“7. Among insects, such as large beetles, Locustidæ, dragon-flies, etc., sufficiently powerful to give good graphic tracings, it can be shown that the inspiratory movement is slower than the expiratory, and that the latter is often sudden.
Fig. 419.—Profile of trunk 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.—After Plateau.
“8. In most insects, contrary to what obtains in mammals, only the expiratory movement is active; inspiration is passive, and effected by the elasticity of the body-wall.
“9. Most insects possess expiratory muscles only. Certain Diptera (Calliphora vomitoria and Eristalis tenax) afford the simplest arrangement of the expiratory muscles. In these types, they form a muscular sheet of vertical fibres, connecting the terga with the sterna, and underlying the soft, elastic membrane which unites the hard parts of the somites. One of the most frequent complications arises by the differentiations of this sheet of vertical fibres into distinct muscles, repeated in every segment, and becoming more and more separated as the sterna increase in length. Special inspiratory muscles occur in Hymenoptera, Acridiidæ, and Trichoptera.
“10. The abdominal, respiratory movements of insects are wholly reflex. Like other physiologists who have examined this side of the question, Plateau finds that the respiratory movements persist in a decapitated insect, as also after destruction of the cerebral ganglia or œsophageal connectives; further, that in insects whose nervous system is not highly concentrated (e.g. Acridiidæ and dragon-flies), the respiratory movements persist in the completely detached abdomen; while all external influences which promote an increased respiratory activity in the uninjured animal, have precisely the same action upon insects in which the anterior, nervous centres have been removed, upon the detached abdomen, and even upon isolated sections of the abdomen.
“The view formerly advocated by Faivre, that the metathoracic ganglia play the part of special, respiratory centres, must be entirely abandoned. All carefully performed experiments on the nervous system of Arthropoda have shown that each ganglion of the ventral chain is a motor centre, and, in insects, a respiratory centre, for the somite to which it belongs. This is what Barlow calls the ‘self-sufficiency’ of the ganglia.” (Miall and Denny.)
Fig. 420.—Transverse section of abdomen of a lamellicorn beetle. The position of the terga and sterna after an inspiration is indicated by the thick line; the dotted line shows their position after an expiration; and the arrow marks the direction of the expiratory movement.
