The three larval instars of Megadytes (Paramegadytes) glaucus (Brullé, 1838) and the third-instar larvae of M. (Bifurcitus) magnus Trémouilles & Bachmann, 1980 and M. (Trifurcitus) robustus (Aubé, 1838) are described and illustrated for the first time, with particular emphasis on the morphometry and chaetotaxy. A key to the subgenera of Megadytes Sharp, 1882 is presented. In a cladistic analysis of third-instar larval characters, Megadytes is resolved as non-monophyletic; the species of Cybistrini studied, except those included in the subgenus Trifurcitus Brinck, 1945, share three synapomorphies: (i) medial projection of frontoclypeus truncate apically, with many apical setae directed forwards; (ii) lateral projections of frontoclypeus project forwards, not flattened; and (iii) median process of prementum rounded apically. The clade composed of the subgenera Megadytes s. str., Paramegadytes Trémouilles & Bachmann, 1980 and Bifurcitus Brinck, 1945 along with Cybister lateralimarginalis (De Geer, 1774) is well supported by three synapomorphies: (i) head capsule subrectangular and (ii) distal third of mandible more strongly projected inwards, (iii) with a ring of long, hair-like setae. The two species of the subgenus Paramegadytes have bilobed lateral projections on the frontoclypeus. Megadytes (M.) marginithorax (Perty, 1830) is characterized by the very narrow notches between the medial and lateral projections of frontoclypeus. No synapomorphies were discovered to group together the two species of the subgenus Bifurcitus.
The three larval instars of Megadytes (M.) carcharias Griffini and M. (Trifurcitus) fallax (Aubé) are described and illustrated in detail for the first time, with an emphasis on morphometry and chaetotaxy of the cephalic capsule, head appendages, legs, last abdominal segment and urogomphi. The ground plan of chaetotaxy of the genus Megadytes Sharp is described and illustrated based on three of the four recognised subgenera. First-instar larvae of Megadytes are characterised by the presence of a large number of additional sensilla on almost every part of the body. Primary chaetotaxy of the subgenera (Bifurcitus Brinck based on third instar) is very similar, with few differences including (1) shape of the setae on the anterior margin of the frontoclypeus; (2) presence or absence of a ring of multi-branched setae on distal third of mandible; and (3) number of setae on the urogomphus. A cladistic analysis of Dytiscidae, based on 169 larval characters and 34 taxa, indicates that: (1) Trifurcitus Brinck deserves generic status; (2) Cybistrini are not closely related to Hydroporinae; (3) the absence of a galea in Cybistrini is a secondary loss independent of that in Hydroporinae; (4) Cybistrini are well supported by many characters (including several aspects of first-instar chaetotaxy).