LIGHT AND MOVEMENT 



45 



inhibited by associations established by conditioned reflexes ; thus the 

 photo -negativity of the cockroach, BlateUa, can be inhibited by training 

 if a hght is placed over its dark shelter (Goustard, 1948-50). It is also 

 to be remembered that the removal of necessary effector organs may 

 inhibit or invert a normal phototactic response even although these 

 have no apparent connection with photoreceptors (the antennae of the 

 cockroach or the wings of the fruit -fly, Drosoj^hila , Goustard, 1949). 



In the second place, these resjjonses are all of biological value and 

 to attain this end they may vary with the strength of the stimulus or 

 change their character if associated with a second stimulus of another 

 nature ; moreover, they may alter in type and even reverse their 

 nature during the life of the animal to meet the needs of a change in 

 environment. 



Thus the usual photo-negative response (the shadow-reflex) seen 

 in so many worms and molluscs is essentially an escape movement 

 from the presence of predators, while the opposite response of the 

 tentacle of the snail is the expression of the fact that a shadow usually 

 signifies food. Some of these responses are very sensitive : thus the 

 acorn-shell, Balanus, responds to a darkening of 5% (v. Buddenbrock, 

 1930). The simplest example of a variation in the response with the 

 strength of stimulus is seen in the protozoon, Euglena, which is photo- 

 positive in weak and negative in strong light so that it orientates itself 

 to favourable mid-intensities (Mast, 1938), or in the fruit-fly, Droso- 

 -phila, which is positively phototactic in illuminations below 9 lux 

 and negatively over 79 lux (Medioni, 1954). A similar variation may 

 occur with the nature of the light ; thus the flat -worm, Planaria 

 lugubris, is said to be positively phototactic to red and negatively to 

 blue light (Viaud, 1949). Again, other environmental circumstances 

 may alter the response. Paramcecium is geo-positive in the light and 

 negative in the dark (Fox, 1925) ; the normal negative phototaxis of 

 the goldfish, Carassius auratus, disappears if the temperature is in- 

 creased by 10° C (Andrews, 1952) ; the normal positive phototaxis of 

 the tsetse-fly, Glossina, becomes negative if the temperature is raised 

 above 40° C even if the temperature in the dark is so high that it drops 

 down dead (Jack and Wilhams, 1937) ; exposure to dry air alters the 

 phototactic reaction of the woodlouse, Armadillidium, from negative 

 to positive (Henke, 1930) ; while the negative response of the ohgo- 

 chsete, Perichmta, when it is extended can be changed to a positive 

 response when the worm is contracted (Harper, 1905). 



An excellent example of a change in response with different combinations 

 of stimuli is seen in the behaviour of Littorina neritoides, a tiny mollusc which 

 inhabits the rocky shores of Etu-opean seas. Fraenkel (1927) showed experi- 

 mentally that it was always geo -negative, photo -negative always when out of 

 the water and when normally orientated in the water, but photo -positive when 



Balanus 



0^ 



Carassius 



■'^=-^ 



Glossina 



Littorina 



