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ANIMAL FLIGHT 129 



]\Iany flies can hum or buzz Cjuite as well with the wings cut 

 off as with them present, apparently through the action of the 

 halteres which in this case appear to be wings transformed 

 into singing organs. 



The song of the crickets, locusts, grasshoppers, katydids and 

 similar insects is produced by the fore wungs, parts of which 

 are modified into very perfect sounding organs operated by 

 the rubbing of the wings together, or by the long hind legs. 



The song of the cicadas and their allies, though it sounds 

 much like that of the crickets and the locusts, at least like 

 that of some of their tropical representatives, is not produced 

 by the wings but by a special apparatus on the under side of 

 the body. In some kinds the piercing shriek they give can 

 only be compared to the whistle of a steam engine, and may 

 easily be heard on a calm day four miles or more. 



The wings of insects are mere outgrowths from the body 

 wall, quite unconnected with the legs. They are thus com- 

 parable to the side extensions of the flying lizards and to the 

 cobra's hood. In many groups, especially in the moths and 

 butterflies and in many flies, hke moth-flies, they bear numer- 

 ous broad scales somewhat resembhng the feathers of a bird; 

 in others they are often sparsely hairy hke a bat's wings. 



Except for bats all the flying mammals are tree-living climb- 

 ing creatures, and in them the wing membrane is stretched 

 between the legs. 



In the bats and birds the wdngs are an adaptation from 

 special climbing organs, somewhat as suggested by the long 

 arms of the spider monkeys. In the bats the flying surface 

 is formed by broad areas of skin stretched between immensely 

 elongate fingers and extending to the hind legs as in the an- 

 cient flying reptiles and in all the other flying mammals. In 

 the birds the flying surface is made up of long feathers which 

 are outgrowths of the skin of the long front limbs. With 

 their very long arms the monkeys and the lemurs climb with 

 great rapidity through the forest trees. With their very long 

 and suitably modified front limbs the bats and birds in much 



