THE ORGANS OF CIRCULATION AND RESPIRATION. 163 



Fig. 92. Transverse section of Abdomen, Bee (Bornbus}. 



6. Contrary to the opinion once general, changes in length 

 of the abdomen, involving protrusion of the segments and sub- 

 sequent 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. 



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, contrarv to what obtains in Mammals, 



mf 



onty 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 tcnax) 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 mem- 

 brane which unites the hard parts of the somites. One of the 



