120 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



objects. On the whole it appears quite improbable, but experi- 

 ments dealing with problems of this kind are very few. Gaffron 

 raised the (question as to whether stroboscopic movement and 

 after-images of movement occur in insects also. The answer 

 appears to be partly in the positive and partly in the negative. 

 The nymph of the dragon-fly Mschna responds to the movement 

 of any small object and will never respond to any kind of food 

 which is stationary. This creature can, however, be made to do 

 so by previous or simultaneous strong movements in the surround- 

 ing field, such as a screen which revolves around the receptacle in 

 which the animal is contained. When such movement has been 

 actuated for about ten minutes, the after-effect lasts for about 

 half an hour after such movement has ceased. Flies which are 

 responding very regularly to a striped revolving screen cease to 

 move if this real movement is replaced by an irregular appearance 

 and disappearance of the stripes at different points on the screen 

 (which produce a semblance to movement in the human eye). 

 Thus, there appears to be no appreciation of stroboscopic move- 

 ment in these insects at any rate. On the other hand, insects 

 like Coccinella, which are known to display light-compass reactions, 

 do respond to the interrupted appearance and disappearance of a 

 pattern if the change be not too fast. Experiments of this kind 

 show that it is necessary to differentiate between movement 

 responses and light-compass reactions which have sometimes been 

 confused with them. 



In connection with the subject of light intensity and its wave- 

 length, it may be noted that the most modern work of this kind 

 has been carried out by Sander (1933) in Gottingen. In his 

 experiments the light energy of twelve different spectral lines, 

 including ultra-violet (366 fifi), was made equal to that of the 

 weakest line, the absolute amount of energy of any line being 

 436-10-^ watt per cm.^ Each one of the twelve different rays 

 was compared with another in the two windows of a small dark 

 cage in which a group of bees was confined. The number of bees 

 passing, during repeated exposures, at the two windows was 

 counted and results plotted in twelve curves. The curves showed 

 two maxima — in the yellow and blue ; and three minima — in 



