Phylogenetic position of Heterorhabditis relative to other notable Rhabditina.

A. At the base of the tree is the free-living microbivorous Panagrellus (Panagrolaimoidea). Lineages in green are semaphoronts of large, diverse clades of microbivorous nematodes whose members associate with invertebrates at some point in their lifecycle, typically via phoresy and/or necromeny [52]–[54]. Heterorhabditis is a transitional taxon, exhibiting ancestral microbivorous traits, but has also evolved obligate pathenogensis and shares most recent common ancestry with obligate mammalian parasites (Strongyloidea; lineages in red). Modified after [127]–[129]. Taxonomy follows the ranking hypotheses and nomenclature of Hodda, 2011 [130]. B. H. bacteriophora nematodes have evolved a mutualism with insect pathogenic P. luminescens bacteria (green) where each partner cooperates to achieve voracious entomopathogenicity. An infective juvenile regurgitating intestinal symbionts (right) out the pharynx is shown. The movement of the nematode head causes slight misalignment of the fluorescent and differential interference micrograph image overlays.