606 BIOLOGICAL EFFECTS OF RADIATION 



time required to travel from the bottom to the top of the vessel. More- 

 over, there were, owing to the inversion of the vessel, radical changes in 

 the locations of the stimuli in the eyes and in the intensity of the illu- 

 mination of the photosensitive elements, and the observed results only 

 very roughly approximated the calculated values. It is therefore 

 obvious that the conclusions reached by Moore and Cole are not well 

 founded. 



Welsh (215) maintains that as the light increases from a low intensity 

 (0.185 foot-candle) the rate of locomotion in the water mite Unionicola,^ 

 increases rapidly to a maximum at about 70 foot-candles and then remains 

 practically constant and that the increase in the rate is the result of 

 increase in both the frequency and the length of the steps. He concludes : 

 "The change in length of stride is due to tonus changes in the leg muscles; 

 this is due to the tonic effect of light, and is directly proportional to the 

 logarithm of the light intensity." But why he considers the change in 

 the length of the steps due to change in tonus rather than to change in the 

 magnitude of contraction of the muscles, is not clear. The mites were 

 fully dark-adapted, and then light-adapted for 2 to 3 min. before each 

 test. They were therefore more nearly completely light-adapted in the 

 tests made in the lower than in those made in the higher intensities. 

 This doubtless accounts, at least in part, for the difference in rate of 

 locomotion observed. Moreover, there was considerable change of 

 light intensity during each test. 



Clark (33) in observations on the whirligig beetle, Dineutes, avoided 

 changes of intensity during the tests, and he ascertained the effect of 

 adaptation as well as the effect of light intensity on rate of locomotion. 

 He reached the following conclusions: In fully dark-adapted specimens 

 the rate of locomotion in light of 4000 meter-candles is at first relatively 

 high; then, as the time of exposure increases, it gradually decreases to a 

 minimum, after which it remains constant. In light of 8000 meter- 

 candles it is at first relatively low; then it gradually increases to a maxi- 

 mum (which is the same as the preceding minimum), after which it 

 remains constant. 



In specimens which are fully adapted to the light in which they are 

 exposed, the rate of locomotion increases (with increase of intensity) to a 

 maximum at 0.5 meter-candle, then remains constant until 8000 meter- 

 candles is reached, and then decreases. The condition of adaptation of 

 the eyes is consequently a very important factor in the relation between 

 the rate of locomotion in insects and luminous intensity. 



As a whole, the evidence in hand does not warrant any precise con- 

 clusions, concerning the quantitative relation between the stimulus and 

 response to light in insects, but it shows that to ascertain this relation 



^ Unionicola is an arachnid, not an insect. I include it because its responses to 

 light are similar to those of many insects. 



