294 
Psyche 
[Vol. 92 
coupled males did not differ significantly in femur length indicates 
that coupled males have higher wing loading (wing area relative to 
body size) than uncoupled males. This wing loading difference may 
be important to a male’s ability to maneuver in the swarm. 
But why should mating pairs remain in the swarm? Males risk 
losing females in fights with other larger males in the swarm and 
females might be injured in such flights. An advantage a male might 
have would occur if mating durations are relatively short compared 
to the time remaining in the swarming period. Males then might 
profit by remaining in the swarm and continuing to advertise for 
other females. If a male has been selected by a female that evening 
he “knows” that he has outcompeted the other males in the same 
swarm. Provided females will mate with males that have already 
mated that evening, males will gain by remaining in the swarm. 
However, pairs were found hovering near the edge of the woods and 
durations of mating last until after swarming has ceased. Another 
alternative is males may remain in the swarm to exercise some 
choice over females that enter. If female size is an indication of her 
reproductive output then males may be waiting to see if a larger 
female enters the swarm. A male would gain reproductively by 
mating with a larger more fecund female. A female, on the other 
hand, might exercise her choice and fly back into the swarm if she is 
coupled to an unwanted male and force the male to prove his 
competitive ability. 
Summary 
Mating swarms of Hilara sp. are made conspicuous by white, 
silken balloons carried by males. Groups of 1-25 males swarm 
beneath branches of broadleaf trees at a modal height of 1.4 m 
(range 0.5-4.5 m). Male activity periods begin about sunset and 
continue for 20 minutes with the beginning and ending of swarming 
correlated with light intensity. Females fly into swarms where 
coupling occurs. Unlike most other swarming Diptera, Hilara sp. 
pairs remain in the swarm while mating. Males are larger than 
females, though coupled males were significantly smaller (wing 
length) than other males in the swarm. 
