PREY CAPTURE IN MANTIDS 67 



Unfortunately the experiments planned to give information about the 

 fixation-deficit after eliminating the hair plates were executed before I 

 knew the distance rule mentioned and the fact that the jerks normally are 

 directed to the sides of the prey. For mere technical reasons the fly (in the 

 first apparatus) was presented at a distance of more than 30 mm. and 

 hence faced by saccadic movements almost exclusively. The head devia- 

 tions (fx), if plotted against the fly deviations (a) showed a considerable 

 spread and, at first sight, it seemed impossible to detect whether the mantis 

 had faced the left or the right side of the fly. The difficulty was overcome 

 by considering the way the data were obtained. As the fly normally was 

 moved from the median position ((7 = 0) to the right or left sides respec- 

 tively (a = ± 60°) while the reverse occurred very rarely, it should be 

 assumed that, in the majority of presentations on the right side of the 

 mantis, the left side of the fly (as seen from the mantis) was faced and vice 

 versa. Hence the regression function of head deviation /x on fly deviation <t, 

 if calculated for each side separately, should result in two parallel re- 

 gression lines separated by an abscissa value corresponding to the width of 

 the fly. Furthermore their slopes should be larger than the one calculated 

 from the total set. This was indeed so in 3 out of 4 sets. The result is given 

 in the following table : 



SLOPES OF THE REGRESSION FUNCTIONS OF 



HEAD DEVIATION (m) ON FLY DEVIATION ((7) 



FOR TWO INDIVIDUAL MANTIDS 



It can be seen that 



(1) In all cases, the slopes obtained after bilateral proprioceptive de- 

 afiferentation are larger than those obtained before. 



(2) In both individuals tested, the differences between the total slopes 

 before and after operation are highly significant with P < 0.0002. 



(3) The means of the left and right slopes (with one exception probably 

 due to an unfavorable accumulation of a values around the median po- 

 sition) are about 10% larger than the totals. 



(4) The left and right slopes after deaft'erentation are close to unity. 



