knife from the phosphorescent tissues; d, dorsal layer of the phosphorescent plate penetrated by irregular tracheal branches, and rendered opaque by numerous urate concretions imbedded in it; v, ventral phosphorescent layer of the plate, with perpendicular tracheal stems whose branches, where they pass into capillaries, bear lumps which stain brown with osmic acid; n, structureless substance (coagulum?) filling the end of the last ventral segment. B, isolated portion of the ventral layer of the phosphorescent plate; tr, tracheal stem surrounded by a cylindrical lobe: p, parenchym cell attached to the cylinder; c, capillary, without the spiral threads; m, coagulum stained brown. C, a tracheal stem of the ventral layer: at the fork of the brown-stained capillaries are lumps stained brown with osmic acid. D, a part of C, more highly magnified, showing the remains of the tracheal end-cells (tc) enveloping the brown lumps (m).—After Emery.
In insects luminosity is mostly confined to a few Coleoptera, and besides the well-known fireflies, an Indian Buprestid (Buprestis ocelata) is said to be phosphorescent; also a telephorid larva. Other luminous insects are the Poduran Anurophorus, Fulgora, certain Diptera (Culex, Chironomus[[60]] and Tyreophora), and an ant (Orya).
The seat of the light is the intensely luminous areas situated either in the head (Fulgora), in the abdomen (Lampyridæ), or in the thorax (in a few Elateridæ of the genus Pyrophorus). The luminous or photogenic organ is regarded by Wielowiejski and also by Emery as morphologically a specialized portion of the fat-body, being a plate consisting of polygonal cells, situated directly under the integument, and supplied with nerves and fine tracheal branches.
In Luciola as well as in other fireflies, including Pyrophorus, the phosphorescent organ or plate consists, as first stated by Kölliker, of two layers lying one over the other, a dorsal one (Fig. 388, d) which is opaque, chalky white, and non-photogenic, and a lower one (v), the active photogenic layer, which is transparent. Through the upper or opaque layer and on its dorsal surface extend large tracheæ and their horizontal branches, from which arise numerous very fine branches which pass down perpendicularly into the transparent or photogenic layer of the organ. Each tracheal stem, together with its short branches, is enveloped by a cylindrical mass of transparent tissue, so that only the short terminal branches or very fine tracheal capillaries project on the upper part of the cylinder. These finest tracheal capillaries are not in Luciola filled with air, but with a colorless fluid, as was also found by Wielowiejski and others in Lampyris.
These transparent cylinders, with the tracheæ within, forming longitudinal axes, resemble lobules. These lobules are so distributed that they appear on a surface section of this plate as numerous round areas in which circular periphery the tracheal capillaries are arranged with the axially disposed tracheal end-cells. These “tracheal end-cells” are only membranous enlargements at the base of the tracheal capillaries (Wielowiejski). The cylindrical lobules are separated from each other by a substance consisting of abundant large granular cells (parenchym cells) among which project the tracheal capillaries. The cylindrical lobules extend to the hypodermis and come in contact only by their lateral faces with the parenchym.
The structure of the upper opaque chalky white layer of the phosphorescent organ is, compared with that of the photogenic lower portion, very simple. In its loose, pappose, mass are no cellular elements, but when treated with different reagents it is seen to be filled with countless urate granules (guanine) swimming in the fluid it contains, the cell plasma appearing to be dissolved, the cells having lost their cohesion.
In comparing the phosphorescent plate or organ of Luciola with that of Lampyris, the general structure, including the clear cell elements of the cylindrical lobules, which envelop the perpendicular tracheal twigs and their branches, and also the granular parenchymatous cells are alike in both, though the arrangement and distribution of the elements in Luciola is more regular, in Lampyris the tracheal stems being irregularly scattered through the parenchym.
Wielowiejski found in the larval and female Lampyris a higher degree of differentiation than in the male, and Luciola has a more differentiated photogenic organ than Lampyris, as seen in the more regular structure of the lobules.
As regards the light-apparatus of Pyrophorus, or the cucujo, Heinemann shows that, as in the Lampyridæ, it consists of distinct cells, and may be regarded as a glandular structure. It is rich in tracheæ and the other parts already described. In still later researches on a Brazilian Pyrophorus Wielowiejski shows that the phosphorescent plate consists of two layers, the upper usually being filled with crystalline urate concretions, and entirely like those of the Lampyridæ, consisting of distinct polygonal cells, among which are numerous tracheal stems, with tænidia, coursing in different directions, when freshly filled with air, and sending capillaries into the underlying photogenic layer. The latter shows in its structure a striking difference in the cellular arrangement from that of Lampyrids. In the upper or non-photogenic layer are tracheal capillaries which pass down into the underlying cellular plate and which are in the closest possible relations with the single cells—a point overlooked by Heinemann.
Physiology of the phosphorescence.—As is well known, the phosphorescence of animals is a scintillating or glowing light emitted by various forms, the greenish light or luminous appearance thus produced being photogenic, i.e. without sensible heat.
Langley rates the light of the firefly at an efficiency of 100 per cent, all its radiations lying within the limits of the visible spectrum. “Langley has shown that while only 2.4 per cent of luminous waves are contained in the radiation of a gas-flame, only 10 per cent in that of the electric arc, and only 35 per cent in that of the sun, the radiation of the firefly (Pyrophorus noctilucus) consists wholly of visible wave-frequencies.” (Barker’s Physics, p. 385.)
The spectrum of the light of the cucujo was found by Pasteur to be continuous. (C. R. French Acad. Sc. 1864, ii, p. 509.) A later examination by Aubert and Dubois showed that the spectrum of the light examined by the spectroscope is very beautiful, but destitute of dark bands. When, however, the intensity diminishes, the red and orange disappear, and the green and yellow only remain.
Heinemann studied the cucujo at Vera Cruz, Mexico. At night in a dark room it radiates a pale green light which shows a blue tone to the exclusion of any other light. The more gas or lamp light there is present, the more apparent becomes the yellowish green hue, which in clear daylight changes to an almost pure very light yellow with a very slight mixture of green. “In the morning and evening twilight, more constantly and clearly in the former, the cucujo light, at least to my eyes, is an intensely brilliant yellow with a slight mixture of red. In a dark room lighted with a sodium light the yellow tone entirely disappears; on the other hand, the blue strikingly increases.” As regards the spectrum he found that almost exactly half of the blue end is wanting and that the red part is also a little narrower than in the spectrum of the petroleum flame.
Professor C. A. Young states that the spectrum given by our common firefly (Photinus?) is perfectly continuous, without trace of lines either bright or dark. “It extends from a little above Fraunhofer’s line C, in the scarlet, to about F in the blue, gradually fading out at the extremities. It is noticeable that precisely this portion of the spectrum is composed of rays which, while they more powerfully than any others affect the organs of vision, produce hardly any thermal or actinic effect. In other words, very little of the energy expended in the flash of the fire is wasted. It is quite different with our artificial methods of illumination. In the case of an ordinary gaslight the best experiments show that not more than one or two per cent of the radiant energy consists of visible rays; the rest is either invisible heat or actinism; that is to say, over 98 per cent of the gas is wasted in producing rays that do not help in making objects visible.”
Panceri also remarks that while in the spectroscope the light of some Chætopteri, Beroë, and Pyrosoma exhibit one broad band like that given by monochromatic light, that of Lampyris and Luciola is polychromatic. (Amer. Nat., vii, 1873, p. 314.)
The filtered rays of Lampyris pass (like Röntgen and uranium rays) through aluminium (Muraoka).
The physiology of insect phosphorescence is thus briefly stated by Lang: “The cells of this luminous organ secrete, under the control of the nervous system, a substance which is burnt during the appearance of the light; this combustion takes place by means of the oxygen conveyed to the cells of the luminous body by the tracheæ, which branch profusely in it and break up into capillaries.”
Emery states that the males of Luciola display their light in two ways. When at night time they are active or flying, the light is given out at short and regular intervals, causing the well-known sparkling or scintillating light. If we catch a flying Luciola or pull apart one resting in the day time, or cut off its hind body, it gives out a tolerably strong light, though not nearly reaching the intensity of the light waves of the sparkling light. In this case the light is constant, yet we notice, especially in the wounded insect, that the phosphorescent plate in its whole extent is not luminous, but glows at different places as if phosphorescent clouds passed over it.
It is self-evident that a microscopic observation of the light of the glow-worm or firefly is not possible, but an animal while giving out its light, or a separated abdomen, may readily be placed under the microscope and observed under tolerably high powers. By making the experiment in a rather dark room Emery saw clear shining rings on a dark background. “All the rings are not equally lighted. Comparing this with the results of anatomical investigation, it is seen that the rings of light correspond with the previously described circular tracheal capillaries, i.e. the limits between the tracheal-cell cylinder and the parenchym-cells. The parenchym-cells are never stained of a deep brown; this proves that its plasma may be the seat of the light-producing oxidation. Hence this process of oxidation takes place in the upper surface of the parenchym-cells, but outside of their own substance. The parenchym-cells in reality secrete the luminous matter; this is taken up by the tracheal end-cells and burnt or oxidized by means of the oxygen present in the tracheal capillaries. Such a combustion can only take place when the chitinous membrane of the tracheæ is extraordinarily fine and easily penetrable, as is the case in the capillaries of the photogenic plate; therefore the plasma of the tracheal cells only oxidizes at the forking of the terminal tracheal twigs and in the capillaries.” (Emery.)
The color of the light of Luciola is identical in the two sexes, and the intensity is much the same, though that of the female is more restricted. The rhythm of the flashes of light given out by the male is more rapid, and the flashes briefer, while those of the female are longer, more tremulous, and appear at longer intervals.
Emery then asks: What is the use of this luminosity? Is it only to allure the females of Luciola, which are so much rarer than the males? Contrary to the general view that it is an alluring act, he thinks that phosphorescence is a means of defence, or a warning or danger-signal against insectivorous nocturnal animals. If we dissect or crush a Luciola, it gives out a disagreeable cabbage-like smell, and perhaps this is sufficient to render it inedible to bats or other nocturnal animals. An acrid taste they certainly do not possess.
It has long been known that the eggs of fireflies, both Lampyridæ and Pyrophorus, are luminous. Both Newport and more recently Wielowiejski attributes the luminosity not to the contents of the egg, but to the portions of the fat-body cells or fluid covering on the outside of the eggs, due to ruptures of the parts within the body of the female during oviposition. The larvæ at different ages are also luminous.
The position of the luminous organs changes with age. In the larvæ of Pyrophorus before moulting, according to Dubois, the luminous organs are situated only on the ventral side of the head and prothoracic segment. In larvæ of the second stage there are added three shining spots on each of the first eight abdominal segments, and a single luminous spot on the last segment. These spots are arranged in a linear series and thus form three luminous cords. In the adult beetles there is a luminous spot in the middle of the first abdominal sternite, but the greatest amount of light is produced by the two vesicles on the hinder part of the prothorax, the position of which varies according to the species.