Unravelling the obscure homology: postembryonic development of chaetotaxic traits in a basal hexapod taxon (Collembola: Tomoceridae)

Homology assessment is essential for systematic zoology, but for many taxa and traits, precise recognition of homology can be challenging with limited evidence. Tomoceridae is one of the earliest branches in the elongate-bodied springtails (Collembola: Entomobryomorpha), and is among the commonest and best-known families in the class. However, the homology of some important traits is still obscure in this group, including its characteristic chaetotaxic traits. In this study, we conducted the first rigorous homology assessment of chaetotaxy in Tomoceridae based on ontogenetic evidence. The postembryonic development of chaetotaxy was investigated in three species of Tomocerus. The primary chaetotaxy of first instar larvae show high similarity within Tomocerinae. The postembryonic development involves extensive transformations between chaetal types. The continuous dwarfism of primary chaetae during development leads to apparent oligochaetosis in adults. The thoracic bothriotricha are transformed from primary macrochaetae. The sensory chaetotaxy is stable during postembryonic development, and shows interspecific differences mainly on the mesothorax and the fourth abdominal segment. This study will promote the future use of chaetotaxic traits in the taxonomic and phylogenetic studies of Tomoceridae, and will potentially illuminate the early evolution of Entomobryomorpha.