In the Cockroach (_Periplaneta_) the sterna are slightly raised during expiration. (See figs. 89 and 91.) [Illustration: Fig. 91.--Transverse section of Abdomen, Cockroach (_P. orientalis_).] (_b_) Terga well developed, overlapping the sterna on the sides of the body, and usually concealing the pleural membrane, which forms a sunk fold. The terga and sterna approach and recede alternately, the sterna being almost always the more mobile. To this type belong Odonata, Diptera, aculeate Hymenoptera, and Acridian Orthoptera. (Fig. 92.) (_c_) The pleural membrane, connecting the terga with the sterna, is well developed and exposed on the sides of the body. The terga and sterna approach and recede alternately, while the pleural zone simultaneously becomes depressed or returns to its original figure. To this type the writer assigns the Locustidæ, the Lepidoptera and the true Neuroptera (excluding Phryganidæ). (Fig. 93.) [Illustration: Fig. 92.--Transverse section of Abdomen, Bee (_Bombus_).] 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. [Illustration: Fig. 93.--Transverse section of Abdomen, Hawk Moth (_Sphingina_).] 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.