Bristly millipedes (subclass Penicillata, order Polyxenida) are minute

diplopods characterised by uncalcified cuticle and a body covered

with unique tufts of bristles. The order is found worldwide and comprises

less than 200 described species divided into three families, with

many of the species and genera being poorly known. The first evolutionary

analysis of the order presented here utilises both molecular

(COI, 18S rRNA and 16S rRNA) and morphological data to examine

monophyly of the families and subfamilies and the evolutionary relationships

between them. Maximum likelihood analysis was based on

molecular data only, whereas parsimony analyses were based on

molecular data as well as combined morphological and molecular

data. The results of these analyses with two different optimality criteria

were incongruent in many aspects. Unlike parsimony, the likelihood

result found strong support for a basal position of the family

Synxenidae and separation of the order into two monophyletic clades

corresponding to the two superfamilies Synxenoidea, containing the

family Synxenidae, and Polyxenoidea, containing the families

Polyxenidae and Lophoproctidae. Parsimony results did not support

the existence of the two superfamilies. Both analyses resolved the

family Synxenidae as monophyletic and Polyxenidae as polyphyletic,

whereas the family Lophoproctidae was shown to be paraphyletic in

likelihood and monophyletic in parsimony analysis. The subfamilies

Monographinae and Polyxeninae were found to be monophyletic in

the likelihood tree but parsimony suggested paraphyly of both. The

results suggest that further revision of the systematics of the

Polyxenida may be necessary. However, a much larger molecular

data set will be necessary to clarify and provide stronger nodal support

for phylogenetic trees and to confirm the relationships, particularly of

the families Polyxenidae and Lophoproctidae. Molecular identification

is likely to be an important tool for this taxonomically challenging

order in future. A simplified key to the genera is provided.