S. Mahunka szerk.: Folia Entomologica Hungarica 60. (Budapest, 1999)

Evaluation of the data

—Adults were collected between 9 April and 13 October. Adults can be attracted by light, but the period of captures at light is shorter, between 31 July and 21 August (three data only). Larvae were collected between 5 May and 20 October. The map of distribution in Hungary (Fig. 4) makes it clear that specimens collected before 1955 most probably came from the Danube (main branch) and either the MosoniDuna or the Rába at Győr. It was not found in the main branch of the Danube among the records after 1988, but its survival in the Mosoni-Duna and in the Rába is confirmed. New records make evidence to the presence of Macronychus quadrituberculatus in other water courses, i. e. in the Bársonyos, Bódva, Dráva, Ipoly, Lajta, Lapines, Pinka, Tisza, Túr, Vörös-patak and Zala. In Austria, it is known only from Burgenland, from the Rähnitz (Jäch 1992) and the Lafnitz (Graf 1997). In 1998, it was also collected from the Lafnitz, but in the lower reaches, at Königsdorf. Collecting methods are unknown in the case of two old specimens. Adults were collected by light in three cases (4.76%); adults were netted or picked from submerged wood or stone in 46 cases (73.02%); larvae were collected in the same way in 14 cases (22.22%). DISCUSSION Larvae of Potamophilus acuminatus and Macronychus quadrituberculatus live in swift-running, clean, well-oxygenated water of rivers. Contrary to other aquatic beetles, adults do not need to visit water surface to renew air supplies, because they breathe by means of plastron - an extremely dense coat of microscopical hairs acting as "physical gills". The plastron only works when the level of dissolved oxygen is high and the water is free of any kind of detergents, so unpolluted, cool running water is crucial for the survival of these species. Larvae stay and feed on submerged, dead wood and adults are also associated with this particular habitat, at least in a certain period of life. Floods carrying floating wood play an important role in their dispersal. Mating underwater was observed several times in the case of Macronychus quadrituberculatus. This species is occassionally found also on stones where it feeds on the mat of organic matter. Both species fly to light in warm periods of the year. Potamophilus acuminatus is sometimes found walking outside the water - this habit is rather unusual among European riffle beetles. Adults of Macronychus quadrituberculatus are easier collected than the larvae which are quite secretive and well-camouflaged, although the larvae were almost always found on wood and stones where adults were also present. Larvae of Potamophilus acuminatus are distinctive and easy to collect, or at least much easier than adults. It seems quite possible that adults of this species spend only a short time in water. In late June and early July of 1999, a few of the observed beetles stayed on the submerged pieces of wood while many (up to several dozens) individuals clinged to the parts emerging above the water surface where mating was observed. The beetles swiftly took to flight when disturbed, and landed on another emerging wood or even on the observer's hand put in water. This habit is similar to that of certain species of Donaciinae (Chrysomelidae) (e.g. Donacia crassipes Fabricius, 1775) living on leaves floating on water surface.