AERIAL AND TERRESTRIAL DIPTERA 
195 
and fro pendulum-motion of the Homalomyiae in our rooms ? Or 
the perpendicular up and down motions of Rhyphus , IIybos, and 
Phora ? In this respect there is much room for investigation 
yet. 
Having done with the generalizations on my subject, I proceed 
now with their application in the different groups and families of 
Diptera. 
I begin my exposition with the Brachycera. 
In each of the superfamilies of the Brachycera , one of the above- 
defined types is prevalent. But at the same time, as I said above, 
each of the superfamilies contains exceptional forms, showing a 
distinct tendency towards the life-habits of the opposite tj-pe. I 
repeal, that in such cases the modification of structure runs paral¬ 
lel with a change of life-habit: a Dipteron, although belonging to 
a prevailingly aerial superfamily, assumes under such a change of 
circumstances some of the characters of terrestrial Diptera, and 
vice versa. 
The most numerous and thriving family among the Tromoptera , 
the Bombyliidae , are principally aerial; they have holoptic heads in 
the male, well-developed wings, a remarkably regulated flight, 
comparatively weak legs, and a moderate development of almost 
imperceptible macrochaetae. But among the Bombyliidae there is 
the heavy Toxophora , a pedestrian among aerial forms, with com¬ 
paratively short wings, but stout legs, and showing an unusual 
development of stout macrochaetae on the thorax, and even a pair 
of ocellar bristles on the head. In complete contrast with Toxo¬ 
phora is another Bombylid, Systropus , aerial to the core, slender in 
shape, entirely bare of hairs and bristles, but, as if in compensation, 
holoptic (I may say) with a vengeance, because the eyes offer the 
rather rare instance of a complete contact of the eyes in both 
sexes. (Compare my paper 158 , 1897 , “ Preliminary Notice,” etc., 
Berl. Ent. Zeit ., p. 368-369, for details about Bombyliidae and 
the exceptional position, among the Tromoptera , of the small and 
specialized families of Nemestrinidae and Cyrtidae.') 
The superfamily Energopoda are essentially pedestrian, and hence 
the species are usually dichoptic in both sexes; they have abun¬ 
dant macrochaetae and strong, variously specialized legs. But 
