THE EXTERNAL ANATOMY OF INSECTS 79 



Musical sounds are produced by many different insects, and in 

 various ways. These sounds are commonly referred to as the songs of 

 insects ; but properly speaking few if any insects sing ; for, with some 

 possible exceptions, the note of an insect is always at one pitch, lacking 

 musical modulations like those of the songs of man and of birds. 



The sound produced by an insect may be a prolonged note, or it 

 may consist of a series of short notes of varying length, with intervals 

 of rest of varying lengths. These variations with differences in pitch 

 give the wide range of insect calls that exists. 



In some cicadas where the chambers containing the musical organs 

 are covered by opercula, the insect can give its call a rhythmic 

 increase and decrease of loudness, by opening and closing these 

 chambers. < 



As most insect calls are strident, organs specialized for the pro- 

 duction of these calls are commonly known as stridulating organs. 

 But many sounds of insects are produced without the aid of organs 

 specialized for the production of sound. The various ways in which 

 insects produce sounds can be grouped under the following heads : 



First. By striking blows with some part of the body upon sur- 

 rounding objects. 



Second. By rapid movements of the wings. In this way is 

 produced what may be termed the music of flight. 



Third. By rasping one hard part of the body upon another. 

 Under this head fall the greater number of stridulating organs. 



Fourth. By the rapid vibration of a membrane moved by a muscle 

 attached to it. This is the type found in the cicadas. 



Fifth. By the vibration of membranes set in motion by th-^ rush 

 of air through spiracles. The reality of this method has been ques- 

 tioned. 



Sixth. By rapid changes of the outline of the thorax due to the 

 action of the wing muscles. 



a. SOUNDS PRODUCED BY STRIKING OBJECTS OUTSIDE THE BODY 



Although the sounds produced by insects by striking blows with 

 some part of the body upon surrounding objects are not rapid enough 

 to give a musical note, they are referred to here for the sake of 

 completeness. 



The most familiar sounds of this kind are those produced by the 

 insects known as the death-watch. These are small beetles of the 

 family Ptinidas, and especially those of the genus Anobium. These 

 are wood-boring insects, frequently found in the woodwork of old 



