8 DIPTERA 
been bestowed on the Empididz.  Rhamfhomyia sociabilis is one of the most striking of the early sum- 
mer species in Washington. By thousands it dances up and down, in and out, in an immense swarm, 
sometimes in the open and sometimes in the shade of a large tree. Some species of Emfis skim back 
and forth in numbers among the tree tops. Reacting to air currents the group rises or lowers in a 
synchronized aerial dance. Other species of Emfis and Rhamfhomyia perform their dance closer to the 
ground, among underbrush or in the open spaces beneath forest trees. The species of Hilara and 
Rhamphomyia that hover over water weave in and out, in leisurely or in rapid erratic flight, barely missing 
the surface. The large metatarsi of the male Hilaras may touch the water but the insects are not wetted. 
In this surface dance the Hilaras may congregate on some small floating object and drift down stream, 
shortly to separate and return. Usually males predominate in the air-dances of the Empids. 
Courtship. — No group of the Diptera offers more interesting and varied details of pygidial 
structure than the Empididaz. With the elaboration of primary and secondary sexual structures in 
Pygidium, legs, wings, mouth-parts and vestiture, there has evolved a complex series of mating habits, 
The significance of many of the cases of sex dimorphism is not understood, since the mating of most of 
the species has not been observed. The procuring of insect prey and the anzmotropic dances just 
described are manifestations of the courtship instinct. Hamm and Poulton have suggested an evolu- 
tionary sequence in the complexity of behavior, the stages in the use of prey being somewhat as follows. 
1. Prey devoured by both sexes independently of mating. Examples, Tachydromia, Hybos, some 
species of Emfis. 
2. The prey provided by the male as a gift anticipatory to copulation is devoured, or sucked by 
the female during amplexus. Examples, Pachymeria, Rhamphomyia, Empis spp. 
3. The prey or object provided by the male is not devoured by the female, but acts as a love- 
charm or stimulus to insure amplexus. Examples, Hilara, Enoflempis. 
Where prey is passed from the male to the female, as in the second and third stages, the act is 
regarded as a physiological necessity for copulation. Since Empididsz are cannibalistic the offering 
of prey to the female may have first functioned to divert her attention from her consort. Many antho- 
philous females have never been seen to imbibe insect juices except at the moment of copulation, 
dropping the prey furnished by their mates as soon as the sexes separate. With these species the use 
of insect prey is to be regarded as a philter, a stimulus rather than nourishment for the female. 
A. T. Hamm has made extensive observations on the mating habits of English Empidide. 
While there are of course species differences in behavior, the descriptions of two types observed by him 
are here given as characteristic. — Empis trigramma is one of the species that does not depend on the 
transfer of a love-token during the wooing process. A male alights on a leaf near a female, flutters the 
wings and raises and waves the front legs. The female responds with the same actions. 'The male 
then rubs the front tarsi together while vibrating the wings, and the female repeats these motions. As 
the two approach each other they caress each other's front tarsi, whereupon the female elevates the tip 
of the abdomen and the male deftly flies to her back and copulation ensues. 
Among the species where the male lures his mate with prey Mr. Hamm observed the male of 
several species of Empis pounce on a fly, holding it with the posterior legs while it inserted the proboscis 
into the neck to pierce the ganglia behind. With this quieted offering the male either zig-zags over 
the resting females, or, in the case where the females are engaged in their aerial dance, flies into the 
circle, bearing the victim encircled by his middle legs. The selected female is chased, overtaken, and 
during a struggle of a few seconds duration the offering is transferred to the female, and the pair settles 
in copula to the herbage below. The male is apt to support his burden by hanging to a leaf by his 
front legs, tightly clasping his mate with the posterior legs. While copulating the female squeezes the 
