CH. V] WASPS 131 



If a wasp be held firmly but allowed to vibrate the 

 wings freely, the movements of the dorsal surface of the 

 meso-thorax can be readily detected by means of a long 

 and light lever rested upon it. Moreover a very slight 

 dorso-ventral pressure upon the meso-thorax completely 

 arrests all movement of the wings. 



It must not be imagined that the actual movement of 

 the wing is so simple as this description indicates. Marey 1 

 and Pettigrew have independently pointed out that the 

 extremity of the wing describes an 8-shaped trajectory 

 during one complete oscillation. The former of these 

 investigators has made it clear that, though it is true that 

 the muscles maintain only the up-and-down movement, 

 the resistance of the air operating upon the more flexible 

 hind portion of the wing induces numerous changes in 

 the inclination of the surface of the wing and thus deter- 

 mines the form of the trajectory. 



The number of vibrations per second is in a wasp 

 about 190. This fact has been determined by causing the 

 wing of a captive insect to record its strokes upon a 

 rapidly revolving cylinder whose surface is blackened with 

 smoke. A tuning-fork, whose number of vibrations per 

 second is known, is made simultaneously to record a 

 tracing on the same cylinder. Thus a direct comparison 

 is possible between the two series of markings. 



The rapid movement of the wings is in part responsible 



for the well-known buzzing of wasps and other insects. 



The slight change in the note heard as an insect is flying 



is not due to any alteration in the frequency of vibration, 



1 Movement. Trans, by E. Pritchard. Heinemann, 1895. 



99 

 A 



