THE EXTERNAL ANATOMY OF INSECTS 81 



While it is only necessary to determine the note produced by 

 vibrating wings to ascertain the rate of vibration, a graphical demon- 

 stration of the rate is more convincing. Such a demonstration has 

 been made by Marey ('69) who fixed a fly so that the tip of the wing 

 just touched the smoked surface of a revolving cylinder, and thus 

 obtained a wavy line, showing that there were actually 320 strokes in 

 a second. This agrees almost exactly with the number inferred from 

 the note produced. 



The music of flight may be, in many cases, a mere accidental result 

 of the rapid movement, and in no sense the object of that movement, 

 like the hum of a trolley car ; but there are cases where the song seems 

 to be the object of the movement. The honeybee produces different 

 sounds, which can be understood by man, and probably by bees, as 

 indicating different conditions. The contented hum of the worker 

 collecting nectar may be a song, like the well-known song of a hen 

 wandering about on a pleasant day, or may be an accidental sound. 

 But the honeybee produces other sounds that communicate ideas. 

 The swarming sound, the hum of the queenless colony, and the note 

 of anger of a belligerent bee can be easily distinguished by the experi- 

 enced beekeeper, and doubtless also by the bee colony. It seems 

 probable, therefore, that in each of these cases the rate of vibration of 

 the wings is adjusted so as to produce a desired note. This is also 

 probably true of the song of the female mosquito, which is pitched so 

 as to set the antennal hairs of the male in vibration. 



While the music of flight is a common phenomenon, many insects 

 have a silent flight on account of the slowness of the wing-movement. 



C. STRIDL'LATING ORGANS OF THE RASPING TYPE 



The greater number of the insect sounds that attract our attention 

 are produced by the friction of hard parts of the cuticula by which a 

 vibrating surface is set in motion. In some cases, as in many of the 

 Orthoptera, the vibrating surface is apart of the wings that is special- 

 ized for this purpose; but in other cases, a specialized vibrating sur- 

 face has not been observed. 



Stridulating organs of the rasping type are possessed by ivpivsm- 

 tatives of several of the orders < if insects; but they are most comm< >n 

 in the order Orthoptera, and especially in the families Arridiid;r. 

 Locustidac, and Gryllid;c, where the males of very many species 

 possess them. Very few other Ortlmptera stridulate; and with feu- 

 exceptions it is only the males that sing. 



