367 



THE STRUCTURE OF THE EYE-SURFACE, AND THE 

 SEXUAL DIFFERENCES OF THE EYES IN DIPTERA. 



By W. Wesche, F.R.M.S. 



Plate 28. 

 (Read March 5th, 1909.) 



The fact that the eyes of the male insect are larger, in the 

 majority of cases, than those of the female is well known, and 

 this difference is generally accepted as a secondary sexual character. 

 We are probably correct in assuming that a greater amount of 

 eye- surface gives room for a greater number of facets, and a 

 greater number of facets gives increased power of vision, and 

 consequently enables the insect more readily to find his mate. 

 We see this character carried to its extreme development in 

 many flies, as in the Pipunculidae, where the head of the male is 

 almost entirely occupied by the compound eyes, and the female 

 has acquired only a lesser measure of the same development. The 

 Cyrtidae have gone a step farther, as in one species, Oncodes 

 gibbosus, L., the female has arrived at an equally extreme state of 

 development with the male. 



Williston * mentions that there are cases in which the female 

 has gained the character in advance of the male, giving a 

 reference to the Cyrtidae and an instance in the Bombylidae. 



Regarded as a sexual character, Osten-Sacken has called 

 the contiguity of the eyes of the male and the separation of the 

 eyes of the female " holoptic." As opposed to this, there are 

 multitudes of species and genera where no such difference exists, 

 the eyes being widely and equally separated in both sexes, as 

 in many of the Mycetophilidae, Chironomyidae, Tipulidae, and 



* ''Antennae of Piptera ; A Study in Philogeny," Bio. Bull., vol. xiii. 

 No. 6, p. 331, November 6, 1907 : New York. 



