200 
AERIAL AND TERRESTRIAL DIPTERA 
plays a part not unlike that of the Empidae among the Energo- 
poda: although terrestrial in their general appearance and their 
leading characters, they display an aerial tendency in their power of 
regulated flight, necessary for their aerial dances, and in the struc¬ 
ture of the head in the male, which I have called p>$eudoholoptic 
( Berl . Ent. Zeit., 1896, p. 867) in order to distinguish them from 
genuine lioloptic heads. 
To return to the aerial Syrphidae , there are forms among them 
which show a pedestrian tendency in the use and development of 
their legs. Such are Xylotae running upon leaves with their stout 
hind legs, and showing a minimum of contact of the eyes in the 
male. Syritta , a relative of Xylota, is seen rummaging with its 
strong hind legs among stamens of flowers; it also hovers around 
them, but is not a high flier; it does not scan a wide horizon, and 
therefore is not provided with an lioloptic head. Other Syrphidae 
exhibit the same pedestrian and terrestrial tendency in the devel¬ 
opment of comparatively numerous macrochaetae. Conspicuous 
among these is the genus Hammerschmidtia (allied to Brachyopa') 
which has the aspect of a pedestrian fly and resembles an Htdomyza ; 
the macrochaetae, for a Syrpliid, are unusually abundant, and the 
contact of the eyes in the male is reduced to a minimum. 
Male holopticism, although very common among Syrphidae , is, 
in this family (as well as in others), not a very deep-seated char¬ 
acter. Some phylogenetically closely allied genera differ from each 
other in regard to holopticism. The connection between this char¬ 
acter and the mode of flight is, in some cases, not easy to explain. 
W e can readily understand the correlation between the head of the 
heavy Microdon , dichoptic in both sexes, and its short wings; both 
characters are indicative of a Dipteron that is not a high flier. But 
it is more difficult to interpret a priori why most Helophili (includ¬ 
ing the genus Bolichogyna Macq. from Chili), why Ascia, Sphe- 
gina , Psarus , Pelecoeera, etc., are likewise dichoptic in both sexes. 
Observation in life alone will solve such questions. 
The Nemocera vera are essentially aerial. But as lioloptic heads 
do not occur among them, the influence exercised by their life-habit 
on their organization does not appear in the structure of the head, 
but in that of other organs, and especially of the antennae, the 
