FUNCTION OF OCELLI. xlv 



species of insects also have a tendency to become 

 blind. Accommodation to circmnstances, also, would 

 appear to occm' in these Lakes, but in a different 

 manner. There are certain larvae whose tracheae 

 become charged with water, instead of air. Dr. Forel 

 states that where these larvte abound, the pup^ rarely 

 occur, and that they have never been seen to rise to 

 the surface to become images. On the other hand, he 

 thinks than these larva3 are capable of perpetuating 

 their kind through the development of sexual organs. 

 If these insects never come to the surface it is clear 

 that they must, like fish, extract the air which is 

 dissolved in the water. ( Vide ' La Faune profonde 

 des Lacs Suisses,' par Dr. Forel de Merges, Basle, 

 1885.) 



In the AphididaB, which are so nearly allied to the 

 Tettigidae, we find several genera, including numerous 

 species, which are quite bhnd. These species are 

 almost wholly subterranean. {Vide ' British Aphidinas,' 

 vols. i. to iv., Eay Society.) 



Very little is certainly known as to the office of the 

 ocelli. By experiment Plateau found that it made 

 little or no difference to certain insects, so far as their 

 flight was concerned, whether their ocelli were or 

 were not covered by a black varnish. F. Muller 

 thought that the function of the ocelli was to focus 

 near objects ; others have suggested that they are only 

 efficient in feeble lights. Spiders, however, must have 

 acute vision, and yet they have no compound eyes. 



As yet no perfectly blind Cicada has been recorded. 



The whole subject of insect vision is most interesting, 

 yet at the same time not a little puzzling. We may be 

 arguing inconclusively, and from unsuitable premisses, 

 that our own visual sensations are identical with those 

 of insects. The images formed from the lenses of the 

 insect eye seem to be formed not as with us, directly 

 upon the pigment of the retina, but in advance of the 

 same. We are also perplexed with the question how 

 from three to four thousand images, all of which are 



