4 DIPTERA 
has a genus with slender femora been produced by such forms as GEdalea or Lactistomyiía. But an 
orthogenetic development of the highly specialized hind legs of these two genera does not necessarily 
bespeak a phyletic relationship between the two, even though the resemblances are continued to such 
details of structure as the setosity of the under side of the femur, the patellar bend at the knee, the 
shortening of the tibia and lengthening of the trochanter and even the compression of the tibia to a flexor 
edge. The similarities must be interpreted as parallel evolution, for GZdalea judged by its three posterior 
veins and dichoptic condition, is more generalized than the thick-legged H ybotinz, and yet possesses 
the specialized anal cell and antenna of the Ocydromiinz. Both of these genera are better regarded as 
highly specialized convergent developments of two subfamilies, the result of an orthogenetic evolution. 
In a group of predatory insects, such as the Empididz, it would be natural to look for raptorial legs, 
andsuch an adaptation does occur in each of the main groups. In Hoflocyrtoma, Scelolabes and Paleoleptopesa 
of the Ocydromiinz the hind legs are fitted for grasping, and in the last genus the adaptation dates back 
to the Lower Oligocene. In P/atypalfus the middle legs are modified, even to the patellar bend, and a 
similar but incipient tendency is noticeable in others of the Tachydromiinz as in certain species of 
Elafhropesa and Tachypesa. In the Hemerodromiinze it is the front pair that is modified, and so success- 
ful has this adaptation proved when on the first pair of legs that the coxze have become lengthened to 
extend the reach. Of the Empidinze, Haflomera, Pachymeria, Enoplempis and certain species of : 
Rhamfphomyia and Hilara present thickened femora. 
It is my belief that the following tabulation of characters shows the principal lines of evolution 
in the Empididz; that development has proceeded from the first set to the second and not in the reverse 
direction except in some cases of retrograde specialization; and that when two related genera are com- 
pared the one possessing a preponderance of the first listed characters is the more generalized and is 
ancestrally older. Quite obviously no present day Empid is the direct ancestor of another, but with due 
reservation, particularly with respect to homoplastic characters the tabulation will prove helpful in 
plotting the phylogeny of the several groups. 
Generalized condition. Specialized condition. 
Head round. Head hemispherical or lengthened. 
Dichoptic. Holoptic. : 
Facets uniform. Area of large facets. 
Eyes bare. Eyes pubescent. 
Face narrowly quadrate. Face broad or excised. 
Antennze three-jointed. Antenne two-jointed. 
Third joint conical. Third joint elongate or short. 
Basal joints simple and similar. First joint setulose, second joint with bristle. 
Style (terminal). Arista (dorsal). 
Proboscis short, vertical. Proboscis long, no labella, inflexed or porrect. 
Palpi two-jointed, bristly. Palpi one-jointed, bare. 
Body delicate. Body heavily chitinized. 
Pollinose. Pruinose, or polished. 
Slightly convex thorax. Hunchbacked. 
With few small bristles. With many strong bristles. 
Metanotum long. : Metanotum short and declivous. 
Metapleurze bare. Metapleurz with hairs. 
Pygidium small and symmetrical, short penis, Pygidium large or asymmetrical, long penis, 
small valves. large valves. 
