.MECHANISM OF RESPIRATION 



455 



muscles; 1, 2, inspirator muscles. 

 A 



" 6. 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). 



" c. The pleural membrane, 

 connecting the terga with the 

 sterna, is well developed and ex- 

 posed on the sides of the body. 

 The terga and sterna approach 

 and recede alternately, while the 

 pleural zone simultaneously be- 

 comes depressed, or returns to its 

 original figure. To this type, 



Plateau awi-n<i flip TooiistidiP FlG - 417. Muscles of right half of abdomen of 



Lldse, Ph) . yganM gtriata, ? : A, E longitudinal dorsal rnus- 



Lepidoptera, and the true Neu- cles ; a, b, longitudinal sternal muscles ; l>, e, oblique 

 roptera (excluding Trichoptera) 

 (Fig. 423). 



"6. Contrary to the opinion 

 once general, changes in length 

 of the abdomen, involving protru- 

 sion of the segments and sub- 

 sequent retraction, are rare in 

 the normal respiration of insects. 

 Such longitudinal movements ex- 

 tend throughout one entire group 

 only, viz. the aculeate Hymenop- 

 tera. Isolated examples occur, 

 however, in other zoological 

 groups. 



" 7. Among insects, such as large beetles, Locustidse, dragon-flies, etc., suffi- 

 ciently powerful to give good graphic tracings, it can be shown that the inspira- 

 tory 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 expira- 

 tory 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 expira- 

 tory muscles. In these 

 types, they form a muscular 

 sheet of vertical fibres, con- 

 necting 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, Acridi- 

 idse, and Trichoptera. 



FIG. 418. Muscles of left half of abdomen of Melo- 

 lontha, : A, , longitudinal muscles (pretracteurs of 

 Straus) ; a, n, true respiratory muscles (expirators). 

 This and Figs. 415-417, after Plateau. 



FIG. 419. Profile of trunk of cockroach (P. orientalifi). 

 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. tk, mesothorax ; 

 Mt. th, metathorax. Reduced from a magic-lantern projec- 

 tion. After Plateau. 



