Ectoparasites: direct impact on host fitness.
Despite being restricted to the host's first line of defence (the integument, away from vital organs), ectoparasite damage has a pronounced impact on host fitness. This generalization can be explained by the reduced dependence of most ectoparasites on their individual host, which minimizes the fitness loss linked to host death. This explanation implies that permanent ectoparasites evolve less 'aggressively' than do either nest or field ectoparasites. This, and other determinants of ectoparasitic virulence are discussed here by Tovi Lehmann.
Lehmann
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Department of Entomology, Forbes 410, College of Agriculture, University of Arizona, Tucson, AZ 85721, USA.