1980] 
Trail — Predation by Argyrodes 
353 
In two cases there was evidence that an Argyrodes had killed 
more than one individual in a communal group. On July 6 a female 
Argyrodes baboquivari was found in a communal group feeding on 
a P. oweni female; a second P. oweni female, sucked dry, was 
present in the webbing nearby. On July 14, a female A. baboquivari 
was found feeding on a P. oweni egg case, with the mother of the egg 
case dead and partially consumed nearby. This implies that Argy¬ 
rodes may remain longer in communal groups, perhaps because 
they represent a large source of potential prey. 
2. Communal behavior and defense against predation. Group¬ 
living is often interpreted as a means of defense against predators. 
(See Brown 1975 and Wilson 1975 for summaries of this literature.) 
Of the many anti-predator strategies made possible by group-living, 
three could conceivably operate in communal spider groups such as 
those of P. oweni. These are enhanced detection of predators by 
groups of individuals; cooperative defense against predators; and 
the “selfish herd” effect. 
The first two of these anti-predator strategies have not been 
observed in P. oweni. Philoponella do not appear to detect Argy¬ 
rodes in their webs. Argyrodes are frequently seen resting unmo¬ 
lested in both solitary and communal webs of P. oweni. No active 
defense behavior has been observed in the solitary or communal 
webs. Argyrodes feeding on one colony member were never ap¬ 
proached by other colony members, and female Philoponella did 
not attack Argyrodes that were preying on their hatchlings. 
The third anti-predator function of groups is the selfish herd 
effect (Hamilton 1971). If a searching predator encounters a solitary 
prey individual, that individual is likely to be attacked. When 
confronting a group, a predator may attack one individual, but the 
others are at least temporarily safe. The larger and denser the group, 
the smaller the probability that one particular individual will be 
taken by a predator. 
My data are insufficient to determine if this is an important factor 
in Philoponella colonies. If an Argyrodes in a communal group 
takes only one or a few prey, then large communal groups may 
confer a certain amount of safety on their members through the 
selfish herd effect. But if the Argyrodes take many prey relative to 
colony size then the selfish herd is more like a collection of sitting 
