Fig. 77.—Tracheal System of Cockroach. Side view of head seen from without, introducing the chief branches of the left half. × 15.
The respiratory organs of Insects consist of ramified tracheal tubes, which communicate with the external air by stigmata or spiracles. Of these spiracles the Cockroach has ten pairs—eight in the abdomen and two in the thorax. The first thoracic spiracle lies in front of the mesothorax, beneath the edge of the tergum; the second is similarly placed in front of the metathorax. The eight abdominal spiracles belong to the first eight somites; each lies in the fore part of its segment, and hence, apparently, in the interspace between two terga and two sterna. The first abdominal spiracle is distinctly dorsal in position.
The disposition of the spiracles observed in the Cockroach is common in Insects, and, of all the recorded arrangements, this approaches nearest to the plan of the primitive respiratory system of Tracheata, in which there may be supposed to be as many spiracles as somites.[147] The head never carries spiracles except in Smynthurus, one of the Collembola (Lubbock). Many larvæ possess only the first of the three possible thoracic spiracles; in perfect Insects this is rarely or never met with (Pulicidæ?), but either the second, or both the second and third, are commonly developed. Of the abdominal somites, only the first eight ever bear spiracles, and these may be reduced in burrowing or aquatic larvæ to one pair (the eighth), while all disappear in the aquatic larva of Ephemera.
From the spiracles, short, wide air-tubes pass inwards, and break up into branches, which supply the walls of the body and all the viscera. Dorsal branches ascend towards the heart on the upper side of the alary muscles; each bifurcates above, and its divisions join those of the preceding and succeeding segments, thus forming loops or arches. The principal ventral branches take a transverse direction, and are usually connected by large longitudinal trunks, which pass along the sides of the body; the Cockroach, in addition to these, possesses smaller longitudinal vessels, which lie close to the middle line, on either side of the nerve-cord.[148] The ultimate branches form an intricate network of extremely delicate tubes, which penetrates or overlies every tissue.
Fig. 79.—Tracheal System of Cockroach. Back of head, seen from the front, the fore half being removed. × 15. The letters A–J indicate corresponding branches in figs. 77, 78, and 79.
