INTRODUCTION. 3 



that, in accordance with the external structure, it forms a striking 

 contrast with the organization of all the other dipterous families. 



The head of the Dolichopodidse in general is more or less he- 

 mispherical. Its sides are occupied by the large, ovate, hairy 

 eyes (naked only in Medeterus), which usually reach so far down 

 that in most genera there are no cheeks (genae) at all, and that 

 on the under side of the head, between the eyes, there is just space 

 enough for the large opening of the mouth, which, in some spe- 

 cies, even encroaches upon a considerable portion of the occipital 

 side of the head. The usually rather broad front bears three 

 ocelli, protected by strong bristles, and several bristles in both 

 corners of the vertex; the remainder of its surface has no bristles. 

 The frontal fissure and lunule* are indistinct. The antenna are 

 three-jointed, of very different structure in different genera, nay 

 even often in the sexes of the same species ; the second joint is 

 sometimes rudimentary. The arista is two-jointed, seldom alto- 

 gether bare, generally with a very short pubescence, more seldom 

 fringed with longer hairs or even plumose ; its dorsal or apical 

 position affords very constant distinctions between different gene- 

 ra ; its length and the other characters belonging to it differ 

 sometimes even in the two sexes of the same species. The face 

 is usually vertical and smooth ; it has often on its inferior third 

 an elevated transverse swelling, or at least, on each side, near the 

 orbit of the eye, a nodule-like elevation ; seldom does the face 

 reach down to the inferior angle of the eye ; it is always without 

 any bristles, although sometimes it is hairy ; its lower edge is 

 rather sharp, and does not coalesce with the border of the mouth ; 

 from under this edge project the single-jointed palpi, incumbent 

 upon the proboscis (except in Aphrosylus, where they hang down 

 on its sides) ; they are usually scale-shaped, more seldom lancet- 

 shaped (for instance, in Orthochile) or round, pallet-shaped (in 

 Diostrocus) ; in the females they are usually considerably larger 

 than in the males ; sometimes however (in Biosiracus), the oppo- 

 site is to be found. 



The proboscis is generally short and (with the exception of 

 Aphrosylus and Orthochile) stout ; its opening is wide, more or 

 less surrounded by the protuberant suctorial flaps, which, by clap- 



* For the explanation of these terms, compare Monographs, Vol. I, p. xii. 

 0. S. 



