THE RESPONSE OF POPILLIA JAPONICA TO LIGHT AND 

 THE WEBER-FECHNER LAW. 



By a. R. MOORE and W. H. COLE.* 

 (From the Riverton Entomological Laboratory, Riverton, N. J.) 



(Received for publication, October 26, 1920.) 



The Japanese beetle, PopiUiajaponica, is a recent importation from 

 Japan and is found in a limited area in central New Jersey. 



During July and the first half of August the beetles may be seen 

 collected in great numbers at the tops of trees, bushes, and weeds. 

 It seems to be immaterial whether the plant is a linden tree 50 feet 

 high or a smar tweed plant a foot high — ^the beetles always occupy 

 the uppermost foliage. This is due to the fact that the direction of 

 movement in these beetles is the additive resultant of two tropistic 

 responses; namely, positive phototropism and negative geotropism. 

 These tropisms determine the head-tail orientation of the body. 



Both in the field and in the laboratory a certain degree of heat and 

 of light is necessary for the active movements of Popillia. Thus, 

 below 23°C. the beetles are generally inactive; 38-39°C. is optimum 

 for their activity, while above 40°C. injurious effects are apparent. 

 At 45°C. activity ceases quickly and permanently. Under the ruby 

 light and in the dark most of the beetles become quiet and show no 

 response to gravity. Occasional individuals which do move show a 

 retarded response. All are roused to activity by illumination from 

 any direction, while a great increase in the strength of the light causes 

 marked acceleration of movement and flying. 



In addition to this kinetic action of light there is also a directive 

 effect, which may be demonstrated by illuminating the beetles first 

 from above and then from below and timing their response in each 

 case. Without exception they ascend the wire gauze of their cage 

 more rapidly in the first instance, although in the latter case the 



* The apparatus for this work was furnished by Rutgers College, New Bruns- 

 wick, N. J. 



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