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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



direction control which exists in the two- winged insects (Diptera) 

 or flies. In these insects the two hinder wings are replaced by two 

 small knobs or "halteres" united to the body by stalks (fig. 10). Dur- 

 ing flight the tips of the halteres swing to and fro in the arc of a circle. 

 When the fly is turned off its course, the halteres continue to swing 



Figure 10. — The hind wings of flies are modified into club-shaped structures known as 

 halteres (indicated by arrows). The halteres vibrate up and down and act as two-dimen- 

 sional gyroscopes, enabling the animal to fly along a steady path. 



in the same plane as before the turn, on the principle of a gyroscope, 

 and the base of the haltere stalk is consequently twisted. This twisting 

 excites a nerve, and the brain thereupon sends appropriate instruc- 

 tions to the muscles that control the wings. 



By this time you will know something about how birds take to the 

 air and how birds, bats, and insects maintain their flight. When the 



