Chapter XXIII: Part I: Morphology and Physiology (19)

Kupffer is likewise of the opinion that the fine tracheæ penetrate into the cells, and Lidth de Jeude asserts that they enter the epithelial cells, “each cell containing several branches.” Kölliker, Emery, etc., maintain, however, that the tracheal endings lie between the cells. Wielowiejski,[63] in describing the line tracheæ of the phosphorescent organs, thinks that the tracheal endings (tracheal capillaries) rarely end blindly, but anastomose with one another, forming an irregular network. The latest observer, Gilson (1893), asserts that tracheal twigs penetrate deeply into the epithelial cells of the silk glands of larval Trichoptera as well as of caterpillars, passing through their protoplasm.

FIG. 398.—Tracheal network of the male glands of _Lampyris
splendidula_: _tec_, tracheal end-cells; _cap_, tracheal
capillaries; at _a_, an expanded matrix.—After Wielowiejski.
]

FIG. 399.—Tracheal capillary end-network (_tr. c. n._) of silk glands
of _Ocneria dispar_: _p_, peritoneal (peritracheal) membrane.—After
Wistinghausen.
]

A late investigator, C. von Wistinghausen, finds in the tracheæ of
the spinning-glands of caterpillars a completely formed network
between the terminal branches of two or several tracheal groups. The
tracheal tubes of this series of terminal branches pass into this
network, which he calls the tracheal capillary end-network (Figs.
398, 400). This last varies in thickness and spreads out under the
membrana propria of the glandular mass over the entire surface of
the large gland-cells and on a level with the tracheal capillaries.
The tracheal endings do not penetrate into the cells, but are
separated from the plasma of the cells by a thin membrane. The
tracheal capillary end-network appears as a system of fine tubes
like the tracheal capillaries, consisting of a peritoneal layer and
a chitinous intima (Fig. 400). The walls of these tubes are
homogeneous, not porous, though readily permeable by the
parenchymatous fluid. The interchange of gases consequently may go
on easier and more vigorously in a system of richly anastomosing
tubules of the net-like mass of tracheal capillaries, than in tubes
ending blindly.

While the diameter of the tracheal capillaries is 0.0016 mm. or 1 µ,
that of the tubules composing the tracheal capillary end-network is
scarcely measurable, but is less than 1 µ.

FIG. 400.—Tracheal end-cells of _Lampyris splendidula_: _tr_,
trachea with tænidia; _tre_, tracheal capillaries.—After
Wielowiejski.
]

These tracheal capillaries also occur on the seminal and other
sexual tubes, on the intestine, on the urinary tubes, on the
fat-bodies, but are most easily detected on the silk-glands.

The latest researches are those of E. Holmgren, who has studied the
branching of the tracheæ in the spinning-glands of caterpillars. He
prefers to call the end-cells “transition cells,” as they lead from
the tracheal tubes proper to the capillary network. This latter is
formed by slender nucleated cells, often with an intracellular
lumen, and, according to the author, probably constituting a
respiratory epithelium. He finds that both large and small tracheæ
may penetrate the gland-cells. (Anat. Anzeiger, xi, 1895, pp. 340–6,
3 figs.; Jour. Roy. Micr. Soc., 1896, p. 182.)

_b._ The spiracles or stigmata

The spiracles are segmentally arranged openings in the sides of the thorax and abdomen, through which the air passes into the air-tubes. In its essential structure a spiracle, or _stigma_, is a slit-like opening surrounded by a chitinous ring, the lips or edges of the opening being membranous and closed by a movable valve of the spiracle attached by its lower edge, which is closed by an occlusor muscle (Fig. 401). The aperture when open forms a narrow oval slit; and in most insects the slit is within guarded by a row of projecting spines or setæ, which form a lattice work or grate to keep out dust, dirt, fluids, etc.

FIG. 401.—Horizontal section of left third stigma and trachea of
_Melolontha vulgaris_, showing the chamber or drum leading into the
trachea: _a_, _a_, external frame or valve protecting the outer
opening of the stigma; _b_, _c_, _c_, inner frame closing the
entrance into the trachea (_l_, _k_); _m_, occlusor muscle closing
the inner orifice.—After Straus-Dürckheim.
]

Krancher[64] has described five leading types of stigmata, not,
however, taking into account those of the Synaptera.

I. _Stigmata without lips_ (Primitive or generalized stigmata).

_a._ The simplest stigma is an aperture which is kept open by a
chitinous ring (Acanthia). The opening may be round or elliptical.
There are no lips nor any movement of the edges to be observed. Such
air-holes occur in the abdomen of bugs (Hemiptera) and beetles
(Coleoptera); within the opening of the stigmata in the same insects
is a funnel-like contraction. Also in the Diptera the abdominal
stigmata are of the same type.[65] The stigmata of the Pulicidae
(Siphonaptera) are more complicated, as the edges of the openings
are provided with setæ (Fig. 402).

FIG. 402.—First abdominal spiracle with a part of the trachea of the
cat-flea: _sp_, spiracle; _t_, trachea.
]

FIG. 403.—Stigma of Melolontha larva, seen from without: _b_, bulla;
_s_, sieve-like plate; _o_, curved slit-like opening.—After Boas.
]

_b._ The stigma consists of a series of minute single stigmata,
which are usually surmounted by a common chitinous ring, and whose
tubular continuations unite within in a common trachea, so that the
single tubes pass off from the stigma like the fingers on the hand.
This form is found in the larvæ and puparia of Diptera.

II. _Stigmata with lips_ (Secondary more specialized stigmata).

_c._ The lips are represented by a single chitinous ring, with
sparse spines. One side of the stigma is a little higher, and partly
overlaps the other posteriorly; this form is peculiar to the
Orthoptera and Libellulidae.

_d._ The lips are roof-like, bent inwards and densely hairy, forming
a peculiar kind of felting. The setæ of the lips are in most beetles
and many Lepidoptera separate, and more or less branched. In
caterpillars, the setæ are so finely branched as to form a loose
felt, or sieve-like arrangement.

_e._ The stigmata are round, with a very broad border and a
concentric middle portion, the structure being complicated. The
concentric middle portion is pouch-like and bears the occlusor
muscle. This form occurs in the larvæ of lamellicorn beetles, and
can be seen with the naked eye, or with a lens, in Oryctes, Cetonia,
and Melolontha (Fig. 403).

_f._ Over the outer opening of the spiracle is an incurved chitinous
projection, on one side of which the trachea takes its origin. It is
thus in the Hymenoptera.

The remarkable grate-like stigma of the lamellicorn larvæ has the
appearance as if the outer closing plate or valve were impenetrable.
The earlier observers considered these stigmata to be open, but
Meinert regards them as closed; Schiödte, however, has observed by
pressing a preserved specimen of a Melolontha larva the alcohol
within passing out in drops, through the grate-like plate, and hence
he considers this a proof that the stigma is permeable (Kolbe).

More recently (1893) Boas has examined the same structure in the
same species of larva as examined by Schiödte, and he finds it to be
open only during the process of moulting. He finds that on each side
of the larva there are nine short and wide stigmatic branches, each
of which is shut off from the exterior by a brown plate; this
consists of a reniform sieve-plate, and of a curved bulla which fits
into the cavity of the plate. The stigmatic branch, however, is
provided with a large external opening, which is homologous with the
stigma, but which is usually closed by the plate and bulla, and is
only open during the moulting; at first it is circular, but later
becomes a cleft. A transverse section shows that the bulla is a
simple tegumentary fold, the outer chitinous layer of which has
become especially firm. The plate forms a horizontal half-roof,
which springs from one side of the tracheal orifice, and is
supported by obliquely set bases, which spring from the adjoining
part of the inner side of the tracheæ. The plate and bars are purely
cuticular structures. (Zool. Anz., 1893; also Journ. Roy. Micr.
Soc., p. 54.)

The tracheal system of libellulid nymphs is not closed; on the other
hand, in the fully-grown nymphs the anterior stigmata occurring on
the dorsal side are large, and the tracheæ arising from them are
thick. These stigmata are permeable by the air. In half-grown and
still younger stages of Æschna the two anterior thoracic stigmata
are undeveloped. In order to breathe, the fully-grown nymph either
rises up on the upper side and elevates the end of the body to the
surface in order to take the air into the rectum, or it rests with
the back of the thorax at the surface in order to breathe through
the large stigmata. The young nymphs take in air only through the
rectum. The young nymphs of Libellula and its allies, on the other
hand, possess large thoracic stigmata, but they prefer to breathe
through the rectum. The fully-grown nymphs of Agrion breathe through
the thoracic stigmata. (Dewitz, in Kolbe.)

=The position and number of pairs of stigmata.=—The spiracles are usually situated in the soft membrane between the tergites and pleurites, but their exact position varies in different groups. In the Coleoptera they occupy on the thorax a more ventral position, and on the abdomen are placed near the edge of the dorsal side, under the elytra. In the dragon-flies, the first pair is situated much more dorsally than the second and third pairs; the following seven pairs are almost wholly ventral and lie concealed in the membranous fold near the external plate. In the Hemiptera, also, the abdominal stigmata, though entirely free and visible, are situated ventrally.

Primarily, in the embryo a pair of stigmata appear on each segment of the thorax and abdomen, except the 10th and 11th, and even possibly in the head, for a pair of stigmata are said to occur in the head of Podurids (Smynthurus) (Lubbock), though this statement needs confirmation. Scolopendrella, however, is known to possess a pair of cephalic spiracles.

From the foregoing statement it will be seen that while in existing winged insects no more than 10 (in Japyx 11) pairs of stigmata are to be found in any one species, yet that 12 segments of the body, in different groups taken collectively, bear them. The primitive number of pairs of spiracles, therefore, in winged insects, was 12, _i.e._ a pair in each thoracic segment, and a pair in each of the first nine abdominal segments. Insects were originally all holopneustic, and gradually as the type became differentiated into the different orders they became peripneustic or amphipneustic, and, in certain aquatic forms, apneustic. (See pp. 459, 461.)

In the still more primitive, probably wingless, ancestors of insects there was a larger number of stigmata. Hatschek, in 1877, discovered a pair of tracheal invaginations in each of the three posterior head-segments of the embryo of a moth, with stigmatal openings in the 1st and 2d maxillary segments.

Thus early in embryonic life every segment of the body, except those bearing the eyes and the last abdominal, bore a pair of stigmata, so that the primitive insect had at least 15, and perhaps more, pairs of stigmata.

The position of the stigmata is subject to much variation, the result of adaptation to this or that mode of life. Examples are those insects which live in dusty situations or usually more or less concealed in the earth, as in most beetles, and in the Hymenoptera. In such beetles, the stigmata are situated in the thin membrane between the segments; in the Hymenoptera, on the upper edge of the segments. In the Siphonaptera, Pediculina, bed-bug, and similar forms, which breathe an air freer from dust, the spiracles lie free on the outside of the body.

“When the stigmata are free and without any protection on the
abdomen, there are other ways by which the entrance of foreign
bodies into the tracheæ is prevented. In such cases the body is
covered with dense hairs, as in most Diptera and Neuroptera, as well
as many Lepidoptera; or there is situated in front of the stigma
either a small fissure which is covered over by a number of hairs
arising from the edge, as in many Orthoptera; or, as in most
insects, a luxurious growth of hairs on the inside of the stigma
forms a thick filter for the air. Thus we see that also in this
respect each species of insect is completely adapted to its
surroundings.” (Krancher.)

FIG. 404.—_A_, thoracic stigma of the house-fly: _Sb_, valve which
closes the opening.
]

FIG. 405.—Diagrammatic figures of the internal apparatus which closes
the trachea, in the stag-beetle: _A_, trachea open; in _B_, closed;
_St_, the stigma, with its grated lips; _Ct_, cuticula of the
body-walls; _Vk_, closing pouch; _Vbü_, closing bow; _Vba_, closing
band; _M_, occlusor muscle.—From Judeich and Nitsche.
]

=The closing apparatus of the stigma.=—Whether the external opening of the stigma is permanently open or closed, communication with the tracheæ may be cut off at pleasure during respiration by an internal apparatus of elastic chitinous bands and rods and the occlusor muscle.

The parts concerned in this operation are: 1. The closing bow; 2. The closing lever or peg; 3. The closing band; 4. The occlusor muscle (Figs. 405, 406).

FIG. 406.—Stigma, with the closing apparatus, of _Smerinthus populi_
(imago), seen from within: _b_, closing bow; _c_, closing band: _o_,
stigmatic opening; _r_, external chitinous ring; _l_, closing lever;
_m_, occlusor muscle; _s_, scales which lie like roofing tiles over
the stigma.—After Krancher.
]

“The first three parts are chitinized; they form a ring around the stigmatic opening, and are united to each other by joints. The bow is usually crescentic and as a rule surrounds one-half of the trachea. On the other side is the closing band which, by different contrivances, representing the closing lever or peg, becomes closely pressed against the closing bow. This lever is usually of the shape of a slender chitinous rod, which causes the closure; but it can also bend rectangularly, become converted into a typical lever as in the Lepidoptera, or it may assume the form of two peg-like processes, which press with their base against the closing bow.” (Krancher.)

“The closure of the spiracular opening is effected by the contraction of the muscles, while the opening is due to the elasticity of the chitinous parts. When at rest the spiracle is naturally open, so that the air in the trachea can directly communicate with the external air. Usually one end of the muscle is attached to the closing peg, and the other end to the closing bow. Where, as in Melolontha, the closing apparatus is provided with two levers, then naturally the muscle binds these two together and brings about by powerful contractions a firm closure of the trachea”; but, remarks Krancher, “this is not the only kind; there are numerous modifications. Besides the form just described, the levers assume the form of valves (Sirex), or of a brush (Pulex); or of a ring (larvæ of Diptera) with a circular muscle attached to it; or of a ring which simply becomes compressed (thoracic stigmata of Diptera).”

_c._ Morphology and homologies of the tracheal system

As first shown by Bütschli, the tracheal system is a series of segmentally arranged tubular invaginations of the ectoderm; a pair of stigmata primitively occurring on every segment of the body except perhaps the most anterior, and the last two or last one, a reduction in their number having since taken place, until in the Podurans none have survived. In the supposed ancestor of myriopods and insects, Peripatus, there are tracheæ; but they are very fine, simple, not-branched chitinous tubes which are united into tufts at the base of a flask-shaped depression of the integument, the outer aperture of which depression is regarded as a stigma. In one species (_P. edwardsii_) these tufts and their openings are scattered irregularly over the body; but in another kind (_P. capensis_) some of the stigmata at least show traces of a serial arrangement, being disposed in longitudinal rows—two on each side, one dorsally and one ventrally, those of each row, however, being more numerous than the pairs of legs. (See p. 9 and Fig. 4, _D_.)

It should be observed that in Peripatus, which does not possess urinary tubes, the segmental organs or nephridia are well developed, hence the tracheal tubes coexisting with them cannot be their homologues. We are therefore compelled to regard the tracheal system as of independent origin, arising in the earliest terrestrial air-breathing arthropod, and not indebted for its origin to any structure found in worms, unless perhaps, as both Kennell and Lang suggest, to dermal glands, since, according to Kennell, certain Hirudinea and many Turbellarian worms possess long, mostly unicellular, glands which spread far through the parenchyma of the body. (Kennell.)

Thus Kennell supposes that the ancestors of the Tracheates had
spiracles on every segment of the body where the internal
organization allowed them to exist. “The reduction of the breathing
holes to a smaller number, and their restriction of a pair only to a
single segment, was brought about partly by adaptation to a peculiar
mode of life,—as insect larvæ especially teach us,—partly also—I may
say mechanically—as a result of the obstruction to their development
made by the growth or excessive development of other organs.” Among
these he reckons the thick, dense cuticula of the integument, the
internal fusion of several segments to form body-regions, and the
arrangement and great development of the muscles in the head and
thorax, etc. (p. 29.)

FIG. 407.—Section through a tracheal pit and diverging bundles of
tracheal tubes taken transversely to the long axis of the body:
_tr_, tracheæ, showing rudimentary spiral fibre; _tr. c_, cells
resembling those lining the tracheal pits, which occur at
intervals along the course of the tracheæ; _tr. o_, tracheal
stigma; _tr. p_, tracheal pit.—After Balfour, from Sedgwick.
]

Kennell has suggested the origin of the tracheæ of Peripatus from
the unicellular dermal glands of annelidan ancestors, since he has
found glands in certain land-leaches of tropical America, which are
provided with enormously long tubular passages united into bundles
and opening externally, these tubes appearing to be slightly
chitinized. Fig. 407 will show the appearance of a bundle of fine
tracheal tubes of Peripatus ending at the bottom of a follicle
formed by a deep invagination of the integument, which may be
regarded as a primitive spiracle. (See Kennell, Ueber einige
Landblutegel des tropical America, Zool. Jahrb. ii, 1886; also Die
Verwandtschaftsverhältnisse der Arthropoden, 1891, p. 25.) We may
add that Carrière supposes from his study of the embryology of the
wall-bee (_Chalicodoma muraria_), published in 1890, that not only
the salivary glands, but also the tentorium, are homologues of the
tracheæ, while other structures than tracheæ may have evolved from
unicellular dermal glands, which are widely distributed. It may in
this connection be observed that some authors derive the book-lungs
or book-leaf tracheæ of Arachnida from the gills of Limulus; hence
if those of Arachnida arose from quite different and more
specialized organs than dermal glands, it is not impossible that the
tracheæ of Peripatus, Myriopods, and insects arose _de novo_, and
then we need not look for any primitive structures in worms from
which they arose.

Although Bütschli in 1870 in his embryology of the honey-bee called
attention to the “great similarity which the eleven pairs of
invaginations in the eleven first trunk-segments in their first
indication (_anlage_) have with the spinning-glands, and also with
the segmental organs of Annelids,” he did not go further than this,
and it is now known that in the 2d maxillary segment open not only
spinning-glands, but in the embryo a pair of stigmata.

Paul Mayer, however, regarded the tracheæ and urinary tubes as
homodynamous structures, and this view was advocated by Grassi
(1885) for the reason that while in the embryo honey-bee there are
ten pairs of stigmata, the first thoracic and two last abdominal
segments wanting them, the germs of the urinary tubes arise in a
corresponding situation on the two last abdominal segments. To this
view Emery (Biol. Centralb., 1886, p. 692) objects that in Peripatus
the nephridia and tracheæ “have nothing to do with the segmental
organs,” as Peripatus besides nephridia possesses both coxal glands
and tracheæ.

Both Kennell and Lang derive the coxal glands of Arthropoda from the
setiparous or parapodial glands of annelid worms, and the recent
endeavor of Bernard to show that the tracheæ arose from setiparous
glands seems to be disproved by the fact that in insects as well as
in other Arthropoda coxal glands with their outlets exist in the
same segments as those bearing stigmata. Reasoning by exclusion, we
are led to regard Kennell’s original view as the soundest.

Patten, however, regards the tracheæ as modified ends of nephridia,
remarking: “Since in Acilius some of the abdominal tracheæ at first
communicate with the cavities of the mesoblastic somites, it is
probable that all the tracheæ represent the ectodermic portions of
the nephridia.” (Origin of Vertebrates from Arachnids, p. 355.)

It is probable, therefore, that the tracheæ first arose as modifications of dermal glands, as in mites and Peripatus, and that at first they were not provided with tænidia (as in Chilopoda), while in later forms tænidia were developed. In the earliest tracheate forms the stigmata were not segmentally arranged, probably appearing irregularly anywhere in the body, but afterwards in the myriopods and insects became serially arranged.

_d._ The spiral threads or tænidia

It is generally supposed that the so-called “spiral thread” forms a continuous thread from one end of a tracheal branch to the other. This was first shown not to be the case by Platner in 1844. Minot has proved that “there is not a single spiral thread, but several, which run parallel to one another and end after making a few turns around the trachea.”

The tænidia we have found to be in some cases separate, independent, solid rings, though when there is more than one turn the thread necessarily becomes spiral. The tænidia of a main branch stop at the origin of the smaller branches, and a new set begins at the origin of each branch. The tænidia at the origin of the branch do not pass entirely around the inside of the peritoneal membrane; in the axils they are short, separate, spindle-shaped bands (Fig. 409).

At one point in the main trachea of the larva of Datana the tænidia
were seen to end singly on one side (at a considerable distance from
any branch or axil) at intervals, with a tænidium situated between
them, making four or five turns; then there is only one band
situated between two ends; this band or thread is succeeded by a set
with five turns between the two ends, this set being succeeded by
one complete ring situated between two ends; in all cases the ends
vary in length, some threads being short and others long, so that
they apparently end anywhere along the circumference of the trachea,
and this arrangement is seen to apparently extend along the whole
length of the trachea. Hence it is seen that as a rule the tænidia
vary much in length, and never, as generally supposed, pass
continuously from one end to another of a tracheal branch, for there
are many spirals in a branch, each making only from one to five
turns, most usually four turns. Fig. 408, part of a trachea of
_Dyticus marginatus_, shows that at a slight bend in a trachea the
tænidia is interrupted, and short, incomplete, wedge-shaped tænidia
(_e_) are interpolated; at _A_, _d_ is seen a split in one of the
tænidia (compare also MacLeod, Pl. 1, Fig. 9). The threads are quite
irregular in width. In the axils of the branches there is, as seen
in Fig. 409, a basketwork of independent, short, often
spindle-shaped tænidia; these are succeeded by longer ones, until we
have threads passing entirely around near the base of each new
branch; these being succeeded by others which make from two to five
spiral turns.

FIG. 408.—Tænidia of Dyticus: _d_, a split tænidium; _e_, _e_, ends of
tænidia.
]

The shape of the tænidia appears to vary to a great extent. In lepidopterous insects we have observed them to be in their general shape rather flat and slightly concavo-convex, the hollow looking towards the centre of the trachea. Minot’s section (Fig. 393) shows that in Hydrophilus they are cylindrical and solid, and Chun states that those of Stratiomys are round, while in Eristalis they are round, with a ridge projecting into the cavity of the trachea; in Æschna the thread is quadrangular. MacLeod states that sometimes it is cylindrical, in other cases flat, likewise prismatic; Macloskie believes that the spiral threads of the centipede are “fine tubules, externally opening by a fissure along their course.”

FIG. 409.—Tænidia of Dyticus in an axil of two branches: _e_, _e_,
ends of tænidia.
]

Stokes confirms Macloskie’s statements, stating that in the hemipterous _Zaitha fluminea_ “the tænidia are fissured tubules formed within and from chitinized folds of the intima, the convexity of the folds looking towards the lumen of the tracheæ.” In Fig. 414, 1, are represented portions of several tænidia showing the fissure, which is sometimes interrupted; at 2 are seen “the formation of what may be called apertures in a chitinous bridge.” Stokes regards the tænidia as “inwardly directed folds of the membrane.” Near the spiracles the tracheal membrane is externally studded with minute papillæ, as shown at 3, where are represented three broad and incomplete tænidia, with the tapering end, or the beginning, of another. Stokes adds, “Here they are only broad grooves, with no appearance of the narrow fissure of the completed tænidium. At 4 is figured a portion of the internal surface of a large trachea near the external orifice, the tænidia being in an incipient stage, evidently forming more or less of a network, as is usually the case next to the stigma” (compare p. 451, and Fig. 414).

FIG. 410.—End of salivary duct in base of proboscis of _Stomoxys
calcitrans_: _a_, incomplete and irregular tænidia; _b_, two tænidia
making incomplete rings near the distal end of the duct.
]

The tracheæ of chilopod myriopods appear to be like those of
insects. A number of authors have failed to detect the spiral
threads in the Julidæ. As to the Arachnida, several observers,
including Menge and Bertkau, have denied the existence of the spiral
thread in the spiders with the exception of the Attidæ; and MacLeod
finds them “scarcely visible” in Argyroneta.

Besides the tracheæ, the salivary duct is kept permanently distended by tænidia, which, however, are not spiral. They usually form incomplete rings, as in Stomoxys, arranged as shown in Fig. 410.

The labella (proboscis) of flies are supported by incomplete chitinous tubes or “pseudo-tracheæ,” the ends of which form the scraping teeth, this being, according to Dimmock, their primary function. Dimmock describes them as cylindrical channels opening on the surface in zigzag slits. These channels are held open by incomplete rings, one end of which is forked. “These rings are apparently arranged so that one has its fork on one side of the opening of the channel, the next ring the fork on the opposite side of the channel, and so on, in alternation. Their true structure is revealed when flattened out.”

FIG. 411.—Abdominal spiracle (left side) of cockroach (_P.
americana_), side view, showing the bow: _p_, lateral pouch of
spiracle (in centre) seen from within. The tessellated structure of
spiracle and trachea shown at _A_, and the margin of the external
aperture at _B_.—After Miall and Denny.
]

The use of the elastic tænidia is to render the tracheæ elastic, and to keep them permanently open, as is the case with the parallel rings of the trachea of the higher vertebrates. The tracheæ are thus rendered firm and solid, at the least expense of chitinous material. The spiral thread, as MacLeod remarks, “is the realization in nature of what engineers call a form of the greatest resistance.”

The tænidia are wanting in the fine endings of the tracheæ (tracheal
capillaries); also in the cockroach, according to Miall and Denny,
they are not developed in the large tracheæ close to the spiracles,
and the intima or wall of the tube has a tessellated instead of a
spiral marking (Fig. 411). The same structure is seen in the Perlidæ
(Nemoura, Gerstaecker, Zeit. f. wissen. Zool. xxiv, Taf. xxiii,
Figs. 5 and 7); also in Æschna (Hagen, Zool. Anz. 1880, p. 159). In
certain fine tracheæ of the eyes of the fly no spiral threads are
developed. (Hickson.) The air-sacs or dilated tracheæ are also
without tænidia.

While in the living insect the main and smaller tracheæ are filled, with air, it is stated by Von Wistinghausen that the fine capillary ends contain a fluid.

_e._ Origin of the tracheæ and of the “spiral thread”

While we owe to Bütschli the discovery of the mode of origin and morphology of the tracheæ, which as he has shown[66] arise by invaginations of the ectoblast; there being originally a single layer of epiblastic cells concerned in the formation of the tracheæ; we are indebted to Weismann[67] for the discovery of the mode of origin of the “intima,” from the epiblastic layer of cells forming the primitive foundation of the tracheal structure.

Weismann did not observe the earliest steps in the process of
formation of the stigma and main trunk of the tracheæ, which
Bütschli afterwards clearly described and figured.

Weismann, however, thus describes the mode of development of the
intima; after describing the cells destined to form the peritoneal
membrane, he says: “The lumen is filled with a clear fluid and
already shows a definite border in a slight thickening of the
cell-wall next to it.

“Very soon this thickening forms a thin, structureless intima, which
passes as a delicate double line along the cells, and shows its
dependence on the cells by a sort of adherence to the rounded sides
of the cells (Taf. vii, 97 _A_, _a b c_). Throughout the mass, as
the intima thickens, the cells lose their independence, their walls
pressing together and coalescing, and soon the considerably enlarged
hollow cylinder of the intima is surrounded by a homogeneous layer
of a tissue, whose origin from cells is recognized only by the
regular position of the rounded nuclei (Taf. vii, Fig. 97, _B_).

“Then as soon as the wavy bands of the intima entirely disappear,
and it forms a straight, cylindrical tube, a fine pale
cross-striation becomes noticeable (vii, 97, _B_, _int_), which
forms the well-known ‘spiral thread,’ a structure which, as Leydig
has shown, possesses no independence, but arises merely from a
partial thickening of the originally homogeneous intima.

“Meyer’s idea that the spiral threads are fissures in the intima
produced by the entrance of air is disproved by the fact that the
spiral threads are present long before the air enters. Hence the
correctness of Leydig’s view, based on the histological structure of
the tracheæ, is confirmed by the embryological development, and the
old idea of three membranes, which both Meyer and Milne-Edwards
maintain, must be given up.”

Weismann also contends that the elastic membrane bearing the “spiral
thread” is in no sense a primary membrane, not corresponding
histologically to a cellular membrane. On the contrary, the
“peritoneal membrane comprises the primary element of the trachea;
it is nowhere absent, but envelops the smallest branches, as well as
the largest trunks, only varying in thickness, which in the embryo
and the young larva of Musca stands in relation to the thickness of
the lumen.”

The trachea, then, consists primarily of an epithelial layer, the “peritoneal membrane,” or the invaginated epiblast; from this layer an intima is secreted, just as the skin or cuticle is secreted by the hypodermis. We may call the peritoneal membrane the _ectotrachea_, the intima or inner layer derived from the ectotrachea the _endotrachea_. The so-called “spiral threads” are a thickening of the endotracheal membrane, sometimes arranged in a spiral manner. For these chitinous bands we have proposed the name _tænidia_ (Greek, little bands).

As to the origin of the spiral thread our observations[68] have been made on the caterpillar of a species of Datana, which was placed in alcohol, just before pupation, when the larva was in a semipupal condition, and the larval skin could be readily stripped off. At this time the ectotrachea of the larva had undergone histolysis, nothing remaining but the moulted endotrachea, represented by the tænidia, which lay loosely within the cavity of the trachea. The ectotrachea or peritoneal membrane of the pupa is meanwhile in process of formation; the nuclear origin of the tænidia is now very apparent.

FIG. 412.—Longitudinal section of a trachea, showing the origin of the
tænidia.
]

FIG. 413.—Origin of the tænidia from nuclei.
]

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.

FIG. 414.—Tænidia and internal hairs of _Zaitha_.—After Stokes.
]

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:—

FIG. 415.—Muscles of right half of the abdomen of _Forficula
auricularia_: _A_, _a_, longitudinal tergal and sternal muscles;
_D_, _E_, oblique muscles; _a_ (in upper figure) vertical
expirator muscles.
]

“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).

FIG. 416.—Muscles of the left half of abdomen of _Staphylinus
olens_; _A_, _B_, longitudinal dorsal muscles; _D_, _E_, oblique
fascia; _a_, longitudinal sternal muscles; _d_, respiratory
muscles (vertical expirators).
]

“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).

FIG. 417.-Muscles of right half of abdomen of _Phryganea striata_,
♀: _A_, _B_, longitudinal dorsal muscles; _a_, _b_, longitudinal
sternal muscles; _D_, _e_, oblique muscles; 1, 2, inspirator
muscles.
]

“_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).

FIG. 418.—Muscles of left half of abdomen of Melolontha, ♀: _A_,
_B_, longitudinal muscles (prétracteurs of Straus); _a_, _a_, true
respiratory muscles (expirators).—This and Figs. 415–417, after
Plateau.
]

“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.

FIG. 419.—Profile of trunk of cockroach (_P. orientalis_). The black
surface represents the expiratory contour, while the inspiratory
is indicated by a thin line. The arrows show the direction of the
expiratory movement: _Ms. th_, mesothorax; _Mt. th_, metathorax.
Reduced from a magic-lantern projection.—After Plateau.
]

“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.)

FIG. 420.—Transverse section of abdomen of a lamellicorn beetle. The
position of the terga and sterna after an inspiration is indicated
by the thick line; the dotted line shows their position after an
expiration; and the arrow marks the direction of the expiratory
movement.
]

FIG. 421.—Cross-section of abdomen of cockroach.
]

FIG. 422.—Cross-section of abdomen of bee (Bombus).
]

FIG. 423.—Cross-section of abdomen of Sphinx.—This and Figs. 420–422
after Plateau.
]

Plateau has made similar observations upon the respiration of
spiders and scorpions; but, to his great surprise, he was unable,
either by direct observation, or by the graphic method, or by
projection, to discover the slightest respiratory movement of the
exterior of the body. This can only be explained by supposing that
inspiration and expiration in pulmonate Arachnida are
“intrapulmonary,” and affect only the proper, respiratory organs.
The fact is less surprising because of the wide zoölogical
separation between Arachnida and insects.

_g._ The air-sacs

In flying insects the tracheæ are in certain parts of the body enlarged into sacs of various sizes. These air-sacs were first observed by Swammerdam in a beetle (Geotrupes) and afterwards by Sir John Hunter in the bee, Sprengel subsequently discovering them in other insects. Those of the cockroach were described and illustrated in a very elaborate and detailed way by Straus-Dürckheim (Figs. 424 and 425). These vesicles are without tænidia. In the locust (_M. femur-rubrum_) there is a pair of very large vesicles in the prothorax (Fig. 396). The five pairs of large abdominal air-sacs arise, independently of the main tracheæ, directly from branches originating from the spiracles. All these large sacs are superficial, lying directly beneath the hypodermis, while the smaller ones are buried among the muscles. We have detected 53 of these vesicles in the head.

In the honey-bee (Fig. 426) and humble bee (Fig. 427) as well as the flies there are two enormous air-sacs at the base of the abdomen. In larval and wingless insects these sacs are entirely absent.

FIG. 424.—Thorax and abdomen of the cockchafer (_Melolontha
vulgaris_), showing the tracheæ and air-sacs.—This and Fig. 425
after Straus-Dürckheim.
]

=The use of the air-sacs.=—It was supposed by Hunter as well as by Newport, and the view has been generally held, that the use of these sacs is to lighten the weight, _i.e._ lessen the specific gravity of the body during flight. It has, however, been suggested to us by A. A. Packard that this view from the standpoint of physics is incorrect. It is evident that the wings have to support just as much weight when the insect is flying, whether the tracheæ and vesicles are filled with air or not, the body of the insect during flight not being lightened by the air in the sacs. The use of these numerous sacs, some of them very spacious, is to afford a greater supply of air or oxygen than that contained in the air-tubes alone, and thus to afford a greater breathing capacity. The sacs are largest in dragon-flies, moths, flies, and bees, which are swift of flight. When we compare the active movements of these insects on the wing with those of a caterpillar or maggot, it will be seen that the far greater muscular exertions of the volant insect create a demand for a sudden and abundant supply of air to correspond to the increased rapidity of respiration; and the enlargements of the air-tubes, rapidly filled with air at each inspiration, render it possible to supply the demand.

FIG. 425.—Head of _Melolontha vulgaris_, showing the numerous
air-sacs, represented only on the left side, front view.
]

FIG. 426.—Tracheal, nervous, and digestive systems of the honey-bee
(the tracheal system on the right side only partially drawn): _tb_,
the large vesicles in the abdomen; _st_, stigmata; _hm_, honey
stomach; _cm_, chyle stomach; _vm_, urinary tubes; _rd_, rectal
glands; _ed_, rectum; _a_, antenna; _an_, eye; _b_{1}_-_b_{3}_,
legs.—After Leuckart, from Lang.
]

The case is thus seen to be very different from that of those fishes
which, having a swimming-bladder, can in the water change the
specific gravity of their bodies. The case of insects is almost
exactly paralleled by that of birds, where, as stated by
Wiedersheim, the air-sacs appear to form integral parts of the
respiratory apparatus: “a greater amount of air can by their means
pass in and out during inspiration and expiration, especially
through the larger bronchi, and consequently there is less necessity
for the expansion of the lung parenchyma.” In other words, the
supply of air in these sacs, as in insects, increases the breathing
capacity of the bird during flight. Wiedersheim’s retention of the
old idea that the specific gravity of the body is lessened (p. 262)
seems, however, to be incorrect, as the weight of the bird’s body is
not diminished by the air contained in the sacs.

_h._ The closed or partly closed tracheal system