Based on a phylogenomic analysis of millipedes in the subterclass Colobognatha, the closest relative of Illacme is Nematozonium of South Africa which shares a long, narrow body shape and characteristics including gonopods.
A study of millipede phylogeny using genomic data reveals that the closest relative of Illacme is Nematozonium from South Africa, both of which exhibit elongated bodies and possess gonopods.