406 Dr. Sharp's account of recent experiments 



A. One orifice being stationary and the other, of the same 

 size, in motion. — The insects directed themselves in- 

 differently to one or the other, no preference being 

 observed. 



B. The two orifices of the same size, but one arranged 

 so as to admit a greater quantity of light than the other. — 

 The insects directed their flight most frequently to that 

 where the light was most intense, whether this were the 

 stationary or the moving one. 



C TJie two orifices of unequal size, one being four 

 inches each way, the other two inches. — The insects then 

 preferred the larger orifice, whether stationary or moving. 



The final series of experiments recorded by Plateau 

 refers to a curious phenomenon, viz., that winged insects, 

 whose eyes are covered by pigment, when they are set 

 at libert}^ fly vertically upwards to a great elevation. 

 Plateau had previously considered that this was due to 

 their still perceiving the light, partly perhaps because 

 the pigment did not completely blind them, and perhaps 

 partly because of a general dermatoptic sense. Some 

 objections having been made to this conclusion by Forel 

 and others, he undertook a fresh set of experiments with 

 Nocturnal Lepidoptera : the specimens were released 

 when it was quite dark, and after their eyes had been 

 covered with black pigment. The result was found to be 

 that the insects under these conditions did not rise 

 vertically, but either flew away horizontally or descended 

 to the ground. 



[In Prof. Plateau's account of this set of experiments 

 I do not find any statement as to whether the com- 

 pound eyes and the ocelli were both covered with pig- 

 ment or only the former. — D. S.] 



In concluding this imperfect resume of Prof. Plateau's 

 interesting researches, I cannot refrain from expressing 

 my thanks to him for having given himself so much 

 trouble in the matter. His observations, I think, show 

 conclusively that whatever may be the other functions 

 of the optical organs, their relation to light is the chief 

 mode by which the insect is guided when on the wing : 

 W'hen walking it seems, however, probable that tactile 

 sensations and specific habit are the chief factors in 

 deciding a particular direction. 



