Larval chaetotaxy and morphometry of Coelostomatini and Sphaeridiini (Coleoptera: Hydrophilidae): Insights into terrestrial adaptations of Dactylosternum Wollaston, Hydroglobus Knisch, and Sphaeridium Fabricius

This study presents a comprehensive comparative analysis of larvae from the Coelostomatini and Sphaeridiini tribes of Sphaeridiinae. The chaetotaxy of the head capsule and head appendages of larvae of Dactylosternum abdominale (Fabricius), Dactylosternum cf. subrotundum (Fabricius), Hydroglobus puncticollis (Bruch) and Sphaeridium scarabaeoides (Linnaeus) is described and illustrated. Morphometric characters from the head capsule and mouthparts are included, along with detailed illustrations and scanning electron microscope photographs of all characters. Morphology and chaetotaxy of these larvae are compared with those of other Sphaeridiinae. Coelostomatini revealed two distinct ecomorphotypes, terrestrial and aquatic, with adaptations reflecting their respective environments. The somatic morphology of Dactylosternum larvae shows convergence with those inhabiting decaying organic matter, such as Sphaeridiini and Megasternini, indicating common adaptive strategies across tribes. However, Dactylosternum cf. subrotundum shows ambiguous characteristics between terrestrial and aquatic morphotypes, suggesting a distinct adaptive strategy in terrestrial environments. No putative shared larval apomorphies were observed to support a phylogenetic relationship between Hydroglobus and Phaenonotum as close as previously assumed. Detailed comparisons among Sphaeridium larval instars reveal variations in chaetotaxy and morphometry of head structures and appendages.