— 45 — 



AH these distinctions clearly prove that here we are dea- 

 ling with the so called ..differentiation into a double eye". 

 As was shown by Dietrich's investigations such a differen- 

 tiation is to be found in different degrees in a very great 

 number of representatives of Diptera. As a rule a bilate- 

 rally differentiated eye is peculiar only to the male, this se- 

 condary change taking place only in the dorsal part of the 

 eye, in the, so called, „Dorsalauge". It is only in Empidae 

 that we find „double eyes" in both sexes, and in some of 

 them, e. g. in TacJujdrotnia miniita, according to Dietrich, 

 the most differentiated part of the eye is its ventral part. 

 Thus, in Diopsidae the „anterioreye" deviates in development, 

 the term here having a relative meaning. The deviation in the 

 structure of one part of the eye from the other is, as a 

 matter of fact, so insignificant, that it is hardly advisable to 

 speak of a special „eye" in this case. Tlie essential point is 

 that one part of tlie eye, the anterior in our case, differs 

 from the other part of the eye in its histological structure 

 and is specialized for a special function, which will be trea- 

 ted in the next paragraph. 



The physiological and biological Significance of the stalked Eyes. 



In literature there are hardly any data concerning the sig- 

 nificance of the eye-stalk in Diopsidae. We know very little 

 of the life of Diopsidae, and, at any rate, nothing that 

 could be of value in explaining the curious development of the 

 stalks. The Diopsidae Jive partly along the shadowy banks 

 of streams and rivers in virgin forests, and partly inhabit 

 the grass of the plains, where they are exposed to the sun. 

 There is no doubt, however, that these insects are rapacious. 

 Such a nature of these insects is evident on account of their 

 strong anterior legs adapted to grasp, the broad femora 

 of which are provided from the interior surface with 

 denticles and destined for catching live prey ^). A similar 

 structure of the anterior legs is found in another well known 

 rapacious fly — Oethera mantis. 



I think that the development of stalked eyes in Diopsi- 



1) In one of the specimens I had at my disposal I found a torn head 

 of an ant, which points to the character of its prey. 



29 



