Fig. 412 represents a longitudinal section through a secondary tracheal branch, showing the origin of the chitinous bands, or tænidia. At t′ are pieces of six tænidia which have been moulted; ectr indicates the nuclei forming the outer cellular layer, the ectotrachea or peritoneal membrane. These nuclei send long slender prolongations around the inside of the peritoneal membrane; these prolongations, as may be seen by the figure, become the tænidia. The tænidia, being closely approximate, grow together more or less, and a thin endotracheal membrane is thus produced, of which the tænidia are the thickened band-like portions. The endotracheal membrane is thus derived from the ectotrachea, or primitive tracheal membrane, and the so-called “spiral thread” is formed by thickenings of the nuclei composing the secondary layer of nuclei, and which become filled with the chitin secreted by these elongated nuclei. The middle portion of the tænidia, immediately after the moult, is clear and transparent, with obscure minute granules, while the nuclear base of the cell is filled as usual with abundant granules, and contains a distinct nucleolus.

The origin of the tænidia is also well shown by Fig. 413, which is likewise a longitudinal section of a trachea at the point of origin of a branch. The peritracheal membrane or ectotrachea (ectr) is composed of large granulated nuclei; and within are the more transparent endotracheal cells; at t′ are fragments of the moulted tænidia. The new tænidia are in process of development at t; at base they are seen to be granulated nuclei, with often a distinct nucleolus, each sending a long, slender, transparent, pointed process along the inside of the trachea. These unite to form the chitinous bands or spiral threads.

Internal hair-like bodies.—In the large tracheæ of Lampyris very fine chitinous bristles project free into the cavity of the tube (Gerstaecher), while according to Leydig there are similar chitinous points in the tracheæ of the Carabid beetle Procrustes. Dugardin had previously (1849) called attention to such hairs, giving a list of the insects in which he observed them. Emery figures a section of the tracheæ of Luciola, “in wendig behaart.”^[69] Stokes has described those of Zaitha fluminea (Fig. 414) as “internal chitinous, hair-like bodies arising from the fold of the tænidia and projecting into the lumen of the tubes.” They are hollow, their minute cavity distinctly communicating with that of the tænidium, from which they arise by an enlarged base. They end in an exceedingly fine point which is occasionally bifid or trifid. In Fig. 414, 4, several are shown attached to the wrinkles of the tracheæ near a spiracle, and at 5 is represented a transverse section of a trachea with three hairs projecting into its cavity.^[70]

Stokes has also described “certain minute, elliptical bodies in the tænidia, each with an internal, presumably glandular, appendage, to all appearance forming part of the tænidium from which it springs.” These are shown in Fig. 414, at 1, 3, and, more in detail, at 6; those at 7, whose thickness is about 1
8000 of an inch, appear as collections of exceedingly minute, rounded apertures in a cushion-like mass. Although not commonly occurring on the tracheal membrane between the tænidia, they may be found there, as at 4.

f. The mechanism of respiration and the respiratory movements of insects

By holding a locust in the hand one may observe the ordinary mode of breathing in insects. During this act the portion of the side of the body between the stigmata and the pleurum contracts and expands; the contraction of this region causes the spiracles to open. The general movement is caused by the sternal moving much more decidedly than the tergal portion of the abdomen. When the pleural portion of the abdomen is forced out, the soft pleural membranous region under the fore and hind wings contracts, as does the tympanum, or ear, and the membranous portions at the base of the hind legs. When the tergum or dorsal portion of the abdomen falls, and the pleurum contracts, the spiracles open; their opening is nearly but not always exactly coördinated with the contractions of the pleurum, but as a rule they are. There were 65 contractions in a minute in a locust which had been held between the fingers about ten minutes. It was noticed that when the abdomen expanded, the air-sacs in the first abdominal ring contracted.

For expanding the abdomen no special muscles are required, since it expands by the elasticity of the parts. For contracting its walls there are two sets of muscles, viz., special vertical expiratory muscles serving to compress or flatten the abdomen (Figs. 415–418), and other muscles which draw together or telescope the segments.

It was formerly supposed that when the abdomen contracted the air was expelled from the body and the tracheæ emptied; that, when the abdomen again expanded by its own elasticity, the air-tubes were refilled, and that no other mechanism was needed. But Landois insisted that this was not enough; as Miall and Denny state: “Air must be forced into the furthest recesses of the tracheal system, where the exchange of oxygen and carbonic acid is effected more readily than in tubes lined by a dense intima. But in these fine and intricate passages the resistance to the passage of air is considerable, and the renewal of the air could, to all appearance, hardly be effected at all if the inlets remained open. Landois accordingly searched for some means of closing the outlets, and found an elastic ring or spiral, which surrounds the tracheal tube within the spiracle.” By means of the occlusor muscle this ring compresses the tube, “like a spring clip upon a flexible gas-pipe.” “When the muscle contracts, the passage is closed, and the abdominal muscles can then, it is supposed, bring any needful pressure to bear upon the tracheal tubes, much in the same way as with ourselves, when we close the mouth and nostrils, and then, by forcible contraction of the diaphragm and abdominal walls, distend the cheeks or pharynx.”

Thus an important point in the respiration of tracheate animals, whether insects, myriopods, or arachnids, is, as Landois claimed, the closure of the spiracles, in order that pressure may be brought upon the air in the tubes, so that it may pass onward into the finest terminations.

The injection of air by muscular pressure into a system of very fine tubes may, as Miall and Denny remark, appear extremely difficult or even impossible. Graham (Researches, p. 44) applies the law of diffusion of gases to explain the respiration of insects, but until physical experiments have been made, we may, with Miall and Denny, “be satisfied that an appreciable quantity of air may be made by muscular pressure to flow along even the finer air-passages of an insect.”

As to the respiratory movements of insects, Plateau is the principal authority, and the following account of the process is taken from his elaborate memoir, and from the statements afterwards contributed by him to Miall and Denny’s “The Cockroach.”

Although many observers have superficially described the respiratory movements of various insects, Rathke was the first one to state precise views as to the mechanism of respiration. His posthumous work, treating of the respiratory movements of the movable chitinous plates of the abdomen, and of the respiratory muscles characteristic of all the principal groups, filled an important blank in our knowledge. But, notwithstanding the skill displayed in this research, many questions still remain unanswered which require more exact methods than mere observations with the naked eye or the simple lens.

Plateau, who was followed a year later by Langendorff, conceived the idea of studying, by such graphic methods as are now familiar, the respiratory movements of perfect insects.

“He has made use of two modes of investigation. The first, or graphic method, in the strict sense of the term, consisted in recording, upon a revolving cylinder of smoked paper, the respiratory movements, transmitted by means of very light levers of Bristol board attached to any part of the insect’s exoskeleton. Unfortunately, this plan is only applicable to insects of more than average size. A second method, that of projection, consisted in introducing the insect, carried upon a small support, into a large magic lantern fitted with a good petroleum lamp. When the amplification does not exceed 12 diameters, a sharp profile may be obtained, upon which the actual displacements may be measured, true to the fraction of a millimetre. Placing a sheet of white paper upon the lantern screen, the outlines of the profile are carefully traced in pencil so as to give two superposed figures, representing the phases of inspiration and expiration respectively. By altering the position of the insect so as to obtain profiles of transverse sections, or of the different parts of the body, and, further, by gluing very small paper slips to parts whose movements are hard to observe, the successive positions of the slips being then drawn, complete information is at last obtained of every detail of the respiratory movements; nothing is lost.”

“This method, similar to that employed by the English physiologist, Hutchinson,^[71] is valuable, because it enables us, with a little practice, to investigate readily the respiratory movements of very small arthropods, such as flies or lady-birds. It has this advantage over all others, that it leaves no room for errors of interpretation.”

“Not satisfied with mere observation by such means as these, of the respiratory movements of insects, the writer has also studied the muscles concerned, and, in common with other physiologists (Faivre, Barlow, Luchsinger, Dönhoff, and Langendorff), has examined the action of the various nervous centres upon the respiratory organs. The result at which he has arrived may be summarized as follows:—

“1. There is no close relation between the character of the respiratory movements of an insect and its systematic position. Respiratory movements are similar only when the arrangement of the abdominal segments, and especially when the disposition of the attached muscles, are almost identical. Thus, for example, the respiratory movements of the cockroach are different from those of other Orthoptera, resembling those of the heteropterous Hemiptera. Those of the Trichoptera are like those of the aculeate Hymenoptera, while the Locustidæ ally themselves in respect to these movements with the Neuroptera and Lepidoptera.

“2. The respiratory movements of insects, when at rest, are localized in the abdomen. As graphically stated by Graber, in insects the chest is placed at the hinder end of the body. If thoracic respiratory movements exist, they do not depend on the action of special muscles.

“3. In most cases the thoracic segments do not share in the respiratory movements of an insect at rest. The respiratory displacements of the posterior segments of the thorax are, however, less rare than Rathke believed. Plateau has observed them in certain Coleoptera (Staphylinus, Chlorophanus, Corymbites), and they are more feebly manifested in Hydrophilus, Carabus, and Tenebrio. Among the singular exceptions to this rule is the cockroach (Periplaneta orientalis), in which the terga of the meso- and metathoracic segments perform movements exactly opposite in direction to those of the abdomen (Fig. 419).

“4. Leaving out of account all details and all exceptions, the respiratory movements of insects may be said to consist of the alternate contraction and recovery of the figure of the abdomen in two dimensions, viz. vertical and transverse. During expiration both diameters are reduced, while during inspiration they revert to their previous amounts. The transverse expiratory contraction is often slight, and may be imperceptible. On the other hand, the vertical expiratory contraction is never absent, and usually marked. In the cockroach (P. orientalis) it amounts to one-eighth of the depth of the abdomen (between segments 2 and 3); in Eristalis tenax to one-ninth (at the 2d segment).

“5. Three principal types of respiratory mechanism occur in insects, and these admit of further subdivision:

“a. Sterna usually short and very convex, yielding but little. Terga mobile, rising and sinking appreciably. To this class belong all Coleoptera, heteropterous Hemiptera, and Blattina (Fig. 420).

“In the cockroach (Periplaneta), the sterna are slightly raised during expiration (Fig. 421).

“b. Terga well developed, overlapping the sterna on the sides of the body, and usually concealing the pleural membrane, which forms a sunken fold. The terga and sterna approach and recede alternately, the sterna being almost always the more mobile. To this type belong Odonata, Diptera, aculeate Hymenoptera, and acrydian Orthoptera (Fig. 422).

“c. The pleural membrane, connecting the terga with the sterna, is well developed and exposed on the sides of the body. The terga and sterna approach and recede alternately, while the pleural zone simultaneously becomes depressed, or returns to its original figure. To this type, Plateau assigns the Locustidæ, Lepidoptera, and the true Neuroptera (excluding Trichoptera) (Fig. 423).

“6. Contrary to the opinion once general, changes in length of the abdomen, involving protrusion of the segments and subsequent retraction, are rare in the normal respiration of insects. Such longitudinal movements extend throughout one entire group only, viz. the aculeate Hymenoptera. Isolated examples occur, however, in other zoölogical groups.

“7. Among insects, such as large beetles, Locustidæ, dragon-flies, etc., sufficiently powerful to give good graphic tracings, it can be shown that the inspiratory movement is slower than the expiratory, and that the latter is often sudden.

“8. In most insects, contrary to what obtains in mammals, only the expiratory movement is active; inspiration is passive, and effected by the elasticity of the body-wall.

“9. Most insects possess expiratory muscles only. Certain Diptera (Calliphora vomitoria and Eristalis tenax) afford the simplest arrangement of the expiratory muscles. In these types, they form a muscular sheet of vertical fibres, connecting the terga with the sterna, and underlying the soft, elastic membrane which unites the hard parts of the somites. One of the most frequent complications arises by the differentiations of this sheet of vertical fibres into distinct muscles, repeated in every segment, and becoming more and more separated as the sterna increase in length. Special inspiratory muscles occur in Hymenoptera, Acridiidæ, and Trichoptera.

“10. The abdominal, respiratory movements of insects are wholly reflex. Like other physiologists who have examined this side of the question, Plateau finds that the respiratory movements persist in a decapitated insect, as also after destruction of the cerebral ganglia or œsophageal connectives; further, that in insects whose nervous system is not highly concentrated (e.g. Acridiidæ and dragon-flies), the respiratory movements persist in the completely detached abdomen; while all external influences which promote an increased respiratory activity in the uninjured animal, have precisely the same action upon insects in which the anterior, nervous centres have been removed, upon the detached abdomen, and even upon isolated sections of the abdomen.

“The view formerly advocated by Faivre, that the metathoracic ganglia play the part of special, respiratory centres, must be entirely abandoned. All carefully performed experiments on the nervous system of Arthropoda have shown that each ganglion of the ventral chain is a motor centre, and, in insects, a respiratory centre, for the somite to which it belongs. This is what Barlow calls the ‘self-sufficiency’ of the ganglia.” (Miall and Denny.)