132 WASPS [CH. V 



but depends upon whether the flight is directed towards 

 or away from the observer. The note becomes higher as 

 the insect approaches and lower as it recedes. A pre- 

 cisely similar phenomenon, due to the same cause, may be 

 noticed in the whistle of an express train approaching, 

 rushing through and receding from a railway station. If 

 an insect be held captive when its wings vibrate the note 

 produced is constant. According to Perez and Bellesme 1 

 there are two distinct sounds in the buzzing. One, a 

 deep noise, is due to the vibration of the wings and is 

 produced whenever a certain rapidity is attained; the 

 other is a shriller sound and is said to be produced by the 

 vibrations of the walls of the thorax to which muscles are 

 attached. Both of these observers agree that the spiracles 

 are not, as has been maintained by some, concerned in 

 producing the sound. 



Shipley and Wilson 2 have described an apparatus on 

 the wing of the mosquito to which the high-pitched note 

 of the buzz of this insect may be due. The organ lies at 

 the extreme base of the wing very close to the articulation 

 with the thorax. It consists of a slightly movable bar A 

 (Fig. 23), which bears on its hinder, free edge a series of 

 well-marked teeth from thirteen to fifteen in number. 

 Posteriorly to this bar there is situated a chitinous blade 

 B, with from thirteen to fifteen sharply defined and 

 slightly oblique elevations. The teeth of the bar A rasp 

 against the ridges of the blade B and a shrill note is 



1 C. R. Acad. Sci. Paris, LXXXVII. 1878. 



2 On a possible stridulating organ in the mosquito (Anopheles maculi- 

 pennis, Meig.), Trans. Roy. Soc. Edinb. XL. pt. ii. No. 13, 1902. 



