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.
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Nancy E. Stamp (1982) studied this question.