LIGHT AND MOVEMENT 



63 



for reference to a distant source of light, flies to its death in the nearby- 

 flame. 



This behaviour is not constant in moths. If a number of these insects is 

 introduced into a room where a candle is situated on a table they will take up 

 positions on the table around the light with their heads turned towards it. 

 As a rule, one by one they take wing ; the first may fly arovmd the flame in 

 diminishing circles until it passes through it to fall in flames into the molten 

 mass of wax beneath ; the next will similarly follow to commit deliberate suicide ; 

 and so on the procession goes, some perishing in the flame itself, others escaping 

 with singed wings to fall on the table when, with wings too charred to use, they 

 may crawl with difficulty up the candle and walk straight into the base of the 

 flame to die. While most fly around the flame in decreasing circles, some may 

 fly straight into it ; others remain upon the table apparently worshipping from 

 afar, while others again wander aimlessly about the room paying no attention 

 to the light. The cause of this variation in conduct is quite unknowTi ; it seems 

 to indicate that the phototactic response is not entirely determmed on a mechanis- 

 tic level. 



The navigational sense in birds is an astonishing example of the 

 accuracy of a modification of the light -compass reaction. It has long 

 been knowai that young birds will undertake their initial migration from 

 one continent to another unaccompanied by their parents and arrive 

 in the correct habitat with extreme precision, and that homing birds 

 such as the pigeon or the gull, released in an unkno\m area in random 

 directions, will rapidly head straight for home in a dh-ect line of flight 

 (Matthews, 1951-55 ; Kramer and St. Paul, 1952 ; Kramer, 1953). 

 The Manx shearwater, Puffinns, for example, transported to America, 

 has homed 3,050 miles across the Atlantic wastes to arrive after \2\ 

 days in its own particular burrow on an island off the west coast of 

 England (Matthews, 1953). It is obvious that in navigational feats of 

 this type visual orientation is quite inadequate and a bi-coordinate 

 orientating mechanism of great accuracy must exist. It is true that 

 many birds show a relatively simple positive phototactic response, 

 flying towards an illuminated patch or the lighted end of a long dark 

 tunnel — a primitive reaction still carried out after ablation of the 

 cerebral hemispheres (j^igeons, Viaud and Marx, 1948) ; but it is 

 equally true that they are capable of executing the most complex type 

 of orientation. 



For years this navigational ability of some birds has excited the 

 curiosity of naturalists. Several explanations have been explored such 

 as an acceleration-displacement recording mechanism or an ability to 

 exploit the earth's magnetic field, but they have all been discredited 

 by experiment ^ ; nor do the structural arrangements apparently exist 

 in the eyes of birds as in the compound eyes of insects to appreciate the 



1 Gordon (1948), Matthews (1951-55), Yeagley (1951), van Riper and Kalmbach 

 (1952). 



Puffinus 



