292 
Psyche 
[Vol. 92 
cantly shorter wing lengths than uncoupled males captured in 
swarms (Table 1). 
Discussion 
Kessel (1955), expanding on Hamm’s (in Poulton 1913) and 
Melander’s (1940) classification, recognized eight stages in the evo- 
lutionary sequence of empidid mating behaviors. The eighth and 
final stage is represented by balloon-carrying species in which males 
swarm and the balloon alone has become the stimulus for courtship 
and mating. Hilara n. sp. thus is a representative of this stage. 
Two of the other five North American balloon flies that have been 
studied are Hilara species. Hilara wheeleri is similar to Hilara n. sp. 
in that males swarm near broadleaf trees at heights less than 2 m, 
balloons are made of silk from tarsal glands. However, dipteran 
prey are always encased in the walls of the balloon (i.e. stage 5 of 
Kessel’s sequence). Hilara granditarsis swarms at heights more than 
3 m high in the branches of coniferous trees and males carry frothy 
balloons that contain no prey (stage 8). H. sartor, a European spe- 
cies, also swarm high in the branches of conifers and have frothy 
balloons without prey (stage 8). 
Congregating on leaves before swarming may occur because both 
males and females respond to the same conspicuous landmark. One 
hypothesis on the evolution of swarming behavior is that going to 
such markers has been an efficient mechanism for unmated females 
to locate mates (Sullivan 1981). Selection then acts on both males 
and females to respond to these conspicuous landmarks. Though 
male Hilara appeared to search for females they did not try to court 
or mate with females on nearby leaves. Once males reach an area 
using long range visual cues they should try to locate specific swarm- 
Table 1. Hind femur and wing length (mm) of Hilara females, coupled and 
uncoupled males.* 
n 
femur length 
x ± S.D. 
wing length 
x ± S.D. 
females 
8 
1.06 ± .05 A 
3.2 ± .1 A 
coupled males 
10 
1.15 ± .06 B 
3.4 ± .1 B 
uncoupled males 
22 
1.20 ± .06 B 
3.5 ± .l c 
•Means in a column with different letters are significantly different (p < .05). 
