600 BIOLOGICAL EFFECTS OF RADIATION 



at right angles, it goes toward a point between the two beams. The loca- 

 tion of this point depends upon the relative intensity of the two beams. 

 The greater the difference, the nearer the more intense beam is this point 

 (Mast, 140; Patten, 183; Dolley, 49). 



Results obtained in observations made on Eristalis tenax by Dolley 

 and Wierda (56) show that with the intensity of the two beams so related 

 that when the insect is oriented the image in one eye is near the anterior 

 end and that in the other eye is near the posterior end, the one receives 

 50 times as much light as the other. That is, these results show that to 

 produce a given effect on turning, an image near the anterior end of the 

 eye must be 50 times as intense as an image of the same size near the 

 posterior end. This indicates that the efficiency of light to induce 

 turning decreases rapidly as the location of the image passes forward 

 in the eye. In an oriented insect under these conditions the two eyes 

 are equally illuminated only when the light in the two beams is equal 

 (Mast, 148). Parker (182) and Cole (39) demonstrated that a given 

 amount of light in a large image has a greater effect on turning than the 

 same amount of light in a small image. It is therefore evident that 

 when an insect is oriented under natural conditions, the two eyes are 

 rarely if ever equally illuminated. 



If photopositive insects on a nonreflecting background are (with a 

 concentrated source of light) illuminated on one side from behind, all the 

 legs on the illuminated side step backward and all on the opposite side 

 forward. If illuminated directly from the right, the front legs on both 

 sides step toward the right. If illuminated directly from in front, all the 

 legs on both sides step forward. This obtains also for insects with only 

 one functional eye. The movement of the legs on both sides is controlled 

 by photic stimulation of either eye and the direction of movement 

 depends upon the location of the stimulus in the eye (Mast, 148, 149). 



Photopositive insects with the front and the middle legs on one side 

 removed orient nearly as precisely as normal insects. If they are laterally 

 illuminated from either direction, the front leg is extended toward the 

 light, attached and then flexed. Thus the anterior end is pulled either 

 toward the operated or toward the normal side, depending upon which 

 side is more highly illuminated. After they are oriented, the front leg is 

 extended nearly directly forward, attached, and then flexed. Thus they 

 proceed slightly sidewise but fairly directly toward the light, with the two 

 eyes continuously unequally illuminated (Mast, 148). 



Insects with the function of one eye destroyed tend to turn continu- 

 ously toward the functional eye if they are photopositive, and toward the 

 blind side if they are photonegative, and some of them lean in the direc- 

 tion in which they turn, the legs on one side being sharply flexed and 

 those on the other side much extended (Holmes, 109; Parker, 182; 

 Rddl, 189; Dolley, 49; Garrey, 75). 



