180 OCELLI OF CAVE-DWELLING SPIDERS. 



Lebert has expressed the opinion * " that in spiders 

 some of their eight eyes — those which are most convex 

 and brightly coloured — serve to see during daylight ; 

 the others, flatter and colorless, during the dusk." 

 Pavesi has observed t that in a cave-dwelling species 

 (Nesticus speluncarum), which belongs to a genus in 

 which the other species have eight eyes, the four 

 middle eyes are atrophied. This suggests that they 

 serve specially in daylight. 



Keturning for a moment to the ocelli of true insects, 

 it seems almost incredible that such complex organs 

 should be rudimentary or useless. Moreover, the 

 evidence afforded by the genus Eciton seems difficult 

 to reconcile with this theory. The species of this 

 genus are hunting ants, which move about in large 

 armies and attack almost all sorts of insects, whence 

 they are known as driver ants, or army ants. They 

 have no compound eyes, but in the place of them 

 most species have a single large ocellus on each side 

 of the head, while others, on the contrary, are blind. 

 Now, while the former hunt in the open, and have all 

 the appearance of seeing fairly well, the latter con- 

 struct covered galleries, and seek their prey in hollow 

 trees and other dark localities. 



Insects with good sight generally have the crystalline 

 lenses narrow and long, which involves a great loss of 

 light. The ocelli are specially developed in insects, 

 such as ants, bees, and wasps, which live partly in the 

 open light and partly in the dark recesses of nests. 

 Again, the night-flying moths all possess ocelli ; while 

 they are entirely absent in butterflies, with, accord- 



* " Die Spinnen der Scliweiz." 



t " Sopra una nuova Specie di Ragui." 



