12 
Psyche 
[Vol. 94 
base of the head capsule for securing an egg. An additional factor 
promoting the caste-bias in host selection observed in this study 
may be that major workers provide more nutrition for developing 
parasite larvae than non-major workers. This would be especially 
the case if P. obtusus larvae develop only in the head, a pattern 
observed in other species of ant-specific phorids (Pergande, 1901). 
Finally, major workers may represent a more reliable resource. 
They are the longest-lived of the worker castes (Porter and Tschin- 
kel, 1985), and their relatively reduced aggressiveness renders them 
least likely of all workers to incur injury or death from fighting. 
Therefore we suggest that the probability of an individual host sur- 
viving the requisite period of parasite development in that host is, 
on average, greater for major workers than for other workers. If so, 
selection would favor phorids that parasitize major workers. 
Acknowledgement 
We wish to thank M. S. Obin for his excellent suggestions and 
comments on the manuscript. We also thank F. C. Thompson, D. 
R. Smith, and J. C. Trager for the identifications of the phorids and 
ant specimens. We are grateful for the assistance received from 
Antonio Pereira, Jorge Rodrigues and EMBRAPA in Brazil. 
Finally, we appreciate the opportunity afforded us by the U.S. and 
Brazilian governments to visit and study in Brazil. 
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