PKOTECTIVE MIMICRY. 229 



The observations of M. Plateau, who has particularly de- 

 voted himself to elaborate experiments upon insect vision, have 

 iin important bearing upon the matter now under discussion.* 

 As regards the compound eyes of Hymenoptera, he has written 

 as follows : " My first experiments on Hymenoptera, 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 ap- 

 peared to guide themselves amongst the obstacles with remark- 

 able 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 discovered 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 fact. 



This explanation may be thus summarised : The Hymeno- 

 pteron 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. 



* My information about these papers has been drawn from an excellent 

 review by Dr. David Sharp, in Tram. Eiit. Soc, 1889, p. 393. 



