Dolomedes

Lore & Background

Dolomedes spiders are covered in hydrophobic hairs that allow them to stand or run on water using surface tension. When submerged, air trapped in these hairs forms a silvery film, enabling them to breathe via book lungs that open into the film. They hunt by waiting at the edge of water, detecting ripples from prey, then running across the surface to subdue it with their foremost legs and inject venom. Their main prey is insects, but larger species can catch small fish. They are nocturnal hunters, as birds are their main predators.

Reader's Guide

Dolomedes species are ecologically significant as predators in aquatic and semiaquatic habitats, controlling insect populations and occasionally preying on small fish. Their unique adaptations—hydrophobic hairs, surface tension locomotion, and underwater breathing via an air film—make them a model for biomimetic studies. In North America, nine species exist, including the six-spotted fishing spider (D. triton) and the arboreal D. albineus. Europe has two species, with the great raft spider (D. plantarius) listed as endangered in Great Britain. New Zealand hosts four endemic species. Their hunting method, using vibration detection on water rather than webs, distinguishes them from most other spiders. The male-biased sex ratio (3:1) and obligate male death after mating in D. tenebrosus highlight unusual reproductive biology. Conservation concerns exist for some European species due to habitat loss.

Did You Know?

• Dolomedes spiders can run across water and submerge themselves, becoming encased in a silvery film of air that allows them to breathe.
• The genus name Dolomedes comes from the Greek word δολομήδης, meaning wily or deceitful.
• Population sex ratios in Dolomedes vary by species and location; no consistent 3:1 male-to-female ratio is established across the genus.
• Some Dolomedes species can catch small fish, with reports of them eating goldfish, sticklebacks, and even an immature smallmouth bass.