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 phylogenetic examination of Colobognatha millipedes indicates that Nematozonium from South Africa is the most closely related to Illacme, sharing morphological features including a long, narrow body shape and gonopods.