236 ANIMAL ‘LIFE 
many sound-making insects are known; but certain other 
insects, which 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 
