Fig. 394.—Testis of Anabrus, showing the ramifications of the tracheæ.—After Minot.
Distribution of the tracheæ.—The distribution of the air-tubes, as Lubbock and also Minot state, depends first upon the shape of the organs, and upon the size of those whose size is variable. Around the large, hollow organs (digestive canal, sexual organs) the tracheæ ramify in all directions, forking so that the branches diverge at a wide angle. In the organs which have muscular walls, like the oviduct, the tracheæ run straight when the walls are distended, but have a sinuous course when the walls are contracted. (Minot.)
“Around the organs of more elongated form the branches of the tracheæ run more longitudinally, as is shown by the air-tubes of the muscles, which also present some peculiarities worthy of especial notice.
“A short, thick trunk arrives at the muscular bundle, and dividing very rapidly, breaks up into a large number of delicate tubes, which penetrate between the muscular fibres, then terminating in tubes of exceeding fineness, which at first sight seem to form a network that might well be called a rete mirabile. A closer examination, however, reveals that it is not a real network, but rather an interlacing confusing to the eye. The longitudinal direction of the tracheæ of the muscles presents a striking contrast to the system of divarication represented in Figs. 13 and 14. The course of the tracheæ of the Malpighian tubes is also very curious. There is one large trachea which winds around the tube in a long spiral, giving off numerous small branches which run to the surface of the tube, upon which they form delicate ramifications. Each tube has but a single main trachea, and I think the trachea continues the whole length of the tube, but of this last point I am not quite sure.” (Minot.)
While in the nymphs of Orthoptera the tracheæ very closely resemble those of the adult, in larvæ of insects with a complete metamorphosis the tracheæ differ very much in distribution from those of the adult. The larval tracheæ are also more generalized and more like those of the original type than the tracheæ of perfect insects. (Lubbock.)
In general there are two main tracheæ, one passing along each side of the body, near the digestive canal, connected with its mate by a few transverse anastomosing branches, and sending off a branch to each spiracle, this arrangement being most simple and apparent in the maggots of Diptera. From these two main branches smaller twigs branch off into every part of the body with its appendages, passing among the different organs, often serving as cables to hold them loosely in place; they also penetrate into the component parts of the organ themselves, passing into the fat-bodies, and among the fibres of muscles, where they become finely attenuated and refined like the capillaries of the vascular system of vertebrates. (Figs. 395, 396.)
Fig. 395.—Melanoplus femur-rubrum, showing distribution of air-tubes (tracheæ) and air-sacs; V, main ventral trachea (only one of the two shown); S, left stigmatal trachea, connecting by vertical branches with D, the left main dorsal trachea; c, left cephalic trachea; oc, ocular dilated trachea. From the first, second, third, and fourth spiracles arise the first four abdominal air-sacs, which are succeeded by the plexus of three pairs of dilated tracheæ, I, II, III, in Fig. 396. Numerous air-sacs and tracheæ are represented in the head and thorax. The two thoracic spiracles are represented, but not lettered.
