124 INSECT MISCELLANIES. 



before reaching the brain. The latter would have 

 perhaps been satisfactory, had it not been refuted 

 by the simple experiment of pushing one of the 

 eyes a little aside, when objects will be seen double, 

 though this cannot alter the meeting of the nerves. 

 Dr. Wells explains it by the coincidence of what he 

 calls the visible direction *. 



Whatever opinion be adopted, it is evident that 

 most creatures can see an object by using one eye 

 only, sometimes better than when both are employed. 

 The celebrated painter, Leonardo da Vinci, upon 

 this principle recommended his pupils always to look 

 at distant objects with one eye onlyt, and we have 

 frequently observed in birds, particularly those which 

 feed on insects (Sylviadce, Merulidce, #c.), that on 

 looking out for prey, they most commonly turn their 

 head on one side, so as to bring only one eye to bear 

 on the object. A thrush always does so when he ex- 

 amines a snail- shell that he means to attack, and a 

 red-breast before he pounces upon a worm. It is no 

 doubt for this very reason that the wryneck (Yunx 

 torquilla) is enabled to move its head in the man- 

 ner from which it derives its popular name ; and 

 many insects, such as the dragon-flies (Libellulina), 

 can turn their heads nearly round about ; though, 

 from the great volume of their eyes, this might al- 

 most be considered superfluous. 



Most spiders have eight, though some only six 

 eyes, and these are so variously arranged, that their 

 positions have been employed by systematic writers 

 for distinguishing the genera and species; and as it 

 may not only be useful for this purpose, but illustra- 

 tive of the subject immediately before us, we shall 

 here give figures of the position of the eyes of a 

 number of spiders. 



* Phil. Trans, for 1792 and 1811. 

 f Mem. d'Acad,, Berlin, 1768, p. 80. 



