Infective juveniles of Heterorhabditis shed the enclosing L2 cuticle in the presence of an insect host and expose a large, anterior, terminal tooth which is used to scrape at and rupture the host's cuticle. This may enable easier entry into some insects that are less vulnerable to Neoaplectana species which do not have a terminal tooth.
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Bedding et al. (1982) studied this question.