AERIAL AND TERRESTRIAL DIPTERA 193 
marked among the Brachycera , where the corresponding two types 
of flies are distinguished by the presence or absence of holoptic 
heads and of macrochaetae. As in the Nemocera vera these two 
characters are wanting, the contrast between the two types is less 
evident, nevertheless (as I am going to show in the course of this 
paper), it exists; but, in the absence of holoptic heads and of 
macrochaetae, the contrast is characterized by modifications in the 
structure of other organs, such as antennae, wings, legs, etc. 
The modifications of the different organs in accordance with the 
requirements of one or the other life-habit are merely adaptive 
and, although they may be very prevalent in a group, and char¬ 
acteristic of it, they are not very deep-seated, and may change or 
disappear among those of the members of the same group the life- 
habit of which is aberrant from the prevailing type. The phylo¬ 
genetic characters, in such cases, are left untouched, and the 
aberrant members of the group remain in the same family, or 
superfamily, in spite of their sometimes quite conspicuous adaptive 
characters. Such adaptive characters may therefore be considered 
as tools in the battle of life , tools which are easily modified, when 
the conditions of the struggle are altered, although the phylo¬ 
genetic characters remain the same. 
Adaptive structui’es are useful either for orientation (such 
are the adaptations of the eyes, of the antennae, or of the macro¬ 
chaetae) or for action (the adaptations of the wings or legs). In 
some cases, organs of orientation may be developed in such a 
manner as to be also useful for action ; for instance, in certain 
Nemocera long antennae may be used, like the long legs, for regu¬ 
lating the flight. 
The males of atrial Diptera, owing to their holoptic heads, are 
better able to scan, during high flight, a wide horizon. Aerial 
Diptera show a minimum of macrochaetae, sometimes none what¬ 
ever, because the requirements of orientation are less complex 
for aerial than for terrestrial life. Holopticism is usually con¬ 
nected with a wing-structure that is especially adapted for a more 
perfect control of the motions; and this faculty of control, in such 
cases, is not confined to the holoptic male, but exists in both sexes. 
Such a wing-structure enables the male to hang, poising in the air, 
13 
