THE SPECIAL SENSES 383 



many sound-making insects are known ; but certain other 

 insects, that make no sound that we can hear, neverthe- 

 less possess similar sound-making organs. 



Sound is produced by mammals and birds by the strik- 

 ing of the air which goes to and comes from the lungs 

 against certain vibratory cords or flaps in the air-tubes. 

 Sounds made by this vibration are re-enforced and made 

 louder by arrangements of the air-tubes and mouth for 

 resonance, and the character or quality of the sound is 

 modified at will to a greater or less degree by the lips and 

 teeth and other mouth structures. Sounds so made are 

 said to be produced by a voice^ or animals making sounds 

 in this way are said to possess a voice. Animals possessing 

 a voice have far more range and variety in their sound- 

 making than most of the animals which produce sounds in 

 other ways. The marvelous variety and the great strength 

 of the singing of birds and of the cries and roars of mam- 

 mals are unequaled by the sounds of any other animals. 



But many animals without a voice that is, which do not 

 make sounds from the air-tubes make sounds, and some 

 of them, as certain insects, show much variety and range 

 in their singing. The sounds of insects are made by the 

 rapid vibrations of the wings, as the humming or buzzing 

 of bees and flies, by the passage of air out or into the body 

 through the many breathing pores or spiracles (a kind 

 of voice), by the vibration of a stretched membrane or 

 tympanum, as the loud shrilling of the cicada, and most 

 commonly by stridulation that is, by rubbing together 

 two roughened parts of the body. The male crickets and 

 the male katydids rub together the bases of their wing 

 covers to produce their shrill singing. The locusts or 

 grasshoppers make sounds when at rest by rubbing the 

 roughened inside of their great leaping legs against the 

 upper surface of their wing covers, and when in flight by 

 striking the two wings of each side together. Numerous 

 other insects make sounds by stridulation, but many of 



