LIGHT AND MOVEMENT 



59 



frequently occur when one eye only is illuminated, are directionally very exact 

 and do not depend on the persistence of the stimulus— a primitive kind of 

 menotaxis. 



The execution of these movements of orientation in insects is the 

 result of a complex series of coordinated reflexes in the wings or legs of 

 both sides, each of which is specifically correlated to the location of the 



Figs. 34-36. — The Orientation of the Robber-fly, Proctaca.\thvs, on a 

 White Background in a Horizontal Beam of Light. 



Fig. 34. — The upper portion of the left eye and the lower portion of the right 

 eye are covered. The insect is leaning to the left and turning to the 

 right towards the light. 



Fig. 35. — One leg has been removed on 

 the right side while the light conies 

 from the left. The insect is seen turn- 

 ing to the left towards the light guided 

 largely by its left front leg. 



Fig. 36. — When the light comes from the 

 right, in order to orientate itself in this 

 direction, the left front leg is thrown 

 over to the right side and is used to 

 pull the animal in this direction (after 

 Mast, 1924). 



stimulus in the eyes. The excitation of a particular retinal area induces 

 a reaction w^hich orientates the insect in a direction such that the 

 continuous turning allows successive retinal points to be stimulated 

 until the fixation ommatidia are reached ; once this orientation has 

 been attained, the reflexes become inoperative, and if any subsequent 

 deviation occurs further reflex re-orientation immediately corrects it. 

 These reflexes are somewhat analogous to the segmental scratch- 

 reflexes in higher mammals, and their effects have been explored 

 experimentally (as by rotatory experiments on a turn-table) in a large 



