Fig. 92.—Transverse sec­tion of Ab­do­men, Bee (Bombus).

Fig. 93.—Transverse sec­tion of Ab­do­men, Hawk Moth (Sphingina).

6. Contrary to the opinion once general, changes in length of the abdomen, involving protrusion of the segments and subsequent retraction, are rare in the normal respiration of Insects. Such longitudinal movements extend throughout one entire group only—viz., the aculeate Hymenoptera. Isolated examples occur, however, in other zoological divisions.

7. Among Insects 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.

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 differentiation 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. (See the tergo-sternal muscles of the Cockroach, fig. [36], p. 76.) Special inspiratory muscles occur in Hymenoptera, Acridiidæ, and Phryganidæ.

10. The abdominal respiratory movements of Insects are wholly reflex. Like other physiologists who have examined this side of the question, the writer 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.