300 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 1 



ent, we know little about the movements of a bat's wings, but as far 

 as we can see, the motion of the bat's wings appears to be similar to 

 the motion of a bird's wings, and it seems likely that the same gen- 

 eral principles would apply to both (pi. 2, right). What is charac- 

 teristic of a bat's flight is, however, its ability to avoid obstacles when 

 flying in the dark. At one time this very useful faculty Avas believed 

 to be due to sense organs on the wings capable of registering the slight 

 alterations in air pressure which must follow when the moving wing 

 came too close to some such solid obstacle. It has been known, how- 

 ever, for many years that a deaf bat frequently collides with obstacles. 

 It has now been discovered that the vocal chords of a bat can give out 

 sound waves of very high frequency, far higher than those to which 

 a human ear can respond : these waves — not to be confused with the 

 squeak of bats which some people can hear — are of 30,000-50,000 

 vibrations per second, and there can be little doubt that a flying bat 

 sends out these supersonic waves and listens to their reflection from 

 surrounding objects. In effect, the bats have evolved a very efficient 

 echo-sounding equipment. We can make a model of such a system. 

 A Galton whistle capable of sending out sound waves at 50,000 cycles 

 per second, is mounted underneath (but shielded from) a telephone 

 capable of responding to waves of this frequency. Then, if no obstacle 

 lies in the path of the whistle's outgoing high waves, the telephone 

 will not respond. But if any object is present that can reflect sound, 

 the outgoing waves will be reflected back and picked up by the tele- 

 phone where they can be made audible to an observer by means of an 

 instrument reducing their frequency to a level to which human ears 

 will respond. A model of this kind will instantly detect the presence 

 of a glass window. 



Among the invertebrates, only insects appear to have solved the 

 problem of sustaining themselves in the air by means of wings. In 

 most cases, a lift equal to the weight of the insect's body develops 

 only when the wings are forced through the air by the insect's own 

 muscular effort; the red admiral butterfly is one of the few insects 

 capable of gliding flight. As with birds, the smaller the insect the 

 more frequently does it beat its wings. A swallowtail butterfly beats 

 about 5 times, a hive bee or a horsefly about 200 times per second. 

 The record, so far as is known, is held by the males of a small midge 

 {F ercepomyia) whose wings vibrate more than 1,000 times per second 

 and produce a very high-pitched note. ISIuscular activity of this 

 order requires a great deal of energy, and most insects can only main- 

 tain active flight if they are reasonably warm and supplied with plenty 

 of food. We can measure the "fuel" (or food) requirements by 

 allowing a small fruit fly {Drosophila) to beat its wings until all the 

 food reserves in its body are exhausted : this will be after 2-4 hours 

 of flight, according to the age of the fly. Flies wearied in this way 



