240 MOVEMENT 



noticed that the character of the sound continually 

 changes. By close attention, the sound can be dis- 

 tinguished as of a higher pitch when the fly approaches 

 the observer, and of lower pitch as it recedes. This 

 suggests that there must be an alteration in the 

 frequency of the wing vibrations. The phenomenon 

 may, however, fairly be compared to the apparent 

 variations in shrillness of the whistle of a moving train. 

 As the train rushes along, the whistle seems to become 

 shriller when it approaches, and deeper when it recedes. 

 This acoustic phenomenon has long ago been ex- 

 plained. However, if we hold an insect lightly in a 



Fig. 168. — The two top lines are produced by the contacts of a drone's wing on a 

 smoked cylinder. In the middle are recorded the vibrations of a tuning-f rk 

 (250 vibrations per second) for comparison with the frequency of the wing move- 

 ments. Below are seen the movements of the wing of a bee. 



pair of forceps it may be shown that when its wings 

 vibrate the sound produced is practically uniform. 



Mechanical Registration of the Movements of Wings 

 in Insects. — The movements of the wings of a captive 

 insect may be recorded directly on a revolving cylinder. 

 If the cylinder previously be blackened with smoke 

 the slightest touch will remove the black and expose 

 the white paper beneath. Fig. 168 was obtained in this 

 way, and shows several interrupted lines traced by the 

 wing of a drone. The tracing was obtained as follows : 

 The insect was held between a pair of forceps in such 

 a way that the extremity of its wing only just came in 

 contact with the surface of the cylinder, and in so 



