Rhantus suturalis

THE "SUPERTRAMP BEETLE" IS COMMON EVERYWHERE, especially in temporary pools that dry out in the summer. These pools are also breeding ponds for mosquito larvae, the food for the larvae of this beetle, which also regularly dives into the garden pond. That saves us a few mosquito bites... The specimen from these photos had landed in an aquarium in the garden. And she laid eggs there, because a few days later little larvae suddenly swam in the container.
The beetles of the genus Rhantus are medium-sized, at first glance dark brown beetles, which, like all true aquatic predators, swim excellently. They are also good flyers. With a magnifying glass you can see the dense black stippling on the golden yellow elytra, which gives the beetle a dark brown appearance, when viewed with the naked eye. This stippling can be found in older names (synonyms) for this beetle: Rhantus pulverosus and Rhantus punctatus - See the »names page«. The black markings on the side edges of the elytra are missing, so the beetle is lightly fringed. All swimming beetles (Dytiscidae) carry an air supply under their elytra, which is refreshed at the water surface via the abdomen tip. Under water, the beetle occasionally squeezes this air from the space under the elytra into a large air bubble, which remains at the rear tip. Oxygen from the water is then absorbed into the bubble by diffusion. Click on the image on the right for more information.

Small pools of mosquito larvae can be found almost everywhere in the world, and with them also the beetles of the Rhantus genus. There are a number of species in the UK and Europe, some of which are very common. Left: the beetle of these pages, Rhantus suturalis, a globally distributed species, is recognizable by the small (sometimes somewhat faint) diamond-shaped spot in the middle of the pronotum. At right: If that spot is a larger oval and is between two other black spots, it is Ranthus rontalis. This species is common in the south-west of the Netherlands and the coastal strip, but rare inland (van Nieukerken, 1992). In the UK it is common in some regions (Foster & Friday, 2011). Far right: Rhantus exsoletus, a third common species, that has no spot in the middle of the pronotum, but a faint black band on the rear edge. Also note the (non-specific) differences in the head spots. I have doubts about the correct identification of this last specimen, it could also be the (rarer) species Rhantus suturellus.

The photos on the right show the three species side by side. (The original photos showed: R. suturalis on the bottom, R. frontalis horizontally on a hornwort and R. exsoletus vertically on a leaf.) The beetles look very similar. It's a pity that the head position is different in the photos, which makes comparison more difficult. As shown in the photo below, R. suturalus is black on the underside, R. frontalis has a partially yellow underside.