1985] 
Forrest — Balloon-carrying empidid 
293 
ing sites that will be most profitable with respect to female-to- 
competitor ratio. 
The time of swarming is restricted to a 20-minute period at dusk 
(Fig. 3). Crepuscular swarming is common in insects presumably 
because climatic factors, especially wind, are more favorable for 
swarming (Sullivan 1981). For balloon flies the change in light qual- 
ity and quantity, particularly at sunset, may influence when the 
balloon would be most efficient in attracting females. Light does 
influence the timing of Hilara swarms since their activity begins and 
ends earlier in the evening on cloudy days (Fig. 3). 
Males are expected to display when reproductive benefits are 
greater than alternative behaviors. That is males should swarm 
when females are most available and attractive to mates. When their 
availability is confined to brief periods females may “force” males to 
display synchronously. This benefits females since when large 
numbers of males display together females have a greater choice 
from potential mates. From this positive-feedback mechanism 
evolves the spree, a temporal analog of a lek (see Walker 1983). 
Swarms are restricted in space as well as time and may therefore 
be compared to a lek where males display and females come to mate 
with males. Most often only a few males in the leks do all or most of 
the mating and females receive nothing more than sperm from their 
mates (Bradbury 1981). That Hilara females remain on vegetation 
during swarming and do not, until late in the swarming period, enter 
the swarm and couple with a male suggests that they may be evaluat- 
ing males in swarms. The cue used in this evaluation remains in 
doubt. Perhaps the balloon is an effective means by which females 
can judge male vigor on the basis of flight patterns made more 
noticeable by the conspicuous balloon. Or once coupled a female 
might “analyze” the composition of the balloon as an indicator of a 
male’s ability to accrue certain resources (Thornhill and Alcock 
1983). This would be very important in species that presented prey 
as nuptial gifts. 
That males are larger than females (Table 1), unusual in insects, 
points to selection for large males. Males that join swarms must 
compete for a limited number of females and, since males collide 
with others and mating takes place in the swarm, large males might 
have an advantage. However, it was found that coupled males had 
smaller wing lengths than those that were uncoupled (Table 1). That 
