Testing the monophyly of Chaetarthriinae (Coleoptera, Hydrophilidae) and the phylogenetic position of Guyanobius with larval characters

The subfamily Chaetarthriinae includes morphologically distinct larvae that are adapted to a diversity of environments. Based on larval characters, cladistic analyses (maximum parsimony (MP) and Bayesian inference (BI) with homoplasy as a partitioning scheme) were performed to test the monophyly of the subfamily and the relationships of the two tribes included in it: Chaetarthriini and Anacaenini. The chaetotaxy of a third instar larva Guyanobius adocetus is described and illustrated in detail, including morphometric characters. This larva is compared to those of the known larvae of the tribe Chaetarthriini belonging to the genus Chaetarthria, and Pseudorygmodus, Crenitis, and Crenitulus from Anacaenini. None of the unconstrained analyses recover Chaetarthriinae as monophyletic. Chaetarthria diverges in an early branch, probably due to a series of unique morphological modifications associated with a riparian lifestyle whereas Guyanobius appears closely related to Anacaenini. Two alternative positions of Guyanobius are revealed: (1) as sister of all Anacaenini (unconstrained MP) or (2) nested within Anacaenini as sister of Crenitis + Crenitulus (constrained MP and unconstrained BI). The genera Paracymus and Tormus (tribe Laccobiini) diverge as two successive branches subordinate to Chaetarthriinae (excluding Chaetarthria) in the unconstrained MP analysis. However, the support is rather weak, and the position of Paracymus and Tormus is an artifact produced by some homoplastic characters. In this regard, homoplasy partitioning resulted a useful technique to solve some artifacts generated by convergent morphologies.