Abstract
AbstractHydrophilidae (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.
Highlights
Larval stages of holometabolous insects frequently exhibit very different life styles from that of the conspecific adults, both in habitat and in food preferences
We examined the molar structure of four hydrophilid taxa for comparative purposes: Rygmodus modestus and Saphydrus suffusus Sharp, 1884, Dactylosternum hydrophiloides MacLeay, 1825 (Sphaeridiinae: Coelostomatini, terrestrial species feeding on decaying plant material) and Helochares (Hydrobaticus) sp. (Acidocerinae, saprophagous aquatic species)
The topology of the cox1 tree seems congruent to the differentiation in the genital morphology among Rygmodus species: R. alienus, which is a member of the R. alienus - R. antennatus (Sharp, 1884) species complex characterized by the wide and short median lobe of aedeagus, was placed as the early-diverging taxon sister to the other sampled species, which are all characterized by the rather uniform genital morphology with the narrow and elongate median lobe of aedeagus
Summary
Larval stages of holometabolous insects frequently exhibit very different life styles from that of the conspecific adults, both in habitat and in food preferences. This ability to occupy multiple niches during the life of a single specimen, orecological divergence , is often considered as one of the reasons for increased diversification rates of Holometabola and for its immense species diversity (Yang 2001; Mayhew 2007; Rainford et al 2014; but see Condamine et al 2016). In many groups of hemimetabolous stoneflies (Plecoptera), larvae are aquatic and predatory or detritivorous, whereas terrestrial adults feed on lichens, fungal spores, pollen or arthropod corpses (e.g. Fenoglio & Tierno de Figueroa 2003; Rúa & Tierno de Figueroa 2013). Dytiscidae with predatory adults and larvae, Elmidae with adults and larvae often living alongside on the same substrate, feeding on algae and detritus scraped from surface, or on microorganisms from decaying wood (Brown 1987; Balke 2005; Kodada & Jäch 2005)
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