CLASSIFICATION OF DIPTERA. 19 



The Nemocera anoniala are characterized by the following distinc- 

 tive features : 



i. Head holoptic, both in the male and female, but chiefly in 



the former. In Bibio, for instance, the ? has non-holoptic 



eyes, 

 ii. Eyes often bisected, the upper facets being the larger. These 



two halves may be separated by a smooth, unfacetted stripe, 



as in the ^ Bibio. 

 iii. Legs well adapted for walking, and often, as in Bibio and 



Simulium, thick. 

 iv. Male and female generally differ very much. 

 V. Peculiar, often sporadic geographical distribution. 



Upon these characters Osten-Sacken has founded the two divisions 

 of the Nemocera. Both from these characters, their curious distri- 

 bution, and their archaic nature, we see that this division is quite a 

 natural one, and is founded on more scientific principles than most 

 entomological classifications. Amongst the Orthorrapha Brachycera 

 he classifies the following families into one group : the Tabanidce, 

 Stratiomyidce, Acanthomeridte, LeptidcB (plus Xylophagidae). For a 

 long time these families have been converging towards one another, 

 and at last have been united under the name (given by him) Eremo- 

 c/iceta. This seems a natural group, and is founded on a chcetotactic 

 character ; namely, the total absence of Macrochaetae. 



Besides the bristleless nature of these forms, they are separated 

 from the Nemocera and remaining Brachycera by the following 

 characters, given by Osten-Sacken : 



1. The eyes in the male are predominantly holoptic. There 



are, however, a few exceptions to this rule given by him ; 

 viz., Xylophagus, some Australian ChiromyzcB, Hermetia^ 

 etc. But Cienoniyia, a close relation to Xylophagus, has 

 holoptic eyes. 



2. The variegated colour of the eyes, especially seen in Stratio- 



myidce. and Taba7iidce. 



3. The extreme variability of the antennae, on the one hand 



related closely to the Nemocera, and on the other to the 

 Cyclorrhapha. 



4. Three well-developed pulvilli are generally present. Legs 



smooth, and having no bristles, as seen in the Asilidce, 

 Bombylidce, etc. 



The following, then, are the families recently grouped by Osten- 

 Sacken : 



