Studies of the metathorax of the trout-stream beetle,Amphizoa lecontei Matthews (Coleoptera : Amphizoidae): Contribution towards clarification of the systematic position of Amphizoidae

Skeleton and musculature of the metathorax and the abdominal nerve cord of Amphizoa lecontei (Coleoptera : Amphizoidae) Matthews were examined and compared to those of other members of Adephaga (Coleoptera). The following characters may be important for phylogenetic analysis. The narrow prescutal area and moderately developed mesophragma with distinctly separated median phragmal lobes in Amphizoa is considered as plesiomorphic, whereas the median phragmal lobes lying closely together or fused is a possible synapomorphy of Hygrobiidae and Dytiscidae. The short 1st axillary sclerite of Amphizoa may be an autapomorphy of the group. The weakly arched notum and the poorly developed metaphragma arc obviously the result of tendency towards flightlessness. The anepisternum meets the middle coxal cavity. The prosternal process does not reach the meta “sternum” (“sternum” = preepisternum + basisternum presternum + spinasternum 2). The specialized pro-meso-meta “sternal” articulation in Hygrobia and Dytiscidae is considered a synapomorphy of both groups. The amphizoid metafurca is distinctly reduced in size. Pouch-like anterolateral excavations are considered plesiomorphic. Strong dorsolateral and weak ventrolateral projections of the metafurca may be a synapomorphy of Hygrobiidae and Dytiscidae. The hind legs of Amphizoa are not adapted for swimming, except for very sparse fringes of thin setae. Swimming hind legs are considered as a synapomorphy of Hygrobiidae and Dytiscidae. The main flight muscles were degenerated in all specimens of Amphizoa examined. Absence of muscle (M) 65 may be autapomorphic for Amphizoidae. Presence of M 72 is a symplesiomorphy of Amphizoidae and Dytiscidae. M 81 and M 83 arc absent. The strong M 82 is considered as plesiomorphic. The moderate size of M 85 is connected with the lack of swimming ability. The elongate abdominal nerve cord in Amphizoa points towards a sistergroup relationship between Hygrobiidae and Dytiscidae in both of which the abdominal nerve cord shows a high degree of fusion.The phylogenetic propositions of Baehr (1979), and Kavanaugh (1986), are not consistent with the interpretation provided herein, whereas the conclusions of Burmeister (1976), Ruhnau (1986) and Beutel (1986) seem to correlate well. Therefore, it is suggested that the Amphizoidae are the sistergroup of Hygrobiidae + Dytiscidae.