Three populations of the pill millipede genus Trachysphaera Heller 1858 are present in Great Britain, one on the Isle of Wight, one in South Wales and one in mid-Wales. To identify and characterize the British Trachysphaera populations, the intraspecific and interspecific variation of the populations in South Wales and on the Isle of Wight were studied and evaluated in a first integrative study of members of Trachysphaera, utilizing barcoding and SEM. DNA was extracted from 28 British Trachysphaera and 10 French T. lobata (Ribaut 1954) specimens, one each of French T. cf. drescoi (Conde and Demange 1961) and T. pyrenaica (Ribaut 1908), and one of Spanish T. cf. rousseti (Demange 1959); the barcoding fragment of the COI gene was amplified and their genetic intra- and interpopulation distances compared with one another using two Italian T. spp. and one Croatian T. schmidti Heller 1858 specimens as near outgroups. To compare the genetic distances with the morphological characters, 15 characters of a total of 13 British Trachysphaera, together with two specimens of T. pyrenaica, two T. cf. drescoi and one of T. cf. rousseti were imaged, using the same individuals utilized for DNA extraction. Albeit both British populations are genetically distant, they are closely related (1.9–2.5% p-distance) to French T. lobata, corroborating results of earlier studies. Between different Trachysphaera species, genetic distance was high (16.7–18.8%). The morphological study showed the non-reliability of key taxonomic characters in Trachysphaera, with genetically identical individuals exhibiting morphological variation, especially on the telopods. The only observed morphological characters constant within and different between species were the number of rows of sclerotized bacilli on the tergites, as well as the shape of the male and female anal shield. Both, barcoding and the morphological study identify the British Trachysphaera populations as T. lobata.
