1977] 
Robinson & Robinson — Flies and Spiders 
155 
less bee ( Trigona cupira Sm.) which was impaled on the bug’s pro- 
boscis. Around the prey item was a large number of small flies all 
apparently feeding. Eight of these flies were captured, many es- 
caped. The flies were identified as Neophyllomyza sp. 
Discussion 
The relationship between Phvllomyza sp. and its host Nephila 
clavipes is one that involves prolonged contact between the two 
species. We suspect that the same flies may remain on the spider 
for days at a time, leaving only to make very short feeding forays. 
Our attempts to paint-mark the tiny flies failed utterly so we cannot 
be certain on this point. In any case, the association seems to us 
to be distinctly more specialized than that described by McMillan 
(1975) for Desmometopa sp. Of course, this is a matter of inter- 
pretation. However, it is possible to suggest an evolutionary path- 
way from commensalism without contact (the reduviid and Argiope 
savignyi associates) through commensalism plus feeding excursions 
onto the host, to commensalism with sustained non-trophic con- 
tact. If the flies were cued into food sources by olfactory stimuli, 
as seems possible, then the pathway would involve a reduction of 
the detection distance. The strategy of waiting on the host must 
involve some interesting mechanism that allows the fly to “evalu- 
ate” the odds on food being available within its own feeding time- 
scale. At some stage the fly may be faced with “deciding” whether 
to remain with a spider on the off-chance that it will catch food or 
using its remaining food reserves to fly off in search of another 
(more successful?) host. In this respect, the fly may be at an ad- 
vantage over the kleptoparasitic theridiid spiders that also asso- 
ciate with N. clavipes. It can probably range over greater distances, 
more quickly, in search of a new host than can the spiders. At 
least three species of theridiids associate with the golden-web spi- 
der; at least one of these regularly shares the host’s meal, at the 
hub of the web (Vollrath, in press). Such kleptoparasitic spiders 
are small, but differ from the milichiids in having mouthparts ca- 
pable of penetrating insect cuticle. 
Spiders of the genus Nephila may be particularly suitable as 
hosts for this kind of associate. They are large, build very efficient 
webs that are operated 24 hours per day, and show a considerable 
degree of web-site tenacity. The other large, diurnal, orb-web spi- 
