Diptera: yemocera vera, N. anomala and JEremochaeta. 449 



and Mexico with bisected eyes; aniong them Plecia plagiata Wied., 

 also a Plecia from Sydney, Australia. At the same time the Pleciae 

 from South-Eastern Asia, whicli I liave seen, for instance Plecia 

 fulvicollis Wied. and P. forcipata 0. S. from Sumatra, do not show 

 any trace of bisection. 



This difference in tlic structure of the eyes in the genus Plecia 

 was unknown to Loew when, in the Berl. Ent. Zeit. 1858, p. 116, he 

 discussed the question of the fusion of Penthetria and Plecia. Does 

 the presence or absence of bisection alone, without support of other 

 characters, justify generic Separation? We have many genera (to begin 

 with Taharms) in which bisection in holoptic heads offen, but not 

 always, occurs, without ever having been used for generic Separation. 

 I merely call attention to a character hitherto much neglected, without 

 pretending at once to solve the involved questions.i) 



Bihio and Dilophus are distributed nearly all over the world, 

 and are represented in each region by a considerable number of 

 species. A centre for the genus Dilophus is Chili, from which twenty- 

 five species have been described (including the genus Acanthocnemis 

 Avhich is but a slightly modified Dilophus). From Australia, on the 

 contrary, we have but a Single well-authenticated Bibio, four Di- 

 lophus and four or five Pleciae. Plecia is principally tropical. — 

 The monotony in form and color among the multitude of species of 

 Bibionidae from all parts of the world is remarkable: the colors 

 are generally black and red, sometimes yellow. Spodius Lw. (Ilespe- 

 rinus Wk.) has been found in Hungary, in the British possessions 

 of North-America, in the White-Mountains, New Hampshire, and on 

 the heights of the Rocky Mountains in Colorado; I also saw a spe- 

 cimen brought by Whymper from the altitude of 11 — 13000 feet in 

 the Andes of Peru. The occurrence oi Hesperinus in Brazil (Sc hi- 

 ner 's Novara, p. 23) requires confirmation. Pachyneura has been 

 found in Swedish Lapland only. The abundance of the Bibionidae 

 (Bibio ■^), Plecia) in different geological strata is wellknown. Loew 

 discovered two species of Plecia and a Single specimen of Dilophus, 

 but no Bibio., in the Prussian amber (Bernstein etc., p. 39). 



The larvae of Bibio and Dilophus with their horny head, a 

 complete set of mouth-organs, and the characteristic, symmetrically 



i) I hope to find oecasion, in a future publication, to euter with 

 more detail into the question of the relation of holopticism with bisection, 

 ancl into other questions connected with the eyes of Diptera. 



2) Are the fossils described by Heer from the lertiary formations 

 as belonging to the genus Bibio real Ribios? Compare Loew, Zeit, 

 f. d. Ges. Nnturw. 1808, vol. XXXIl, p. 181, sqq. 



XXXVII. Heft IV. 29 



