Meruidae, or comb-clawed cascade beetles, are a recently discovered monotypic family of Adephaga endemic to Venezuela. The larvae of Meruidae are described for the first time, based on material of Meru phyllisae Spangler & Steiner, 2005, collected together with adults in southern Venezuela. External morphological features, including chaetotaxy, are reported for the mature larva and an assessment made of the polarity of larval characters of phylogenetic utility in Adephaga. Larvae of Meruidae possess a mixture of primitive and derived character states, and they are unique within the Adephaga in that here the mandibles are asymmetrical, the respiratory system is comprised of only two pairs of spiracles (= oligopneustic), the claws are pectinate, and the abdominal sternite VIII is prolonged overlapping the abdominal sternite IX. A parsimony analysis based on 18 informative larval characteristics was conducted with the program PAUP*. The most parsimonious trees confirm Meruidae as a relatively basal lineage within the Dytiscoidea. Both Meru Spangler & Steiner and Noteridae are hypothesized to have diverged anterior to Amphizoidae, Aspidytidae, Hygrobiidae, and Dytiscidae.
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