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work bears on European Diptera only, it was successful 

 in demonstrating the great variety existing in the histological 

 structure of the eyes even in families closely related to each 

 other, and in the different sexes of the same species. The 

 variations in the histological structure of the eyes may con- 

 cern the most different elements: e. g., the dioptric apparatus, 

 the number, position and form of rhabdom cells, the develop- 

 ment of pigment etc. The sexual dimorphism of the facetted 

 eyes, found chiefly in Orthorrapha, consists in the for- 

 mation of so called „double eyes" in the males (except in 

 Empidae, in which both sexes possess double eyes). 



Dietrich provides a physiological explanation for many 

 of the named modifications of the eye, his point of view 

 being that the form of the eyes is in connection with the 

 conditions of life to which each given species is subjected. 



It is quite probable that the formation of stalked eyes in 

 Diopsidae has also a certain biological foundation. First of 

 all, the question arises whether the adaptation of the eye of 

 Diopsidae to certain special functions is confined to the 

 formation of the stalk, or together with this there take place 

 also changes in the histological elements of the eye. The 

 present work is devoted chiefly to the solution of this question. 



External morphology of the eye. 



The facetted eyes of Diopsidae are situated on the 

 distal ends of long immobile stalks which present outgrowths 

 of the head arising from the sides of the forehead (fig. A). 



For comparison I had some species of the genera Diopsis 

 L. and Diasemopsis Rond. The degree of development of the 

 stalks varies slightly in different species. In some species the 

 stalk is short and broad, not exceeding the width of the head 

 in length (e. g., Diojjsis ornata Wstw.). In a great number 

 of species, however, the stalk is longer than the head and 

 thorax taken together (e. g., Diasemopsis varians, fig. A), in 

 this case it is always very thin. 



Generally, in the species with long eye-stalks the head is 

 of a relatively smaller volume, than in the species with short 

 stalks. This gives an impression that in the latter case the 

 stalks are developed at the expense of the material used for 

 the formation of the head. 



