DIFFICULTY OF SUBJECT. 177 



Eeanmiir, Marcel de Serres, Duges, and Forel also 

 have shown that in insects which possess both ocelli 

 and compound eyes, the ocelli may be covered over 

 without materially affecting the movements of the 

 animal ; while, on the contrary, if the compound eyes 

 are so treated, they behave just as if in the dark. For 

 instance, Forel varnished over the compound eyes of 

 some flies (Musca vomitoria and Lucilia csesar), and 

 found that, if placed on the ground, they made no 

 attempt to rise ; while, if thrown in the air, they flew 

 first in one direction and then in another, striking 

 against any object that came in their way, and being 

 apparently quite unable to guide themselves. They 

 flew repeatedly against a wall, falling to the ground, 

 and unable to alight against it, as they do so cleverly 

 when they have their eyes to guide them. Finally, 

 they ended by flying straight up into the air, and quite 

 out of sight. It seems, indeed, to be a very general 

 rule that insects of which the eyes are covered, 

 whether they are totally blinded, or whether the ocelli 

 are left uncovered, fly straight up into the air — a very 

 curious and significant fact of which I think no 

 satisfactory explanation has yet been given. 



Plateau * regards the simple eyes, or ocelli, as rudi- 

 mentary organs of scarcely any use to the insect. Forel 

 also states, as the result of his observations, that wasps, 

 humble bees, ants, etc., find their way both in the air 

 and on the ground, almost equally well without as with 

 the aid of their ocelli. 



I confess that I am not satisfied on this point. In 

 such experiments great care is necessary. M. Forel's 

 interesting experiments with ants, whose compound eyes 



♦ Bull, de VAcad. Roy. de Btlgique, t. x., 1885. 



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