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found in the colonies demonstrating that the spiders successfully 
reproduce under these conditions. 6) Spiders appear to be capable 
of emigrating with the ant colony because several spiders were pres- 
ent in a colony which was excavated less than a week after it had 
emigrated. 7) The best evidence that these spiders are specially 
adapted for life in harvester ant colonies is that the spiders appar- 
ently adopt the odor of their host colony. Spiders and ants from the 
same colony generally ignore one another; workers occasionally 
displayed mild aggression but the spider was never captured or 
injured. However, when I introduced spiders into foreign harvester 
ant colonies, they were almost without exception attacked and 
killed within minutes or even seconds. Spiders introduced into 
laboratory colonies of Trachymyrmex and Aphenogaster ants were 
similarly attacked and killed. 
It would be interesting to know how these spiders disperse among 
harvester ant colonies, especially how they locate a colony. They are 
very susceptible to desiccation so it seems unlikely that they would 
disperse in the daytime. Gaining entrance to a foreign colony would 
also be a problem since the entrance is sealed at night and guarded 
by hostile ants in the day. 
Many spiders are ant predators (MacKay 1982b, Porter and 
Eastmond 1982), but relatively few are symbionts (Table 1). Per- 
haps, this is because spiders have a distinct preference for live prey 
and are less well adapted to subterranean life than many arthro- 
pods. Furthermore, unless a spider actually preys on the ants them- 
selves, colonies must contain rather large and stable communities of 
symbionts to support a specialized symbiotic predator. The number 
of symbiotic spider-ant relationships which exist in spite of these 
seeming impediments (Table 1) is interesting but perhaps not sur- 
prising in view of the fact that spiders and ants are among the most 
abundant of all arthropod groups. 
Summary 
An undescribed spider of the genus Masoncus (Linyphiidae: Eri- 
goninae) was commonly found in colonies of the harvester ant 
Pogonomyrmex badius. This miniature spider is a predator of Col- 
lembola and all of its life stages were present in the ant colonies. 
