There are two chief morphological tracheal systems: 1. The open or normal and primitive (holopneustic) type, and 2. The closed, or secondary and adaptive, i.e. apneustic, type. The open system is characterized by the presence of the stigmata. Through them the air directly enters into the tracheal tubes, whose delicate walls allow the exchange of gases in the blood. This type occurs in all sexually mature individuals, and also in the greater number of larvæ.
The closed or apneustic tracheal system is distinguished either by the want of stigmata, or, if present, they are not open, and do not function, so that the tracheæ cannot communicate with the air. In such cases the direct oxygenation of the blood is effected through the delicate integument, especially over the surface of the body in general, or in certain specialized places where the gill-like expansions of the skin are rich in tracheæ; such outgrowths, generally tubular or leaf-like, are called by Palmén tracheal gills.
This closed form of the tracheal system only occurs in the larval stage of aquatic or parasitic insects, as in the Plectoptera (Ephemeridæ), Perlidæ, Odonata, and Trichoptera, besides single genera of other orders, i.e. among Coleoptera, Gyrinus, Pelobius, Cnemidotus, and the young larva of Elmis; in the aquatic caterpillar of Paraponyx; in certain Diptera (Corethra, Chironomus, etc.), and some of the parasitic Hymenoptera (Microgaster).
Palmén has discovered that in the nymphs of Ephemeridæ, Perlidæ, Odonata, and the larvæ of most Trichoptera the tracheal branch (stigmatal branch) sent from the longitudinal trachea to where the thoracic stigmata would be situated if present, or where their vestiges only exist, are aborted, becoming simple solid cords not filled with air (Fig. 436, vf, and 447, f, funiculus or stigmatic cord). In the imago, however, they resume their function, connecting with the open functional stigmata. In Corethra, in its earliest stages, the entire tracheal system is, like the stigmatic branch, a system of solid cords and empty of air. (Palmén.)
Embryology shows that these stigmatal branches are well developed, and are formed at the same time as the stigmata. It was also shown by Dewitz, in a posthumous paper (1890), that in the young larval stage of the Odonata and Ephemeridæ the tracheal system is at first an open one, and in some of the families (Libellulidæ, Agrionidæ, and Ephemeridæ) thoracic stigmata are seen at a very early stage. From numerous experiments Dewitz concludes that in the young stages of Odonata and Ephemeridæ there is an open tracheal system; certainly in very young nymphs the thoracic spiracles allow the air to pass out. Fully grown nymphs of Æschnidæ, Libellulidæ, and Agrionidæ are capable not only of forcing the air out, but also, like the perfect insect, of inhaling it. Moreover, he proved that the gills of Ephemeridæ and Agrionidæ are not indispensable for the maintenance of life, as the insects can live without them, breathing either through the skin or by the rectum, or in both ways. It would seem that while in freshly hatched or very young larvæ of aquatic insects of different orders the skin is so delicate as to allow of dermal respiration, in after life, when the skin becomes thicker and denser, these expansions (gills), provided with a very thin and delicate skin, of a necessity grow out from the walls of the body.
It thus appears that the closure and total or partial abolition of the stigmata are in adaptation to aquatic life, and that such insects have descended from terrestrial air-breathing winged forms. This is an important argument against the view that the wings are modified tracheal gills.
In this connection may be noticed the closure of the 2d and 3d thoracic stigmata in holopneustic insects. We have found on laying open the body of a Sphinx larva that a large number of tracheal branches are seen to arise from the prothoracic and from the first pair of abdominal stigmata. Now between these points there are no spiracles or any external signs of them, there being in Lepidoptera no mesothoracic or metathoracic spiracles. Yet the main lateral trachea between the prothoracic and first abdominal segments deviates from its course and bends down to send off a small shrivelled stigmatal branch or cord to a place where, did a spiracle exist, we should look for it. In the larva of Platysamia cecropia, a similar vestigial stigmata branch is present.
In the larva of Corydalus, also, a trachea as large as the main longitudinal one takes its origin and passes directly under the main trachea. Now both tracheæ send a stigmatal branch opposite to where the mesothoracic stigma should be, if present, i.e. on the hind edge of the segment.
Verson, moreover, has found in the freshly hatched silkworm vestiges of meso- and metathoracic stigmata, each consisting of a circle of high hypodermal cells radially arranged around a common centre. The stigmatal branch is long, but shrivelled; its peritoneum is widened out into several berry-like saccules filled with cell-elements. In profile these rudimentary stigmata appear as a series of high hypodermal cells, which form the basis of a short blind tube.
