on the vision of Arthropods. 401 



Part IV. 



Vision hy means of compound eyes. a. Anatomico-physio- 

 logical summary, h. Comparative experiments on 

 insects and on Vertebrata. 



In this fourth section Prof. Plateau passes to the 

 subject of compound eyes. Discussing in a prefatory 

 manner the present state of anatomical and physiological 

 knowledge, he inclines to adopt the views of Patten, that 

 the perception of the compound eye is by means of a 

 retinidium located in the crystalline cone, or in a corre- 

 sponding situation in such insects as do not possess a 

 true crystalline cone ; and he considers if this be the 

 case there can be no true perception of form or image 

 because of the great extent in the vertical direction of 

 the retinidium. 



After this more than sixty pages are devoted to an 

 account of his own experiments, and to descriptive and 

 critical remarks on the observations of other naturalists 

 as to the powers of visual perception of insects with 

 compound eyes. 



Plateau's first series of experiments was conducted in 

 the room lighted on one side only, and consisted of 

 placing small moveable objects — more especially vertical 

 sheets of cork of white or neutral colour — in the way of 

 insects to see if they were avoided, and also of placing 

 the insects in the centre of the labyrinth or maze already 

 alluded to, and observing their ways of making their 

 escape from it. 



Gryllotalpa vidgaris, when placed in the maze, actually 

 ran against each piece that was in its way, whether the 

 piece were light or dark, shaded or illuminated. The 

 same insect, when placed on the floor of a room, did not 

 apparently see the pieces placed in its way, even when 

 these were of white colour and fully illuminated. 



Periplaneta orientalis was tested, and it was found that 

 it proceeded with its long antenme extended, and walked 

 directly towards the obstacles until these were touched 

 with the antennae : when this occurred the object was 

 explored by the antenna, and when its limits were deter- 

 mined by these it then passed the obstacle, the track 

 being of the nature indicated by ccc, fig. 1. But this 

 insect distinguishes between a dark object and an object 

 reflecting much light, for it explores the latter delicately 



