“5. Three principal types of respiratory mechanism occur in insects, and these admit of further subdivision:
“a. Sterna usually short and very convex, yielding but little. Terga mobile, rising and sinking appreciably. To this class belong all Coleoptera, heteropterous Hemiptera, and Blattina (Fig. 420).
“In the cockroach (Periplaneta), the sterna are slightly raised during expiration (Fig. 421).
“b. Terga well developed, overlapping the sterna on the sides of the body, and usually concealing the pleural membrane, which forms a sunken 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 acrydian Orthoptera (Fig. 422).
Fig. 417.-Muscles of right half of abdomen of Phryganea striata, ♀: A, B, longitudinal dorsal muscles; a, b, longitudinal sternal muscles; D, e, oblique muscles; 1, 2, inspirator muscles.
“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, Plateau assigns the Locustidæ, Lepidoptera, and the true Neuroptera (excluding Trichoptera) (Fig. 423).
Fig. 418.—Muscles of left half of abdomen of Melolontha, ♀: A, B, longitudinal muscles (prétracteurs of Straus); a, a, true respiratory muscles (expirators).—This and Figs. 415–417, after Plateau.
“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 zoölogical groups.