194 
AERIAL AND TERRESTRIAL DIPTERA 
and the female (although dichoptic) to hover over a flower, even 
to touch it, without alighting. The legs of aerial Diptera are 
principally fitted for alighting, and much less for any other pur¬ 
poses ; they are, for this reason, less strong and less specialized 
than those of the terrestrial type. 
Terrestrial Diptera show the opposite characters. The heads in 
both sexes are generally dichoptic. The macrochaetae are much 
more numerous and stronger. The flight is more headlong, less 
subject to control. The legs are fitted for running, grasping, dig¬ 
ging, and other exercises, and are for this reason stronger and 
provided with bristles, hairs, and other useful and sometimes orna¬ 
mental appendages. 
Little has been done as yet to establish a causal connection 
between the different modes of flight of Diptera and the mechanism 
of their wings. I have attempted to pave the way for such re¬ 
searches in my two papers, “ On the Terms Tegula, Antitegula, 
Squama, and Alula ” (in the Berl. Ent. Zeit., 1896, p. 285-288), 
and “ On the Terms Calypteratae and Acalypteratae, Calypta and 
Calyptra, as they have been used in Dipterology” (in the same 
serial, 1896, p. 285-288). But these papers of mine were merely a 
beginning of such an investigation, and especially a contribution 
to the nomenclature and literature of the subject. 
On the relation between the venation and the mode of flight, a 
happy hint has been thrown out by Zeller, more than sixty years 
ago, in the following passage: “ The mode of flight of the true 
Bombglius is not unlike that of Sgrphus. They hover for some 
time in the same place, jerk off suddenly, and then gradually sink, 
and thus approach the flowers they intend to visit.” Such a con¬ 
trol of the power of flight is secured in both of these genera by 
a similar structure of their venation; the fourth and fifth veins, 
just before reaching the margin of the wing, form an angle, the 
distal sections of which run parallel to the margin, and end in the 
preceding vein. A similar parallelism of veins with the margin 
secures the same control of the motions of the Mgdaidae and the 
Nemestrinidae. But the mechanics of many other peculiar modes 
of flight of certain Diptera still remain unexplained. On what 
structures, for instance, of wings or veins, depends the singular to 
