152 
Psyche 
[June 
Nephila clavipes and flies of the genus Phyllomyza 
We first found flies associated with Nephila clavipes in January 
1976, on Barro Colorado Island, Canal Zone, Panama. We sub- 
sequently found similar flies associated with this spider at a number 
of localities in the Canal Zone and elsewhere in Panama. Flies 
from four adult female N. clavipes at four different sites were col- 
lected. They were identified (see acknowledgments) as belonging 
to the genus Phyllomyza and all belonging to the same (unde- 
termined) species. All eleven insects were females. The flies rest 
on the dorsal surface of the spider and usually aggregate on the 
cephalothorax. Figure 1 shows eight flies resting on this area. 
The flies remain on the spider for long periods of time and are 
virtually inactive. When we set out to determine what the flies 
were doing sitting on the body of the spider we ran into a major 
practical problem. The Drosophila-sized flies were really too 
small to observe with the unaided eye. This problem was solved 
by adapting a stereo-binocular microscope for horizontal use, 
mounted on a camera tripod (Robinson & Smythe, 1976). With 
this device, under field conditions we could watch the insects under 
10X or 20X magnification. It became apparent that on the cepha- 
lothorax of the spider the flies were not doing anything other than 
grooming themselves, sporadically shifting position and occasion- 
ally defecating. The bodies looked entirely normal and there was 
no evidence of oviposition or of penetrative feeding on the spider 
itself. (The mouthparts of milichiids could clearly not be used for 
piercing the spider’s cuticle and sucking its internal fluids, but at 
this stage we did not know what the flies were.) Eventually we 
decided to feed the spider. This went through all phases of its 
predatory behavior without disturbance to the flies. Prey capture 
involved rushing out to attack the prey, biting it, wrapping it in 
silk, removing it from the web, transporting it back to the hub 
and there wrapping it once again before hanging it and feeding 
(details in Robinson & Robinson, 1973, for Nephila maculata ap- 
ply broadly to N. clavipes). This predatory sequence involves a 
great deal of violent movement, in space and of the spider’s legs, 
throughout which the flies simply sat tight. 
At the hub the spider passed secretions into the prey and after 
about eight minutes the whole surface of the insect was covered in 
a film of liquid. At this stage the flies left the body of the spider 
and clustered on the surface of its prey. There they could be seen 
