ECHOES AS MESSENGERS 



parallel to each other are at just the correct angle so that 

 they float in the surface film. But more specialized still 

 are the hairs located on one particular joint of each an- 

 tenna, the second from the base. These specialized hairs 

 are more than flotation devices; at the base are sensitive 

 nerves that are stimulated by the most minute move- 

 ments of the hairs relative to the remainder of the beetle. 

 Eggers surmised from the microscopic structure of these 

 hairs and nerves (see Fig. 5) that they were used to 

 detect motion of the water surface, and he therefore 

 experimented with them directly. In some beetles he 

 damaged the second segments of the antennae, cut 

 off the hairs on this portion, or damaged only the 

 nerves leading from the bases of these hairs into the 

 central nervous system of the insect. When these water 

 beetles were placed on the surface of an aquarium in 

 the dark, they acted as bewildered as a bird fluttering 

 against a windowpane and collided at random with the 

 walls. 



Other experiments have shown that the sense organs 

 of insects can respond to very weak vibrations. A move- 

 ment of as little as 4 X 10"® centimeter is detected by 

 the sensory nerves attached to fine hairs on the surface 

 of some insects which are generally similar in structure 

 to the whirhgig water beetles. There is thus no reason 

 to be amazed that water beetles can feel the surface 

 waves generated by their own swimming or walking 

 movements. What is amazing is their ability to discrimi- 

 nate the jiggling that results from reflected waves from 

 all the other vibrations that must be affecting the same 

 hairs and the same sensory nerves. This is a problem 

 which the beetles may avoid to a considerable extent by 

 their habit of swimming intermittently, with frequent 

 pauses during which they may perhaps be feeling the 

 "reverberations" of the water waves their swimming has 



55 



