THE INSECTS— ARTHROPODS 223 



formed by a part of the thoracic exoskeleton that extends back over the 

 abdomen. Within this cavity is a membrane that can be stretched and 

 relaxed to make it vibrate at a variable pitch, and the cavity acts as a 

 resonance chamber to amplify and reflect sound. 



The humming of insects is an incidental sound accompanying - the vi- 

 bration of the wings in flying, but should be mentioned under insect 

 voices because it is sometimes used as a means of communication among 

 insects. The pitch of the hum depends on the rate of vibration of the 

 wings ; a house fly hums in the key of F in the middle octave of a piano. 

 When this particular piano string is struck it vibrates 345 times per sec- 

 ond. That means that the house fly's wings must beat up and down 345 

 times per second. This can be checked by allowing the fly's wings to 

 beat against a rapidly revolving smoked drum and counting the strokes 

 made in one second. In bees the humming varies with the temper of 

 the hive, and a beekeeper can tell by the key of the hum whether a hive 

 is contentedly going about its business or is possibly preparing to swarm. 

 Recent research shows that mosquitoes use the hum of their wings as a 

 means of communication. Greatly amplified recordings have been made 

 which show that they have danger signals, mating calls, and numerous 

 other sounds with a definite meaning. 



Insect Reproduction 



Reproduction among insects is as highly specialized and variable as 

 the other body functions, so only a survey of some of the most common 

 methods can be presented here. The problem of sexual attraction is an 

 interesting study among this group of animals. Odor is used by many 



Photo by Winchester 



Fig. 15.10. Male and female Cecropia moths. Note the more extensive branching of 

 the antennae of the male. This sexual difference seems to be correlated with the re- 

 production season when the female exudes an odor which is picked up by the male 

 through the antennae enabling him to find her in the dark. 



