604 BIOLOGICAL EFFECTS OF RADIATION 



side to side and then crawl much more rapidly than they did in the lower 

 intensity (Mast, 140). 



Various attempts have been made precisely to ascertain the relation 

 between luminous intensity and activity in insects. These attempts 

 concern the rate of deflection toward one side with the two eyes unequally 

 illuminated, and the rate of locomotion with the two eyes equally 

 illuminated. 



a. Rate of Deflection with the Two Eyes Unequally Illuminated. — Dolley 

 (49) found in observations on Vanessa with one eye covered that in 

 vertical illumination on a black background, the rate of deflection varies 

 inversely with luminous intensity if the source of light is small, and 

 directly if it is large and if there is much reflection from the background. 

 The direct relation has been observed in a number of other insects. 

 Garrey (75) says, e.g., that the robber fly on a glass cylinder surrounded 

 by a much larger one of white mat paper illuminated from above, takes a 

 spiral course as it proceeds toward the light and that "the brighter the 

 light, the greater the number of spiral circuits required to make the 

 ascent"; and Minnich (170) obtained similar results in observations on 

 the honey bee. But they present no details concerning this relation. 

 Clark (35), however, in observations on Dineutes, found that the rate of 

 deflection does not increase uniformly as the intensity increases. He 

 says that as the intensity increases, the rate of deflection increases to a 

 maximum and then remains constant, and that on a black background 

 the rate of increase is much more rapid than on a white background 

 and the maximum not so high. He concludes that the rate of deflection 

 depends upon the state of adaptation as well as upon the amount of 

 reflection from the background and the size of the source of light. 



Mast (140) maintains that if opposite sides of fly larvae are unequally 

 illuminated, the degree of deflection from the more highly illuminated 

 side varies directly with the difference in the intensity of illumination 

 of the two sides. Patten (183) confirms this. 



Mast (148), in observations on Eristalis tenax in a field of light con- 

 sisting of two horizontal beams which crossed at right angles, measured 

 the angle of deflection from the median in relation to the relative intensity 

 of the beams. The results obtained show that in reference to the effect 

 on deflection, the rhabdomes in the posterior portion of the eye are much 

 more sensitive than those in the anterior portion, but they have little or 

 no bearing on the quantitative relation between stimulus and response. 



Yagi (232) on the basis of results obtained in the walking stick 

 Dixippus, concludes that "photic excitation is proportional to the 

 logarithm of the intensity." This insect, on a vertical wall, assumes, in 

 response to gravity, a vertical axial position. Yagi measured the angle of 

 deflection from the vertical, in response to lateral illumination of different 

 intensities. He maintains that this angle varies directly with the inten- 



