56 
Psyche 
[June-Sept. 
of the genus have the ability to produce viscous bubbles while 
they are engaged in quieting their prey by jabs with the 
sharp proboscis. This propensity has been elaborated into 
an instinctive courtship adaptation in Hilara sartor. 
Professor Aldrich and his student, L. A. Turley, in 1889 
recorded the balloon-making habit of the males of a species 
of Empis, which later I described as Empis aerobatica. In 
company with Professor Aldrich on Moscow Mountain, 
Idaho, I have watched the behavior of this species, and again 
on May 10, 1925, near the junction of the Salmon and Snake 
rivers in Idaho, I found a considerable gathering of the 
flies performing their balloon- juggling act some fifteen feet 
above ground. 
The Empididae are typically rapacious, predatory flies, and 
especially at the mating season are to be found malaxating 
their victims with repeated thrusts of the proboscis. Poul- 
ton and Hamm have suggested an evolutionary sequence in 
the development of this habit, possibly somewhat as follows : 
1. The prey is devoured by both sexes independently of 
pairing. 
2. The prey, caught by the male, is given to the female 
upon mating, as food to divert her attention from any can- 
nibalistic tendency while she is astride him. 
3. The prey is enveloped by the male in a froth secreted 
by the proboscis so that it becomes more enticing to the 
female. The prey need not be sucked by the female but acts 
as an aphrodisiac. 
4. A web or balloon is formed of froth alone as the in- 
dispensible stimulus to copulation. 
Hilara sartor in Europe and Hilara granditarsis in 
America represent the culminating stage in this sequence, 
probably independently developed, while Empis aerobatica 
has advanced the third stage until the frothy balloon is vastly 
larger than the microdipteron that formed its nucleus and 
even bigger than the Empis itself. 
