OCELLI 111 



significance, and represent reversions to an ancestral condition, will 

 immediately suggest itself in the light of foregoing remarks. Such an 

 explanation, however plausible, remains an open question in the absence 

 of any knowledge of the directing causes involved. 



Little is known with respect to the functions of the dorsal 

 ocelli. The fact that the lens is strongly biconvex indicates that 

 their visual powers are limited to the perception of very near 

 objects. The small number of visual elements which compose 

 an ocellus is strongly suggestive that any image-forming capacity 

 it may possess would be of a crude and indefinite kind. The 

 perfection and complexity of the structure of ocelH in many 

 insects argue that they are functionally important organs, but 

 it needs to be emphasised that the power of perceiving even only 

 crude images apparently necessitates a somewhat complex visual 

 organ. 



It has been suggested that the dorsal ocelli are adapted for the 

 perception of very near objects and for vision in darkness or 

 subdued light. This view^ seems, however, to have no proper 

 evidence based upon actual experiment. In so far as ants and 

 bees are concerned, it has been shown by Miiller (1931) that when 

 the compound eyes of these are painted over the creatures no 

 longer exhibit the capacity of orientating themselves in relation 

 to light. A growing opinion favours the idea that dorsal ocelli 

 are general stimulatory organs which have the property of " toning 

 up " the nervous system and increasing general sensitivity of the 

 brain. It is now believed that apart from any imperfect photo- 

 tactic function the ocelli exercise a photokinetic influence in, at 

 any rate, day-flying insects, by helping to maintain the sensitivity 

 of the visual centre of the brain to light stimuli. It is possible, 

 therefore, that the dorsal ocelli perform this function as imjDortant 

 accessories to the compound eyes. Here again the experimental 

 evidence is rather meagre, but Bozler (1925) has shown that in 

 Drosophila the response to light stimuli, received through the 

 compound eyes, is more acute and lasting w^hen the dorsal ocelli 

 are in their normal condition than when painted over with an 

 opaque substance. A similar function has been ascribed to the 

 ocelli of Lepidoptera by Friedrich (1931). 



