These spiracles, according to Hinds, are remarkable honeycomb-like structures, and perforations to the tracheal tubes have not been demonstrated.
Tracheal system reduced or absent.
I', Tracheal aperture in somite 3 of the opisthosoma.
To admit of the free inflow and outflow of currents of water necessary for respiration, which is effected by means of filamentous abdominal tracheal gills, the two ends of the tube are open.
In a second type they are situated at the ends of tracheal strands and consist of groups of richly protoplasmic cells belonging to the epidermis (as in the leaves of many ferns), or to the subjacent tissue (the commonest type in flowering plants); in this last case the cells in question are known as epithem.
In other cases the epithem may be absent altogether, the tracheal strand debouching directly on the lacunae of the mesophyll.
The tracheids or vessels, indifferently called tracheal elements, together with the immediately associated cells (usually amylom in Pteridophytes) constitute the xylem of the plant.
During the process the thin walls are stretched and the turns of the spiral become pulled apart without rupturing the wall of the tracheid or vessel, If the pitted type of tracheal element were similarly stretched its continuously thickened walls would resist the stretching and eventually break.
The parenchyma is often arranged in tangential bands between the layers of sievetubes and tracheal elements.
The xylem parenchyma is often found in strands associated with the tracheal elements.
The stereom is furnished either by cortical cells or by the tracheal elements, in a few cases by fibres which arc probably homologous with sievetubes.
In the secondary tissues of Dicotyledons we may have, as already described, considerably more differentiation of the cells, all the varieties being referable, however, on the one hand to the tracheal or sieve-tube type, on the other to the parenchyma type.
The limit of each years increment of secondary wood, in those plants whose yearly activity is interrupted by a regular winter or dry season, is marked by a more or less distinct line, which is produced by the sharp contrast between the wood formed in the late summer of one year (characterized by the sparseness or small diameter of the tracheal elements, or by the preponderance of fibres, or by a combination of these characters, giving a denseness to the wood) and the loose spring wood of the next year, with its absence of fibres, or its numerous large tracheae.
External tympaniform membranes exist, with great variations, between the specialized one or two last tracheal and some of the first bronchial rings.
The essential feature is that the proximal end of the inner membranes is attached to the last pair of tracheal rings; outer tympaniform membranes exist generally between the 2nd, 3rd and 4th bronchial semi-rings.
Omitting the paired tracheo-clavicular muscles, we restrict ourselves to the syringeal proper, those which extend between tracheal and bronchial rings.
The chitinous layer is usually strengthened by thread like thickenings which, in the region close to the outer opening of the tube, form a network enclosing polygonal areas, but which, through most of the tracheal system, are arranged spirally, the strengthening thread not forming a continuous spiral, but being interrupted after a few turns around the tube.
The tracheal system in Hexapods is very complex, 1 x, forming a series of longitudinal trunks with nexions transvers II), anastomosing finest sub-division and extendingdi by g by the After Miall and Denny, The Cock- nest su-vson and re roach, Lovell Reeve & Co.
The subsequent expansion of the body causes fresh air to enter the tracheal system, and if the spiracles be then closed and the body again contracted, this air is driven to the finest branches of the air-tubes, where a direct oxygenation of the tissues takes place.
Many insects have aquatic larvae, some of which take in atmospheric air at intervals, while others breathe dissolved air by means of tracheal gills.
Other aquatic larvae have the tracheal system entirely closed, and are able to breathe dissolved air by means of tubular or leaf-like gills.
Developed tracheal system.
Young resembling parents, but aquatic in habit, breathing dissolved air by thoracic tracheal gills.
Aquatic larvae with distinct maxillulae, breathing dissolved air by abdominal tracheal gills.
Aquatic larvae with caudal leaf-gills or with rectal tracheal system.
A remarkable fossil from the Scottish Coal-measures (Lithomantis) had apparently small wing-like structures on the prothorax, and in allied genera small veined outgrowths - like tracheal gills - occurred on the abdominal segments.
But a survey of the Hexapoda as a whole, and especially a comparative study of the tracheal system, can hardly leave room for doubt that this system is primitively adapted for atmospheric breathing, and that the presence of tracheal gills in larvae must be regarded as a special adaptation for temporary aquatic life.
In most respects, the shortened abdomen, for example, they are more specialized than the Thysanura, and most of the features in which they appear to be simple, such as the absence of a tracheal system and of compound eyes, can be explained as the result of degradation.
These vary in form: in some species they are entire plates, in others they are cut up into numerous divisions, in all cases traversed by numerous tracheal ramifications.
The dorsal skeletal elements of the thorax and of the anterior six abdominal segments unite with the wing-cases to form a large respiratory chamber, containing five pairs of tracheal gills, with lateral slits for the inflow and a posterior orifice for the outflow of water.
We are compelled to take a similar view of the agreement between the tracheal air-tubes of Arachnida and other tracheate Arthropods.
In specialized forms these pulmonary sacs are wholly or partly replaced by tracheal tubes.
63,stg), as in the Amblypygi, or with the posterior pair, rarely the anterior pair as well, replaced by tracheal tubes.
The posterior pulmonary sacs (except in Hypochilus) replaced by tracheal tubes; the anterior and posterior pairs replaced by tracheal tubes in the Caponiidae.
Respiratory organs tracheal, opening upon the ventral surface of the 2nd and 3rd, and sometimes also of the 4th somite of the opisthosoma.
St, Thoracic tracheal aperture.
1 2, Anterior tracheal aperture of the opisthosoma in somite 2 of the opisthosoma.
Plate correspondingrespectively S, Prosomatic tracheal aperture to appendages I, II, III, and between legs IV and V.
S' and S",Opisthosomatic tracheal d, Second plate of the prosomaapertures.
(Original.) soma agrees in form and number of somites with the abdomen of a Hexapod, and the tracheal stigmata present certain agreements in the two cases.
S, Prosomatic stigma or aperture of the tracheal system.
2, Second opisthosomatic sternite covering the second pair of tracheal apertures sp I .
Sp2, The third pair of tracheal apertures.
2 and 3, Sternites of the second and third somites of the opisthosoma, each showing a tracheal stigma.
Respiratory organs tracheal, opening by a pair of spiracles in the prosoma above the base of the fifth appendage on IV III I composed, at least in many cases, of eleven somites, the 1 1 th somite very small, often hidden within the loth.
Respiratory organs in the form of tracheal tubes opening by a pair of stigmata in the 2nd and 3rd somites of the opisthosoma.
Respiratory organs tracheal, opening by a pair of stigmata situated immediately behind the basal segments of the 6th pair of appendages on what is probably the sternum of the 2nd opisthosomatic somite and also in some cases upon the 5th segment of the legs.
,?, VI B, Ventral view of the prosoma and of the first somite of the opisthosoma, with the appendages I to VI cut off at the base; a, tracheal stigma; mx, maxillary processes of the coxae of the 3rd pair of appendages; g,genital aperture.
Prosoma and opisthosoma; a, tracheal stigma; b, last somite.
- Opisthosoma consisting of ten segments defined by integumental grooves, each of the anterior four of these furnished with a single pair of dorsally-placed spiracles or tracheal stigmata.
A, Lateral view with appendages III to VI removed; 1, plate covering the whole dorsal area, representing the fused tergal sclerites of the prosoma and opisthosoma; 2, similarly-formed ventral plate; 3, tracheal stigma.
So in some respects is the lifehistory, with a true larval preparatory stage, unlike the parent form, and living an aquatic life, breathing dissolved air by means of a paired series of abdominal tracheal gills.
They resemble the May-flies in their " hemimetabolous " lifehistory; the young insects are markedly unlike their parents, inhabiting fresh water and breathing dissolved air, either through tracheal gills at the tip of the abdomen, or by a branching system of air-tubes on the walls of the rectum into which water is periodically admitted.
Short and reticulately-pitted tracheal cells, similar to tracheids, often occur in the circummedullary region of cycadean stems. In an old stem of Cycas, Encephalartos or Macrozamia the secondary wood consists of several rather unevenly concentric zones, while in some other genera it forms a continuous mass as in conifers and normal dicotyledons.
Tracheae are essentially tubes like bloodvessels - apparently formed from the same tissue elements as bloodvessels - which contain air in place of blood, and usually communicate by definite orifices, the tracheal stigmata, with the atmosphere.
The immediate cavities or pits into which the tracheal stigmata open appear to be in many cases ectodermic in sinkings, but there seems to be no reason (based on embryological observation) for regarding the tracheae as an ingrowth of the ectoderm.
With the characters of the grade: add the presence within the body of fine unbranched tracheal tubes, devoid of spiral thickening, opening to the exterior by numerous irregularly scattered tracheal pits.
In higher specialized forms these branchial processes become first of all limited to five segments of the mesosoma, then sunk beneath the surface as pulmonary organs, and finally atrophied, their place being taken by a well-developed tracheal system.