252 
Psyche 
[September-December 
Two additional factors may be important in increasing the 
predation risks of courting and mating animals. In many species, 
a pair in copula probably finds it more difficult than usual to 
escape a predator, given that it has been detected. The other 
factor is that courtship and mating may be rather demanding 
tasks for the nervous systems of the participants. This may de- 
crease the probability that the participants will detect the approach 
of a predator and respond in time to escape. Casual observations 
on P. johnsoni indicated that, when a pair was involved in court- 
ship or mating, they were less responsive to movements of the 
observer. This was especially true of males. 
It will be predicted that predation presents a greater risk to 
P. johnsoni courting and mating outside nests, compared to those 
courting and mating at nests. This is part of a more general hy- 
pothesis that individuals inside nests are subject to less preda- 
tion than ones outside nests. The predicted safety is relative, 
not absolute, since the data presented here implicate certain 
large gnaphosid spiders as predators on P. johnsoni in their nests. 
Occupancy by H. hesperolus of the nests of P. johnsoni females 
and subsequent predation on courting P. johnsoni males might 
be a case of aggressive mimicry (Wickler, 1968), in a limited 
sense. It would be of much interest to verify that this behavior 
occurs in the field. Also, it would be of value to attempt to evalu- 
ate whether this particular advantage to the gnaphosid is great 
enough to constitute a significant selection pressure favoring 
occupancy of P. johnsoni nests. Other factors are certainly 
possible, since organisms such as isopods and anystid mites also 
occupy P. johnsoni nests, and for these the predatory function 
is very unlikely. In considering the importance of this type of 
predation for P. johnsoni males, it should be noted that the pro- 
portion of P. johnsoni nests, in the field, occupied by large gna- 
phosids is less than one per cent (unpublished data). In addi- 
tion, males evidently discriminate between P. johnsoni and H. 
hesperolus nests. 
In conclusion, the following are some ways in which the spider’s 
nest may provide relative safety from predation: 
1. The silk of the nest may form a physical barrier between 
the occupant and the predator. 
2. The silk of the nest may provide a sensory barrier, both visu- 
al and chemotatic, between the occupant and the predator. Preda- 
