t 



EVASION OF BATS BY MOTHS — ROEDER AND TREAT 461 



available. In plate 5, C, the "buzz" was picked up by one ear only, 

 presumably because during this part of its performance the chirps of 

 a bat are much less intense. 



It is tempting to estimate just how close the bat must be before the 

 moth fails to get information on its location. If it is assumed that a 

 bat is first detected at 100 feet and approaches on a straight path at 

 right angles to the moth's course while making chirps of constant 

 loudness, the differential tympanic nerve response would diminish 

 throughout the approach and disappear completely when the bat was 

 15 to 20 feet away. However, we have not yet determined how much 

 of the information that we are able to read out of its auditory mecha- 

 nism is actually utilized by the moth in its normal behavior. 



THE EVASIVE BEHAVIOR OF MOTHS 



Although the evasive behavior of moths in the presence of bats must 

 have been witnessed hmidreds of times, it is hard to find an adequate 

 account of the maneuvers of either party. The contest normally takes 

 place in darlaiess, and, even when it is illuminated by a floodlight, the 

 action is too fast and complex to be appreciated by the eye. The flight 

 path of the bat and its ability to intercept and capture its prey have 

 been studied by Griffin (1958) and his students. More recently, Web- 

 ster (in press) has shown by means of high-speed sound motion 

 pictures that bats become adept at using echoes to plot an interception 

 course with an object moving in a simple ballistic trajectory. Many 

 people have noted the seemingly erratic dives and turns made by moths 

 when bats are near, and similar behavior has been described when 

 moths are exposed to artificial sources of ultrasound (Schaller and 

 Timm, 1950; Treat, 1955). 



In an effort to learn more about the behavior of moths under field 

 conditions their flight was tracked photographically as they reacted to 

 a series of ultrasonic pulses simulating bat cries. The sounds were gen- 

 erated by the equipment used in the experiment shown in plate 3, figure 

 1. The pulses were similar in form to those shown, although longer in 

 duration (6 msec). Each pulse ranged from 50 to 70 kc. with a rise 

 and fall time of about 1 msec. Pulse sequences up to 50 per second 

 could be released on closure of a switch. The sounds were emitted by 

 a plane-surfaced condenser loudspeaker mounted so as to project a 

 fairly directional beam over an open area of lawn and shrubs illumi- 

 nated by a 250- watt floodlight. 



The observer sat behind the somid generator and floodlight, holding 

 in one hand the cable release of a 35 mm. camera set on "bulb," and in 

 the other the switch controlling the onset of the sound-pulse sequence. 

 Many moths and other insects flew out of the darkness into this flood- 

 light arena. A number were attracted directly to the light and were 



