on the vision of Arthropods. 407 



There are one or two points as to which, I think, 

 further and more special experiments are needed. I may 

 mention that I think a set of observations should be 

 made to test to what extent the covering the optic organs 

 with pigment is effectual in excluding light from them. 

 It is admitted that it is only partially effective ; and it 

 appears to me that when this is the case there is not 

 sufficient ground for concluding that the insect is guided 

 by a dermatoptic sense. 



I think, too, that observing and delineating the actual 

 tracks made by particular species when escaping from 

 the labyrinth would be of considerable value ; the tracks 

 shown by Prof. Plateau in his figures being, as he tells 

 us, diagrammatic. 



The question of the actual direction taken by insects 

 in flight appears to me by no means a simple matter. It 

 must be looked on as a resultant of several conditions. 

 Different species have different habits in regard to the 

 height and direction of their ordinary flight, and this 

 must very greatly influence the direction they take when 

 endeavouring to escape. When an insect has been 

 subjected to such an unpleasant experience as being 

 captured and having its eyes covered with blackened oil, 

 it is only natural to assume that it would, when the 

 bonds were taken from it, attempt to simply get away. 

 The habitual direction in which an insect would start to 

 fly away when it was liberated in a natural condition 

 must be taken into consideration. If the usual way of 

 a particular species be to rise a little and then change 

 its direction in accordance with influences of light and 

 shade that may come into play after it has started, it is 

 not surprising that it should continue to rise when (the 

 function of its eyes being interfered with by pigment) 

 the normal sources of orientation do not affect its 

 nervous system. 



Perhaps I may be allowed, in conclusion, to summarise 

 the general impressions I have derived from the study 

 of Prof. Plateau's valuable experiments as follows : — 



1. That insects in motion are guided largely by the 

 direction of light and the existence of lights and shades. 

 That when walking they are guided by a combination of 

 light-impressions, with specific habit (that is, going 

 upwards or downwards, towards the light or away from 

 the light), and by tactile impressions; these latter not 

 acting when the insect is in flight. 



