Larval morphology of the Japanese species of the tribes Acidocerini, Hydrobiusini and Hydrophilini (Coleoptera: Hydrophilidae)

Abstract: Larval morphology of the Japanese representatives of the tribes Acidocerini, Hydrobiusini and Hydrophilini (Coleoptera, Hydrophilidae, Hydrophilinae) is described. Eleven species assigned to seven genera are studied: Agraphydrus narusei (Satô, 1960) (first and third instars), Enochrus (Holcophilydrus) simulans (Sharp, 1873) (all instars), E. (H.) umbratus Sharp, 1884 (third instar), E. (Methydrus) japonicus (Sharp, 1873) (all instars), Helochares (Helochares) pallens (MacLeay, 1825) (all instars), H. (Hydrobaticus) anchoralis Sharp, 1890 (first instar), H. (Hydrobaticus) nipponicus Hebauer, 1995 (all instars), Hydrobius pauper Sharp, 1884 (second and third instars), Hydrochara affinis (Sharp, 1873) (first and third instars), Hydrophilus (Hydrophilus) acuminatus Motschulsky, 1854 (all instars), and Sternolophus (Sternolophus) rufipes (Fabricius, 1792) (all instars). Three aspects of larval morphology were evaluated for each species studied: (1) general morphology; (2) chaetotaxy of the head capsule and its appendages; (3) morphological transformations between instars. Primary chaetotaxy is rather stable among taxa and instars examined, thus we were able to distinguish primary and secondary sensilla even in the majority of the second and third instar larvae studied. Secondary chaetotaxy shows slight intraspecific variation. Presence/absence of secondary sensilla may be useful at least for distinguishing the first larval instar from later instars, and often for distinguishing all three larval instars. The arrangement of primary and secondary sensilla shows differences among the tribes and genera studied, and between species in Enochrus Thomson, 1859 and Helochares Mulsant, 1844. The pattern of character transformations between larval instars is rather constant among taxa examined. Generic keys of aquatic Hydrophilidae, tribes Acidocerini and Hydrophilini from Japan, and key to Japanese species of Enochrus and Helochares with known larvae are provided.

Key words: Coleoptera, Hydrophilidae, Hydrophilinae, Acidocerini, Hydrobiusini, Hydrophilini, larva, morphology, chaetotaxy, morphological transformation, taxonomy, Japan