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 only,| and we have fre- 

 quently observed in birds, particularly those which 

 feed on insects (Sijlviadce, Merulidce, S^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 ( Yiinx 

 iorquilla) is enabled to move its head in the manner 

 from which it derives its popular name ; and many in- 

 sects, such as the dragon-flies (Libellulina), can turn 

 their heads nearly round about ; though, from the 

 great volume of their eyes, this might almost be con- 

 sidered 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 be not only useful for this purpose, but illustrative 

 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. 

 ■i Mem. d'Acad. Berlin, 1768, p. 80. 



