Siphoniulids have been classified in various, conflicting placements within the Helminthomorpha ("worm-like" millipedes) since their initial description, and their relation to the rest of millipedes is still unresolved. When first described, the family Siphoniulidae was placed in the "Suborder" Colobognatha, a group that is now recognized as a larger grouping including the orders Platydesmida, Polyzoniida, and Siphonophorida. The following year, the American entomologist Orator F. Cook considered Stemiulids as "suborder Siphoniuloidea", closely related to Julidans and Spirostreptidans (a grouping termed Diplochaeata). In 1979, Hoffman placed Siphoniulida (now considered an order) as Helminthomorph "incertae sedis", meaning the placement within Helminthomorphs was undetermined, due to the absence of male specimens. With only two known species, Siphoniulida is the smallest order of millipedes, followed by Siphonocryptida with three to six species. In more recent years, millipedes have been studied by cladistic and modern phylogenetic methods, yet Siphoniulida remains enigmatic. In the first cladistic study of millipedes, Enghoff could only place Siphoniulids as "incertae sedis" within the Helminthomorpha, but "probably... a specialized subordinate taxon within some juliform or colobognathan order". However, in the first morphological study of millipede phylogeny incorporating full details of "Siphoniulus" anatomy, Siphoniulida did not appear closely related to the Juliformia nor Colobognatha, but rather appeared as an outgroup to all other helminthomorphs, and the internal classification of Helminthomorpha was poorly resolved and significantly differed from Enghoff's. In a subsequent study combining anatomical data with DNA sequence data from other groups, Siphoniulida again appeared as basal within Helminthomorpha.