AERIAL AND TERRESTRIAL DIPTERA 203 
The Blepliaroceridae, although pre-eminently aerial, offer the 
peculiarity of having the eyes of a nearly identical structure in 
both sexes. That is, in different genera, the eyes in both sexes are 
either holoptic or dichoptic. We do not know yet whether this 
difference is connected with a difference in the behavior of these 
genera. The eyes of this family offer some other exceptional and 
unexplained structures (including bisection). 
The Orphnephilidae, provided with holoptic heads in both sexes, 
have the characters of aerial Piptera, although they do not fly very 
high. Their flight is rather slow, and appears to be well under 
their control, as becomes holoptic Piptera. 
To conclude: The study of the two types of life-habit of 
Piptera, in connection with corresponding modifications in their 
structure, of which I have given a very imperfect outline, I con¬ 
sider as a necessary complement of the study of C'haetotaxy as a 
method of description. Investigations in that direction will, at the 
same time, open new avenues for the inquiry into the evolution of 
Piptera during geological ages. A hint of this kind is introduced 
by me in this chapter in the paragraph on Xylopluigus and 
Coenomyia, two apparently very different types, the phylogenetic 
relationship of which is, however, explained by their evolutional 
history. 
In my “ Introduction ” (p. 22) I expressed regret that I had 
“never had the good fortune to reside for any sufficient length of 
time in a place in which, or within a reasonable distance of which, 
there was a large dipterological collection,” containing forms of all 
continents; and it is in preparing the present chapter, replete as it 
is with generalizations, that I feel how much I have lost by such an 
isolation, and how much more satisfactory my work would have 
been if I had been able to illustrate it by numerous examples of 
exotic forms. 
Before concluding this chapter I deem it useful to state dis¬ 
tinctly that, although a mere dilettante, I firmly adhere to that 
group of naturalists who believe all phenomena of life and organic 
growth to be susceptible of being described, but not explained. For 
this reason in my “ Introduction ” (p. 15) I compared the evolution 
of organic forms with the “ Growth of the tree of Life,” because 
