How they become aware of their surroundings 



certain that this is their true purpose. There is still the problem of what 

 sounds they are meant to receive, and whether other inseas of the same 

 group can make such sounds. 



The insect's voice is very poor compared with our own. We use our 

 breath to vibrate a pair of vocal chords, and by varying the tension of 

 the chords and the resonance of our throat, nose and mouth we have 

 a tremendous range of pitch and timbre. Apart from the buzz, hum or 

 whine produced by the vibration of its wings, an insect generally can 

 make only sounds of a scraping or scratching kind, by rubbing together 

 two parts of its body. This is called stridulation, and although in prin- 

 ciple it is akin to musical instruments of the violin family, the crude 

 construction and small size of the insect's apparatus makes the refined 

 and subtle tone of the violin quite beyond its powers. 



Stridulation is particularly associated with the grasshoppers and their 

 allies, where nearly always it is the male that makes the sound. Stridula- 

 tion may result from rubbing together specially roughened areas of the 

 wings, or by rubbing a leg against a wing. The sound is different in 

 different groups, and varies from a dry crackling to a high-pitched noise 

 like the 'singing' of a tea-kettle. Among the common grasshoppers of the 

 countryside there are differences of song, which have been studied 

 by means of tape-recordings. Besides the differences of pitch and quality 

 that arise from variations in size and structure between different species, 

 there are also differences of rhythm. One entomologist claimed to be 

 able to distinguish between two species of grasshopper in the field by 

 watching the movements of the legs through field-glasses. 



Like their allies, the long-horned grasshoppers, crickets do not use 

 the legs when stridulating, but rub the right hindwing against the left, 

 when specially roughened, file-like veins produce the piercing chirp. 



Stridulating Orthoptera are good ventriloquists, and cicadas (Hemip- 

 tera-Homoptera) are even better. If you hear a cicada 'singing' in a 

 bush and walk slowly towards it, you will get quite close and then the 

 sound will suddenly seem to come from quite a different direction. It is 

 hard to tell whether there is any real illusion of 'throwing the voice', 

 such as a ventriloquist practises, or whether it is just that the cicada 

 that you are tracking falls silent and shams dead as you come near, 

 while another takes its place. 



Cicadas (Fig. 27) are extremely noisy, and produce a great volume 

 of sound by vibrating a drum-like organ called a tymbal. This is pulled 

 inwards by a powerful muscle, and then released, making a sound like 

 that boys make when they attach a string to the centre of a tin can, and 

 either pluck or bow the string. Sound production is again confined to 



65 



