— 47 — 



the facetted eye is not at all more highly developed, or adap- 

 ted to a more intensive function. The direction of the stalks 

 corresponds to the direction of the optic nerves, as it is 

 usual in flies. It gives the impression that in Diopsidae the 

 optic nerve has become elongated in the same direction that 

 it had from the very beginning. The result of such an elon- 

 gation of the optic nerves was that the eyes diverged from 

 each other and became widely separated. In the circumstan- 

 ces given the object which is at a certain distance in front 

 is seen not only with one, but with both eyes, therefore 

 it is quite possible that the insect is capable of binocular 

 vision in the sense that the object seen twice is identified. 

 The possibility of this kind of vision is still more supported 

 by the fact that the facetted eye is directed in its anterior 

 and median portions somewhat medially, and bears the lar- 

 gest facets here. If we assume the presence of binocular vision, 

 it will be clear that the more both the facetted eyes' diverge, 

 the more plastic the vision becomes. It is true that the bi- 

 nocular vision is limited to the forward direction, whereas 

 from above and from below, or from behind the convexity 

 of the eye is too small to allow any object in these directions 

 to fall into the field of view of both eyes simultaneously. 



Explanation of Figures on the Plate. 



a. antenna. o. eye. 



c. cornea. oc. ocellus. 



f. p. layer of nerve-bundles. P'. chief pigment cell. 



g. 0. e. exterior optical ganglion P". collateral pigment cell, 

 g. 0. i. interior optical ganglion. p. d. eye-stalk. 



m. b. basilar membrane. psc. pceudocone. 

 n. P'. nucleus of chief pigment r e t. retinula. 



cell. 



n. P". nucleus of collateral pig- rh. rhabdomere. 



ment cell. 



n. ret. nucleus of retinula. st. gr. granulous layer. 



n. s. nucleus of, Semper. st. m. molecular layer, 



nv. ret. retinular fibre. Tr. trachea. 



Fig. 1. End of right eye-stalk with eye of Diopsis varians, dorsal aspect. 

 Sketched with the help of a binocular microscope 3% 



Fig. 2, The same, ventral aspect. 



Fig. 3. Right eye of D. pulehella. Sagittal section from above and 

 from behind (dp), downwards and forwards (va). The approximate direction 



29* 



