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Psyche 
[Vol. 89 
It is also possible that by not hunting at nest sites, mud-daubers 
more effectivley conceal the location of their nest. Such a mecha- 
nism has been proposed to explain why raptors usually do not hunt 
near their own nests (Durango, 1949). Mud-dauber larvae fall prey 
to a variety of parasitoids and inquilines (Rau and Rau, 1916; 
Krombein, 1967). Hunting away from the nest site can reduce the 
probability of parasitization if the following assumptions are met: 
(1) The parasite encounters the host species at sites where the host 
species hunts; (2) The parasite trails the host species back to the 
nest; (3) The host species can evade the trailing parasite, the proba- 
bility of so doing increasing with the distance over which the host 
species is trailed. 
Certain host-parasite systems involving mud daubers and sarco- 
phagid flies meet the above assumptions. Flies in the tribe Milto- 
grammini are larviporous parasites of many aculeates, including 
sphecid wasps (Allen, 1926). The genera Amobia and Senotainia 
include species of mud-dauber parasites that follow prey-laden 
wasps to their nests (Chapman, 1959; Cole, 1969). The adult flies are 
nectivorous, and it is likely that they encounter foraging wasps on 
vegetation. Prey-laden mud daubers often fly at reduced speeds, and 
their maneuverability is similarly impaired (Obin, pers. obs.). They 
are presumably easier to follow at such times. Furthermore, a wasp 
with prey assures a trailing fly that the wasp is nesting, that a cell is 
being provisioned and is consequently open, and that there will be 
food available in that cell. Sarcophagid flies have been observed 
trailing C. calif ornicum females to their mud nests over distances of 
3-5 m. The pursued wasps often took circuitous routes to their 
nests, and in certain instances left the site altogether before reaching 
their nest (Obin, unpublished data). Whether such behavior results 
in successful evasion is not known, but it does suggest that wasps 
may require flight distances greater than those observed in order to 
evade trailing Miltogrammini. If so, selection may, on average, 
favor wasps that do not hunt close to their nest. 
Interactions between spiders and mud-daubers may not be exclu- 
sively antagonistic, and the selective advantage accruing to wasps 
that do not hunt at nest sites may be a consequence of a site-specific 
mutualism between these two traditional enemies. During the study, 
various parasites of mud-daubers were observed in webs at nest 
sites. These included bombyliid and sarcophagid flies as well as 
