AND THE SEXUAL DIFFERENCES OF THE EYES IN DIPTERA. 375 



opposite extremity of the abdomen (Diopsis apicalis). In the 

 females it is considerably less, and species are found with many 

 degrees of extension, in some the stalks of the eyes being stout 

 and short. It is a widely distributed family, as examples are found 

 in North America, in West and South Africa, and in Ceylon. 



The lenses are of the usual type found in the Muscidae, with- 

 out any opaque structure ; but the eye-surface is greater (and 

 this in both sexes) than in Diptera of similar size. 



5. The Pubescence on the Eyes. 



This is found in a large number of cases equally developed in 

 both sexes, as in genera of the families Mycetophilidae, Dolicho- 

 podidae, Syrphidae, and Muscidae ; but there are many species 

 in which it is a sexual character. 



In the Nemocera, when pubescence is present as a non-sexual 

 character, the hairs are short and stiff, sharply pointed, and 

 obviously socketed in the subquadrate space between four lenses, 

 and a similar condition can be seen in many Dolichopodidae in 

 the Brachycera. But in Bibio, where the pubescence is a male 

 sexual character, the eyes of the females being bare, they are thin, 

 weak, but very numerous, and of extreme and unusual length. 



In Haematopota it is a sexual character, as the eyes of the 

 female are only very faintly pubescent or bare, while the greater 

 part of the eyes of the males are thickly haired. H. pluvialis, S > 

 has the chitinous facets of the eyes thinly haired, but where the 

 structure has disappeared the hairs are long and numerous — two 

 to each facet, as against one to four facets in the chitinous area. 

 The eyes of II. ]rtuvialis, ? , are bare, and H. crassicornis, Whlb., 

 ? , and H. italica, Mg., ? , have only a few scattered hairs on 

 these organs. 



In the Anthomyid genus Hyetodesia there are some cases in 

 which the pubescence on the eyes of the females is shorter or much 

 less than on the eyes of the other sex. Hyetodesia dispar, Fin., 

 II. umbratica, Mg., H. perdita, Mg., H. erratica, Fin., H. basalis, Z., 

 H. rufipalpis, Mcq., and H. lasiophtkalma, Mcq., are examples. 



