THE ANALYSIS OF BEHAVIOUR IN ANIMALS i6i 



in the shade, insects with one eye blackened move in very 

 different paths on the two sides : *' On the shaded side the 

 spirals are parallel and the pitch is acute, but in the Hght of 

 the other side the fly's path is more nearly horizontal, as 

 would be expected from the different conditions of muscle 

 tonus resulting from light of different intensities " (Garrey). 

 Loeb (1890) first showed that if rotated on a turn-table flies 

 describe circus movements in the opposite direction to the 

 movement of the table. These, as later shown by Lyon (1900), 

 do not occur when both eyes are blackened. In Carrey's 

 experiments flies were rotated on a revolving cylinder 

 illuminated from above. The normal fly circles towards the 

 opposite direction, in its ascent thus describing a spiral path. 

 If the speed is sufficiently increased a horizontal path may be 

 induced. When one eye is blackened, the forced motion is 

 intensified if the cylinder is rotated towards the same side and 

 the vertical component is nullified with a much slower motion. 

 When rotated with the blackened eye in the opposite direction 

 to the motion the circus movements are diminished and at 

 a certain speed the fly ascends vertically. 



The conclusion that different regions of the eye in 

 insects are, as implied in Mast's observations, related to 

 different reflexes involved in orientation is of special interest 

 in connexion with a phenomenon studied by Parker (1922) 

 in young turtles. Newly hatched loggerhead turtles find 

 their way from their nests to the sea in consequence of at least 

 three factors, one depending on gravity as shown in their 

 tendency to move down slopes, one which is a response to 

 localised retinal images in that they move towards regions 

 of the horizon which are open and clear rather than interrupted, 

 and finally a response to colour, since they move towards blue 

 areas. The first of these may be described as geotropism, 

 but the last two are types of reaction rather more complex 

 than those to which the term " photo tropism " is customarily 

 applied. 



In most animals it is but rarely that one set of stimuli 

 predominates over all others to the extent that light does in 

 many insects. More frequently the normal orientation and 



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