A large salivary gland and reservoir lie on each side of the œsophagus and crop. The gland is a thin foliaceous mass about  1/3 in. long, and composed of numerous acini, which are grouped into two principal lobes. The efferent ducts form a trunk, which receives a branch from a small accessory lobe, and then unites with its fellow. The common glandular duct thus formed opens into the much larger common receptacular duct, formed by the union of paired outlets from the salivary reservoirs. The common salivary duct opens beneath the lingua. Each salivary reservoir is an oval sac with transparent walls, and about half as long again as the gland. The ducts and reservoirs have a chitinous lining, and the ducts exhibit a transverse marking like that of a tracheal tube. When examined with high powers the wall of the salivary gland shows a network of protoplasm with large scattered nuclei, resting upon a structureless chitinous membrane.

The salivary glands are unusually large in most Orthoptera.129 In other orders they are of variable occurrence and of very unequal development.

The Cæcal Tubes.

There are eight (sometimes fewer) cæcal tubes arranged in a ring round the fore end of the chylific stomach; they vary in length, the longer ones, which are about equal to the length of the stomach itself, usually alternating with shorter ones, though irregularities of arrangement are common. The tubes are diverticula of the stomach and lined by a similar epithelium. In the living animal they are sometimes filled with a whitish granular fluid.

Similar cæcal tubes, sometimes very numerous and densely clustered, are attached to the stomach in many Crustacea and Arachnida. The researches of Hoppe Seyler, Krukenberg, Plateau, and others have established the digestive properties of the fluid secreted in them, which agrees with the pancreatic juice of Vertebrates.

The Malpighian Tubules.

The Malpighian tubules mark the beginning of the small intestine, to which they properly belong. They are very numerous (60–70) in the Cockroach, as in Locusts, Earwigs, and Dragon-flies; and unbranched, as in most Insects. They are about ·8 inch in length, and ·002 inch in transverse diameter, so that they are barely visible to the naked eye as single threads. In larvæ about one-fifth of an inch long, Schindler130 found only eight long tubules, the usual number in Thysanura, Anoplura, and Termes; but the grouping into six masses, so plainly seen in the adult, throws some doubt upon this observation. In the adult Cockroach the long threads wind about the abdominal cavity and its contained viscera.

In the wall of a Malpighian tubule there may be distinguished (1) a connective tissue layer, with fine fibres and nuclei; within this, (2) a basement-membrane, between which and the connective tissue layer runs a delicate, unbranched tracheal tube; (3) an epithelium of relatively large, nucleated cells, in a single layer, nearly filling the tube, and leaving only a narrow, irregular central canal. Transverse sections show from four to ten of these cells at once. The tubules appear transparent or yellow-white, according as they are empty or full; sometimes they are beaded or varicose; in other cases, one half is coloured and the other clear. The opaque contents consist partly of crystals, which usually occur singly in the epithelial cells, or heaped up in the central canal. Occasionally, they form spherical concretions with a radiate arrangement. They contain uric acid, and probably consist of urate of soda.131 In the living Insect the tubules remove urates from the blood which bathes the viscera; the salts are condensed and crystallised in the epithelial cells, by whose dehiscence they pass into the central canals of the tubules, and thence into the intestine.

The Malpighian tubules develop as diverticula from the proctodæum, which is an invagination of the outer integument and its morphological equivalent. They are, therefore, similar in origin to urinary organs opening upon the surface of the body and developed as invaginations of the integument, like the “shell-glands” of lower Crustacea, and the “green glands” of Decapod Crustacea. The segmental organs of Peripatus, Annelids, and Vertebrates do not appear to be possible equivalents of the excretory organs of Arthropods. They arise, not as involutions, but as solid masses of mesoblastic tissue, or as channels constricted off from the peritoneal cavity, and their ducts have only a secondary connection with the outside of the body or with the alimentary canal.

Digestion of Insects.

The investigation of the digestive processes in Insects is work of extreme difficulty, and it is not surprising that much yet remains to be discovered. Plateau has, however, succeeded in solving some of the more important questions, which, before his time, had been dealt with in an incomplete or otherwise unsatisfactory way. The experiments of Basch, though now superseded by Plateau’s more trustworthy results, deserve notice as first attempts to investigate the properties of the digestive fluids of Insects.

Basch set out with a conviction that where a chitinous lining is present, the epithelium of the alimentary canal secretes chitin only, and that proper digestive juices are only elaborated in the chylific stomach, or in the salivary glands. The tests applied by him seemed to show that the saliva, as well as the contents of the œsophagus and crop, had an acid reaction, while the contents of the chylific stomach were neutral at the beginning of the tube and alkaline further down. From this he concluded that the supposed deep-seated glands of the chylific stomach secreted an alkaline fluid, which neutralised the acidity of the saliva. Finding that the epithelial cells of the stomach were often loaded with oil-drops, he concluded that absorption, at least of fats, takes place here. The chylific stomach, carefully emptied of its contents, was found to convert starch into sugar at ordinary temperatures. The saliva of the Cockroach gave a similar result, and when a weak solution of hydrochloric acid was added, Basch thought that the mixture could digest blood-fibrin at ordinary temperatures.

Plateau’s researches upon Periplaneta americana,132 modified by subsequent experiments upon P. orientalis,133 and by still more recent observations, lead him to the following conclusions134:—

1.—The saliva of the Cockroach changes starch into glucose; but the saliva is not acid, it is either neutral (P. orientalis) or alkaline (P. americana). Any decided acidity found in the crop is due to the ingestion of acid food; but a very faint acidity may occur, which results from the presence in the crop of a fluid secreted by the cæcal diverticula of the mesenteron.

2.—The glucose thus formed is absorbed in the crop, and no more is formed in the succeeding parts of the digestive tube.

3.—The function of the gizzard is that of a grating or strainer. It has no power of trituration. If the animal consumes vegetable food rich in cellulose, a substance not capable of digestion in the crop, the fragments are found unaltered as to form and size in the mesenteron. If it is supplied with plenty of farinaceous food, such as meal or flour, the saliva is not adequate to the complete solution and transformation of the starch, and the intestine is found full of uninjured starch granules, which must have traversed the gizzard without crushing.

4.—The cæcal diverticula secrete a feebly acid fluid. To demonstrate its acidity an extremely sensitive litmus solution, capable of indicating one part in twenty thousand of hydrochloric acid, must be used. The fluid secreted by the cæca emulsifies fats, and converts albuminoids into peptones.

In all Insects digestion is effected in the following way (which is particularly easy of demonstration in Carabus and Dytiscus). The crop is filled with food coarsely divided by the mandibles, and the gizzard being shut to prevent further passage, the fluid secretion of the cæca ascends to the crop, and there acts upon the food. Digestion is effected in the crop, and not beyond it. This is clear beyond doubt. In Decapod Crustacea also it is very easy to prove that the fluid secreted by the so-called liver ascends into the stomach (which corresponds to the crop, together with the gizzard of the Insect). To satisfy ourselves on this point we have only to open a Crayfish during active digestion.

When digestion in the crop is finished, the gizzard relaxes, and the contents of the crop, now in a semi-fluid condition, pass into the mesenteron, which is devoid of chitinous lining, and particularly fitted for absorption.

5.—There are no absorbent vessels properly so called, and Plateau has long thought that the products of digestion pass by osmosis directly through the walls of the digestive tube, to mix with the blood in the perivisceral space. If we may rely upon what is now known of the process in Vertebrates, we should be led to modify this explanation. It is very likely that in Insects, as in Vertebrates, absorption is effected by the protoplasm of the epithelial cells, which select and appropriate certain substances formed out of the dissolved food. Not only do the epithelial cells transmit to the neighbouring blood-currents the materials which they have previously absorbed, but they subject certain kinds to further elaboration. The protoplasm of the epithelial cells of Vertebrates is capable of forming fat. Thus, a mixture of soap and glycerine, injected into the intestine of a Vertebrate, is absorbed by the lacteals in the form of oil-drops. Modern physiologists allow, too, that part of the peptone is similarly changed into albumen, without transport to a distance, by the activity of the epithelial lining.

These facts explain why Plateau was unable to isolate the secretion of the epithelium of the chylific stomach of Insects. The cells are not secretory, but absorbent; and the secretion vainly sought for does not actually exist.

CHAPTER VIII.

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.

Heart of the Cockroach.

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.

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 observation141 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.

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.

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

Cornelius144 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.

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.

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

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 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.

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.

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.

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

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

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 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.

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. 87C 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. 87D, 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.