408 ON FUNCTION OF COMPOUND EYES AND OCELLI. 



sects saw near objects with their ocelli. Plateau satis- 

 fied himself that the movements of insects are not 

 affected by the ocelli being covered over, and hence 

 concluded that they are rudimentary organs. The 

 complexity of their structure, however, seems fatal to 

 this conclusion. 



Forel confesses that the use of the ocelli still re- 

 mains an enigma, but he is disposed to think that they 

 enable their possessors to see in comparative darkness. 

 He observes ' that they are specially developed in in- 

 sects which require to see both in bright light and also 

 in comparative obscurity. Aerial insects do not gene- 

 rally require or possess ocelli. 



Lebert expresses 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 colourless, during the dusk. Pavesi 

 has observed^ that while the species of Nesticua possess 

 normally eight eyes, in a cave-dwelling species {Nesti- 

 cus speiuncarum) there are four only, the four middle 

 eyes being atrophied. This suggests that the four 

 central eyes serve specially in daylight. 



Sense of Smell. 



In my previous memoirs I have recorded a few 

 experiments which convinced me that ants are gifted 

 with a very highly developed sense of smell, and that 

 this resides in the antennae. Forel, Graber, Lefebvre, 

 Perris, and other recent writers have come to the same 

 conclusion, and there can, I think, be no reasonable 

 doubt that in very many insects the antennae serve as 

 organs of smell. At the same time it does not neces- 

 sarily follow that the sense of smell should be confined 



» Z. 0. p. 181. 



* Die SjAniien der Schrveiz, p. 6. 



• ' Sopra una nuova specie di Ragni appartenente alle collezioni 

 del Maaeo Civico di Genova,' Ann. JUus. Oi*. 1873, p. 344. 



