Braconid wasps,Apanteles euphydryidis, attended webs of their lepidopteran hosts,Euphydryas phaeton, for hours, with one-third of each hour spent searching for caterpillars. Encounters with larvae on the outside of webs were frequent and usually resulted in parasitoids turning away. Head-jerking exhibited by caterpillars was effective in knocking parasitoids away. Ichneumonid wasps,Benjaminia euphydryadis, traveled from web to web, spending less than 1 min per web. Overall, the defensive mechanisms exhibited by the caterpillars and their distribution on and in webs were effective in deterring parasitoids.