on the vision of Arthropods. 403 



obstacles. NecropJiorus vespillo and Telephorns lividus, 

 when tried, knocked against all the obstacles in their 

 way. Geotriqjes sylvaticus explored objects it came 

 against by means of its antennae, and when these were 

 cut off, used one of its front legs for the purpose of 

 exploring an obstacle. 



Hymenoptera were tried, and gave most instructive 

 results. I shall translate literally Professor Plateau's 

 remarks (Part IV., pp. 39, 40), on these clever and 

 active insects : — 



"My first experiments on H^^menoptera, both deprived 

 of their wings and unmutilated, whether moving in the 

 maze or tested by means of the vertical obstacle at the 

 end of a stick, astonished me profoundly. 



" These insects appeared to guide themselves amongst 

 the obstacles wdth remarkable certainty, avoiding the 

 barriers when these were at a distance, and apparently 

 behaving in every respect like creatures possessing good 

 powers of sight. 



" Certainly, if I had contented myself with a few 

 superficial observations, I should have been persuaded 

 that Hymenoptera are an exceptional group possessed of 

 definite vision. 



" This illusion — a very pardonable one — was due to 

 the rapidity of action of the creatures on which I made 

 my first observations. The strangeness of the results 

 having induced me to make fresh experiments, I dis- 

 covered some species whose ambulatory movements were 

 less rapid. This enabled me to analyse the details, and 

 to detect the explanation, as simple as it was certain, of 

 the facts. 



" This explanation may be thus summarised : The 

 hymenopteron directs its course, with but few exceptions, 

 straight towards the light, that is, towards the windows. 

 In such conditions the obstacles forming the maze, or 

 those placed at the end of a stick, give rise, according 

 to their position, to a shadow cast in front of them, a 

 shadow which is, in fact, of a double nature, a faint 

 one, or penumbra, and a darker and narrower shade 

 (PI. XIII., fig. 2). 



"If in pursuing a straight course the creature haps 

 on a gap between the obstacles, it naturally takes advan- 

 tage of this, as indeed a beetle would do ; but if across 

 its road there intervenes an upright obstacle, the 



