Morphology and biology of the flower‐visiting water scavenger beetle genus Rygmodus (Coleoptera: Hydrophilidae)

Abstract

Hydrophilidae (water scavenger beetles) is well known as an aquatic beetle family; however, it contains ca. 1,000 secondarily terrestrial species derived from aquatic ancestors. The New Zealand endemic genus Rygmodus White is a member of the hydrophilid subfamily Cylominae, which is the early‐diverging taxon of the largest terrestrial lineage (Cylominae + Sphaeridiinae) within the Hydrophilidae. In this paper we report that Rygmodus beetles are pollen‐feeding flower visitors as adults, but aquatic predators as larvae. Based on analyses of gut contents and a summary of collecting records reported on museum specimen labels, adult Rygmodus beetles are generalists feeding on pollen of at least 13 plant families. Rygmodus adult mouthparts differ from those of other (saprophagous) hydrophilid beetles in having the simple scoop‐like apex and mola with roughly denticulate surface, resembling the morphology found in pollen‐feeding staphylinid beetles. Larvae were found along the sides of streams, under stones and in algal mats and water‐soaked moss; one collected larval specimen was identified using DNA barcoding of two molecular markers, mitochondrial cytochrome oxidase 1 (cox1) and nuclear histone 3 (H3). Larvae of two species, Rygmodus modestus and Rygmodus sp., are described in detail and illustrated; they closely resemble ambush‐type predatory larvae of the hydrophilid tribe Hydrophilini in the head morphology. Rygmodus is the only known hydrophilid beetle with adults and larvae inhabiting different environments.

Larval chaetotaxy and morphometry of Oosternum costatum (Coleoptera: Hydrophilidae) including a discussion of larval characters with phylogenetic relevance

“…Similar pore-like sensoria have been observed in larvae of some Hydrobiusini and other genera (e.g. Rygmodus White, 1846, Laccobius Erichson, 1837, Enochrus Thomson, 1859 in a similar position in second and third instar larvae, probably a convergence (MINOSHIMA et al 2011(MINOSHIMA et al , 2017(MINOSHIMA et al , 2018Archangelsky, unpubl. data).…”

Deceptive pollination of Calanthe by skippers that commonly act as nectar thieves

“…The mutually beneficial relationship between plants and pollinators is common in ecosystems (Fenske et al 2018;Minoshima et al 2018). However, a considerable number of angiosperm species do not provide rewards for pollinators, and have instead developed deception mechanisms to entice insects to visit their flowers and complete pollination without receiving anything in return.…”

First known larva of omicrine genus Psalitrus d’Orchymont (Coleoptera, Hydrophilidae)

“…Within Sphaeridiinae, the larvae of Megasternini and Protosternini do not have distinct teeth on the nasale (Archangelsky 1997(Archangelsky , 1999Fikáček et al 2015Fikáček et al , 2018a; the larvae of Sphaeridiini bear a simple, low, median projection (Archangelsky 1997); the nasale of Coelostomatini is more or less variable, ranging from that with a simple median projection resembling that of Sphaeridium Fabricius, 1775 (e.g., Dactylosternum cacti (LeConte, 1855) in Archangelsky 1994) to closely aggregated teeth-like projections resembling that of Omicrus (e.g., Phaenonotum exstriatum (Say, 1835) in Archangelsky et al 2016b). In comparison to Cylominae, which is a sister taxon of Sphaeridiinae, distinct teeth on the nasale are common in the known aquatic or semi-aquatic larvae of Cylominae (Anticura Spangler, 1979, Cylomissus Broun, 1903, Cylorygmus Orchymont, 1933, and Rygmodus White, 1846Minoshima et al , 2018Seidel et al 2018), and only the terrestrial genera Andotypus Spangler, 1979 andAustrotypus Fikáček, Minoshima &Newton, 2014 do not have distinct teeth on the nasale (Fikáček et al 2014). It seems probable that the reduction of the toothed nasale corresponds to specialized prey preference and feeding behaviour in many terrestrial hydrophilid larvae.…”

“…The reduction of larval legs is an evolutionary trend of terrestrial Sphaeridiinae, especially in the Megasternini + Sphaeridiini clade, in which the segments of legs are more or less reduced, appearing as a minute tubercle (Oosternum and some Cercyon) to minute two segments (some Cercyon and Armostus) or five-segmented but short, rod-like (Sphaeridium) (Archangelsky 1997(Archangelsky , 1999(Archangelsky , 2018Minoshima 2018). The presence of legs in Omicrini is a plesiomorphy as supposed by Short and Fikáček (2013) based on the fact that terrestrial and aquatic cylomine larvae and acidocerine larvae have well-developed legs (e.g., Archangelsky 1997;Minoshima and Hayashi 2011;Fikáček et al 2014;Minoshima et al 2018). This is the first detailed description of the larval chaetotaxy of Omicrini.…”