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Specieswatch: how whirligig beetles evolved to occupy an environmental niche

When alarmed and to avoid predators the whirligig beetle (Gyrinus substriatus) swims rapidly in circles. There are about 700 species worldwide but this 6mm version is the most common in the UK and can be seen in hundreds gyrating on ponds and waterways.

The fossil record shows whirligig species have been around for millennia and evolved to occupy a niche in the environment: the surface of fresh water. The beetle’s two pairs of eyes, one pair scanning underwater and the other above, are simultaneously checking for prey and predators.

The beetles exude a waxy substance that repels water to aid swimming speed and makes them slippery, so a useful escape mechanism from fish or birds.

Their bodies are streamlined into a shape called an ellipsoid with front legs for grabbing prey and two pairs of paddle-shaped rear legs for fast swimming. They carry an air bag under their wings for surviving underwater and fly between ponds and rivers to find new habitat.

Adults live in groups for protection, with the hungriest usually swimming on the outside and taking the greatest risks while looking for insects struggling in the water or small creatures to eat. After mating females lay their eggs on waterweed. The larvae also hunt until large enough to pupate.

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