PREY CAPTURE IN MANTIDS 



57 



The result is seen in Fig. 5. With the exception of one series carried out 

 with two individuals which, even in the unloaded state, had a considerable 

 deviation from the normal frequency of misses, a significant decrease in 



20 30 ^0 50 60 



loa d [mg cm] — *■ 



10 20 30 W 50 60 

 load [mgcmj -»- 



Fig. 5. Result of loading the head with an extraneous mechanical force. The 

 angular momentum caused by the load is so directed as to turn the head to the right 

 (for method see text). Ordinate: frequency of misses and left tendency, respectively. 

 Abscissa: angular momentum in mg.-cm. (mg. here is a unit of force, not of mass). 

 Solid circles, first experiment (2 individuals) ; open circles, second experiment (3 

 individuals) ; double circles, third experiment (3 individuals). In the last named 

 the standard error is plotted. Figures at the values: number of strokes (n). 



performance is reached not earlier than at a load of 50 mg. cm. That is a 

 remarkable achievement if we consider that the head of Parastagmatoptera 

 weighs 25 mg. and hence there is a load of twice the head weight at a 

 distance of twice the head diameter. Thus we may be certain that loads 

 which can be expected to occur in normal life (for instance, if the animal 

 is catching the second fly while eating the first) are perfectly ruled out by 

 the mechanism at hand. 



Introduction of an Hypothesis 



Now we have gained sufficient information to put forward an hypothesis 

 concerning how the mechanism works. If we only had to account for the 

 first result, of deafiferentation, we would assume that the hair plates pro- 

 vide for the disputed additional information about head position. If we 

 knew only the second result, of head fastening, we certainly should be 

 advised to the contrary, namely to assume that there is no such influence 

 at all. But it may be concluded from the third that the truth will be some- 

 where between ; and the last result could give an idea of how this compro- 

 mise is expected to work. 



Evidently the mechanism is very sensitive to or easily upset by changes 



