1982] 
Obin — Spiders Living at Wasp Nesting Sites 
331 
chrysidid and mutillid wasps. These observations suggest that spider 
predation may reduce the parasite load at mud-dauber nest sites. 
Since mud-daubers are usually adept at recognizing and maneuver- 
ing on webs and retreats (Eberhard, 1970; Coville, 1976), spiders at 
nest sites may pose only a limited threat to wasps. We have observed 
individuals of all three species of wasp successfully nidify and 
provision nests positioned such that the wasp flew through or 
walked behind a web on each trip to and from the nest. Wasps 
became entangled in webs when their regular flight path was dis- 
rupted during agonistic encounters or when they attempted to pro- 
vision very large spiders. Empirical evaluation of the relative costs 
(e.g., probability of predation, costs associated with increased flight 
distance to foraging patches) and benefits (reduced parasitism) of 
hunting away from the nest site is in progress. One predication of 
the “reduced parasitism” hypothesis is that a small percentage of 
mud daubers at any site may “cheat” — i.e., may occasionally prey 
on spiders at nest sites. Relative to other wasps in the population, 
these wasps would enjoy reductions in the time and energy costs of 
hunting and transporting prey, while at the same time benefitting 
from the “parasite umbrella” afforded by spiders active at nest sites. 
The relative frequencies of cheating and non-cheating (hunting 
away from nest sites) may perhaps be maintained by frequency- 
dependent selection (Fisher, 1930) in an Evolutionarily Stable 
Strategy (Maynard Smith and Price, 1973). 
The Effects of Prey Size and Availability on Mud- Daubers. Vari- 
ous authors have suggested that spider size constrains prey collec- 
tion by Chalybion (Muma and Jeffers, 1945), Sceliphron (Muma 
and Jeffers, 1945; Eberhard, 1970) and Trypoxylon (Cross et al., 
1975). Selection should favor wasps that minimize both the risks 
and metabolic cost of (1) immobilizing and (2) transporting large 
spiders. Do wasps refrain from attacking large prey that they can 
incapacitate but not readily transport, or is the upper range of prey 
size found in nests a reflection of the wasp’s inability or reluctance 
to paralyze prey above a certain size? Measurements of spiders 
dropped by provisioning mud daubers indicate that the wasps suc- 
cessfully incapacitate spiders that exceed the upper range of prey 
size noted in nests, but fail in their ability to transport or cache 
them. 
