PROTECTIVE MIMICRY. 229 



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

 voted himself to elaborate experiments upon insect vision, have 

 an 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 uunmtilated, 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 Hyrneno- 

 pterou 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. Ent. Sac., 1889, p. 393. 



