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INVERTEBRATE PHYSIOLOGY 



85%, the difference being insignificant. Yet the difference between each 

 slope and 100% is highly significant. Thus I don't see how to avoid the 

 assumption that, at the end of a jerk, the head has a position very near to 

 that which it would have if the circuit had reached its final steady state. 

 In view of our problem it is particularly interesting what happens if the 

 prey comes nearer to the reach of the forelegs. Here again, especially at 

 the beginning of the experiment, a tendency to face the sides of the fly is 

 observed in some cases. But in the period before the stroke is released, the 

 picture normally changes (see Fig. 10). There are still two pathways, but 



o 

 o 



+10" 



g -10' 



Y 



-m 



-20' 



-10° 0° +10' 



<^ Fly I Pro: horax 



+20' 



Fig. 10. Same procedure and same mapping as in Figs. 8 and 9, but the fly is pre- 

 sented at a distance smaller than 30 mm. The head followed the fly exclusively by 

 continuous movements. Open circles and full lines : head positions while fly is moving 

 from left( — ) to right ( + ) ; solid circles and broken lines : head positions while fly is 

 swinging from right ( + ) to left( — ). The fixation-deficit is equal to 1 minus the slope 

 of the curve and hence about 15%. 



they differ by not more than about 2° and, if the prey moves from left to 

 right, the head at o- = is about one degree right from the center of the fly 

 and vice versa, thus being slightly in advance. We get an estimate of the 

 achievement produced here while considering the width of the fly and the 

 fact that the best ommatidia of this insect have angular apertures of about 

 one degree. The lines have slopes of 82 to 88%, in accordance with the 

 first result. Thus, the fixation-deficit being about 15%, the total circuit 

 amplification can be calculated to be nearly 6. 



