i6o 



COMPARATIVE PHYSIOLOGY 



unequal muscle tension reflexly excited by unequal illumination 

 of the eyes leads an animal to turn into the position in which 

 (moving towards or away from the source of light) the two eyes 

 are equally illuminated, it follows also that blackening of the 

 eye should lead to circus movements. Such circus movements 

 have been shown by Loeb and many other workers — Parker, 

 Holmes, Lyon, among others — in various insects, and are 

 well illustrated by Carrey's (19 19) experiments on the Robber 

 fly (Protacanthus). These experiments support Loeb's 

 hypothesis and researches on the unilateral removal of the 

 cerebral ganglia in insects (cf. Matula, 191 1), 

 and others confirm the view that the ner- 

 vous mechanism of muscle tonus is pre- 

 dominantly unilateral. However, crossed 

 ^^/i reflexes are also involved in the forced 



v^^<. movements of insects, since Mast (1924) 



has shown that when the posterolateral 

 border of one eye is illuminated the limbs 

 of one side move forward and those of the 

 opposite side backwards, the front feet 

 towards the light and the hinder ones away 

 from it, thus showing that the location as 

 well as the intensity of the stimulus in the 

 photoreceptors of either side is involved in 

 the reflexes which underlie orientation. 

 This does not invalidate the fundamental 

 conception underlying Loeb's contribution 

 to the problem, though it shows that a 

 complete analysis of the phenomenon is a rather more 

 intricate task than he himself supposed. 



FHes with normal eyes ascend either a plane or cylindrical 

 surface vertically. When one eye is blackened they ascend 

 a plane surface obliquely, veering towards the unblackened 

 eye (Fig. 40). If made to ascend a cylindrical surface equally 

 illuminated on all sides, the insect with one eye blackened 

 ascends with a spiral motion towards the seeing eye, the 

 number of spirals depending upon the intensity of illumina- 

 tion. If the cylinder is obliquely placed so that one side is 



Fig. 40. — Effect of 

 unequal illumi- 

 nation on climb- 

 ing insect. 



