The egg case and larvae of all three instars of the cascade beetle Tritonus complanatus Short, 2008 (Coleoptera: Hydrophilidae) are described based on specimens found co-occurring with adults and associated with them by comparing histone 3 nDNA sequences. The morphology of the larva is congruent with the phylogenetic position of Tritonus Mulsant, 1844 in the Paracymus-group of the Laccobiini, but also exhibits characters shared with larvae of Hydrobiusini, and some presumed adaptations to the hygropetric life style. The larva has open mesothoracic and abdominal spiracles situated on top of long spiracular tubes in the first instar, likely working as ‘spiracular snorkels’, and on low tubercles in later instars, along with a well-developed spiracular atrium. Similar spiracular morphology was found in the larva of the terrestrial laccobiine genus Tormus Sharp, 1884, and a brief examination of larvae of few other genera (Oocyclus Sharp, 1882, Hydrobius Leach, 1815) reveals that the peripneustic spiracular system (mesothoracic plus 8 abdominal functioning spiracles) may be more widespread in larval Hydrophilidae than currently believed.