AERIAL AND TERRESTRIAL DIPTERA 
199 
Spezia and Sinigaglia in Italy. These fossil species represent two extreme types, 
the interval between which was filled up by numerous transitional forms of the 
Tertiary period. The two Californian species, the only living remains of the 
genus, are representative forms of those two extreme types (“ es sind zwei sehr 
seliarf geschiedene Typen,” says Heer),all the intervening genera and species in 
the three parts of the world being extinct. 
Bold as the juxtaposition may seem of these two cases of the animal and the 
vegetable kingdom, it may nevertheless serve as a well-founded illustration of the 
working of the law of evolution. (About Sequoia my source of information is 
0. Heer, “ Ueber die Sequoien,” Yortrag in der bot. Section d. Schweiz. Nat. 
Gesellsch., reproduced in the periodical Gartenjlora. My separate has no date, 
but it cannot be later than 1879.) 
The Mydaidae, an ancestral superfamily among the Orthor- 
rhapha Brachycera , seem to hold an intermediate position between 
the aerial and pedestrian types. They are dichoptic in both sexes, 
and most of them have well-developed legs; on the other hand 
they have macrochaetae, and their venation, with the curved veins 
running parallel to the hind margin, indicates a superior power of 
regulating their flight. My experience with living Mydaidae is 
confined to Leptomy das panther inns Gerst., which I have watched in 
1876 about Lone Mountain, in San Francisco. As far as I remember, 
its flight was rather slow, steady, and well-regulated, not far above 
the soil, and not unlike that of a Bombylius. 
The two types of life-habit exercise the same influence on the 
organization of the Cyclorrhapha Athericera , as they do on that of 
the Orthorrhaplia Brachycera. Representatives of the former sub¬ 
order are subject to similar modifications of structure in con¬ 
sequence of their adaptation to external influences. 
The Syrphidae are pre-eminently aerial and holoptic, showing a 
minimum of macrochaetae, and possessing a remarkable power of 
regulating their flight. At the other end of the series of Cyclor¬ 
rhapha , the so-called Acalyptrata are decidedly terrestrial and pedes¬ 
trian, with dichoptic heads in both sexes, more or less numerous 
and characteristic macrochaetae, a diminished power of flight, etc. 
Between the two extremes of Syrphidae and Acalyptrata are the 
Calyptrata or Muscidae , which show a prevalent terrestrial and 
pedestrian character in the abundance of macrochaetae, in the di¬ 
choptic heads in both sexes, and in the headlong character of their 
flight. But among the Calyptrata , the group of Anthomyiidae 
