2. The respiratory activity of resting Insects is localised in the abdomen. V. Graber has expressed this fact in a picturesque form, by saying that in Insects the chest is placed at the hinder end of the body.
3. In most cases the thoracic segments do not share in the respiratory movements of an Insect at rest. Among the singular exceptions to this rule is the Cockroach (P. orientalis), in which the terga of the meso- and meta-thoracic segments perform movements exactly opposite in direction to those of the abdomen. (See fig. 89, Ms. th., Mt. th.)
Fig. 89.—Profile 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.
4. Leaving out of account all details and all exceptions, the respiratory movements of Insects may be said to consist of alternate contraction and recovery of the figure of the abdomen in two dimensions—viz., vertical and transverse. During expiration the diameters in question are reduced, while during respiration 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).
5. Three principal types of respiratory mechanism occur in Insects, and these admit of further subdivision:—
(a) Sterna usually stout and very convex, yielding but little. Terga mobile, rising and sinking appreciably. To this class belong all Coleoptera, Hemiptera Heteroptera, and Blattina (Orthoptera).