Diving beetles are generally aquatic and live submerged in water during larval and adult stages. A few groups have colonised hygropetric habitats and fewer species still can possibly be referred to as terrestrial. Here we describe six new Copelatine species that were mainly found in dry shallow forest floor depressions in the eastern and northeastern lowland humid forests of Madagascar. Three new species are described in each of the two genera Copelatus and Madaglymbus: Copelatus amphibius sp. nov., Copelatus betampona sp. nov., Copelatus zanatanensis sp. nov., Madaglymbus kelimaso sp. nov., Madaglymbus menalamba sp. nov., and Madaglymbus semifactus sp. nov. Diagnosis, description, known distribution, ecology, and conservation notes are provided for each species. All species are illustrated with a dorsal habitus image, ventral and lateral views of the male penis, and parameres. Photographs of the unusual terrestrial habitats where the species were found are provided. Madaglymbus menalamba sp. nov. is also documented with macrophotos and videorecordings of the terrestrial locomotion and behaviour in the field. Although these species should not be classified as terrestrial, or even semi-terrestrial Dytiscidae, they seem to be specialists of very ephemeral aquatic habitats and stay put instead of disperse when the habitat dries up. It is hypothesised that this lifestyle and behaviour on Madagascar is restricted to the high-precipitation humid forest regions mainly in the east. It may also represent a transition step, or stepping-stone, towards becoming fully terrestrial, a step that the few known terrestrial Dytiscid taxa once passed through. It is very likely that this type of habitat is overlooked for aquatic beetles, not only in Madagascar, and the six species herein described may be just the "tip of the iceberg".
Keywords: Copelatus; Madaglymbus; Analalava; Betampona; Marojejy; Masoala; Nosy Mangabe; humid forest; new species; overlooked habitat; protected area; rainy season; semi-terrestrial.