AND THE SEXUAL DIFFERENCES OF THE EYES IN DIPTERA. 373 



being found in the female Pipunculidae. The Phoridae, Lon- 

 chopteridae, and Asilidae seem to have retained the dichoptic 

 eyes, and examples of both conditions are met with in the 

 Empidae. 



There are no examples in the Muscidae which have such 

 extreme development as the Pipunculidae, although the majority 

 of males are distinguished by the holoptic character ; but in the 

 Acalyptrates many have retained the dichoptic eyes. 



2. The Divided Eye of the Male. 



This I know of only in the Bibionidae, although there are 

 certain Coleoptera (the Gyrinidae) which have a somewhat 

 similar structure. It is, however, not a sexual character. 



In Bibio and Dilophus the lower portion of the compound eye 

 is divided from the upper, and appears as a separate part, with 

 its lenses looking downwards. There is, however, another 

 difference, which is not so easily notice;!, and that is that the 

 facets of the lower portion are considerably smaller in size than 

 those of the upper. A measurement of the parts in Bibio 

 hortidanus gave me a proportion of eight units for the upper 

 facets, as against live units in the lower facets ; five units 

 appears to be the normal size, as all parts of the eye of the 

 female gave this measurement. The eyes of Dilophus febrilis 

 were also measured, and gave similar proportions and results. 



The facets in the eye of the male were also measured in 

 two undetermined species, one from New Zealand, and a similar 

 proportion was found to exist. 



Dilophus nigrostigma, a very large species (proportionally), 

 shows a band of chitin which separates the two divisions of the 

 eye, and I have given a figure of a small portion (Fig. 12). 



3. Variations in the Size of the Lenses (Facets) of the Eyes. 



This is a character which, until I found it on the head of a 

 single female of Leptogaster (an Asilid), I thought was confined 



