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certain types of plants "compute" the average length of sunlight per 

 day to find out when to flower, or when to shed their leaves. Others 

 use the length of night, and still others the average light-to-darkness 

 ratio. Another example is the sense of time, which is poor in most 

 humans. Certain animals, for example, cockroaches, have a much more 

 highly developed sense of time than man does. 



In some phenomena, such as bird homing, it is not well understood just 

 what sensory cues or information the animal does use. In others, 

 particularly echo-location, an understanding was developed only after 

 physical analogs had been constructed. Before this, it was beyond 

 human conception to design the proper experiments, even though these 

 experiments could have been readily carried out. To put it in a some- 

 what different fashion, human intuition is often a poor guide to experi- 

 mental design. Someone must not only develop the proper ideas but 

 also be persuasive enough to interest his peers. 



In the following section, the ability of bats to use echo-location 

 in flight, in capturing prey, and in avoiding obstacles is set forth. Many 

 years ago a few persons, perhaps by chance, hit on the correct solution 

 to how a bat senses its surroundings, but these solutions were discarded 

 by their contemporaries as absurd. Pasteur said that chance favors the 

 mind prepared by study and experimentation. We might add, it also 

 favors the man who lives in an age in which his contemporaries are like- 

 wise prepared. 



Besides bats, other mammals and some birds use echo-location. These 

 are also discussed in this chapter. Bees use sensory information for 

 direction-finding and homing; this involves time and orientation senses, 

 and also perhaps the ability to detect polarized light. Making a beeline 

 for home is discussed in Section 4. The concluding section of this 

 chapter deals with bird navigation and homing. 



2. Echo-Location in Bats 



In many families of bats, the sense of vision is poorly developed, hence 

 the colloquial expression, "blind as a bat." It has been shown by direct 

 experimentation that bats fly, hunt, and avoid obstacles, as well when 

 blindfolded as when their eyes are open. Anatomically, the visual 

 portion of the bat brain is very poorly developed, whereas the acoustic 

 or auditory portion makes up a major part of the brain. This suggests 

 that they sense their surroundings through auditory stimuli. Indeed, 

 deafened bats, or ones with their ears covered over, cannot fly well, 

 avoid obstacles, or hunt in the same fashion as normal bats. Covering 

 the bat's mouth (and nose) also interferes in a like manner with its 



