2i8 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



This cavity is closed below by the membrane already 

 spoken of, vr\ but above on each side it is closed by a 

 very tense membrane like a drumhead (Fig. 140, B, dr). 

 This drumhead is stiffened by radiating ribs which are a 

 little convex outward. A slender tendon coming from 

 a muscle, m, in the cavity is attached to the center of 

 radiation of the ribs. By the action of the muscle the 

 stiff convex drumhead is drawn in with a clack, and 

 again springs out with a clack when the muscle is re- 

 laxed. The muscle contracts and relaxes with a rapid 

 vibratory motion, and this gives rise to the characteristic 

 clattering noise of these creatures. 



It is hardly necessary to say that the humming and 

 buzzing sounds so common in insects while flying are 

 due wholly tojthe rapid vibration of the wings. 



Animals in other and lower departments are not 

 known to make sounds intended to be heard. 



