The morphological variation of three representative isolates of the 'Irish group' of Heterorhabditis was examined. First generation hermaphrodites were characterised by having a blunt and mucronate tail. Females (second generation) and third-stage infective juveniles were also distinguished by the morphology of the tail and the presence of a refractile projection in the tail tip. Males were characterised by the position of the excretory pore and by the value of ratio SW. These morphological features do not fit the description of currently recognised Heterorhabditis species, and provide additional evidence in support for the consideration of the Irish group as a new species. A description of this species, as H. downesi n. sp., is provided.
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