Review of the genus Microlaimus de Man, 1880 with an illustrated guide for species identification

Among the representatives of the family Microlaimidae, the genus Microlaimus is the most species richeness. However, the most recent literature on this genus presents species lists that diverge, in terms of composition and the number of species considered valid for Microlaimus. The morphological characteristics of this genus overlap with those of other genera of Microlaimidae, making the taxonomy of this genus complex. In the present study, we reviewed the species of the genus Microlaimus, as well as species included in genera of Microlaimidae that are morphologically similar to this taxon. Groups of species that share certain characteristics were created and represented in an illustrated guide for intraspecific identification of Microlaimus. The position of the amphidial fovea in relation to the anterior end of the body, provided important taxonomic information that was used to distinguish Microlaimus species. The presence or absence of cuticular ornamentation, the size of the spicules, the absence/presence of gubernaculum and the amphidial fovea position in relation to the anterior end of the body were characteristics used to separate the groups. Furthermore, the morphology of the male copulatory structures, as well as the composition of the cephalic arrangement (papillae, setiform papillae and setae) and the length of the cephalic setae in relation to head diameter, were also relevant for the characterization of the species.