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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

April 1999

Most fishing spiders barely exceed three inches in legspan, but they're plenty big enough to make a very hairy memory.© Don Blegen

Dockside hunters

Fishing spiders stalk prey on land, on water and below the surface.

Don Blegen

Most fishing spiders barely exceed three inches in legspan, but they're plenty big enough to make a very hairy memory.

© Don Blegen

After a long afternoon pursuing the elusive walleye, we were headed back to the cabin and supper. Corey cut the outboard and coasted the boat into the dock. The dock supports were made of skinned pine logs faded white by weathering which were anchored by rock-filled cribs. Two huge, very dark spiders stood out against the white logs. Each was easily the size of a child's hand.

Corey, afflicted perhaps with a touch of arachnophobia, looked them over as the boat drifted in. "Disgusting!" he muttered under his breath. The spiders seemed out of place here in the North Country. Spiders that big should be in some tropical rain forest or maybe a desert. As the boat bumped the dock, one of the spiders scuttled out of sight around a log, like a squirrel around a tree. The other one leaped to the water and danced across the surface to a clump of reeds and disappeared. They were fishing spiders.

Fishing spiders (or raft spiders as they are called in Europe) should not be confused with water striders. Water striders are insects, and fishing spiders are arachnids. Both are predators, and both can skim across the water as if it were a solid surface. But fishing spiders are much, much bigger, and leave a much more lasting impression.

People who have had a good look at a fishing spider are impressed and tend to use superlatives when describing their encounters. Like any good tale of a missed buck or lost lunker, there is an inevitable temptation to exaggerate. Though fishing spiders reach tarantula proportions in many stories, in fact, a big female will barely exceed three inches in legspan – not as big as a tarantula, but plenty big enough to make a very hairy memory.

Several species are found around the northern hemisphere. At least three species (all belonging to genus Dolornedes) are commonly found in Wisconsin, along streams and on ponds and lakes. They may stray some distance from water, moving over shoreside and emergent vegetation. To a fishing spider, docks and bridges are just another kind of tree, suitable for hunting food and a quick escape route from enemies. The spiders can run down prey on land or solid surfaces like any hunting spider. They can also chase prey on the water's surface and even dive beneath the surface to capture prey or escape enemies.

Each of the spider's eight legs is tipped with a hydrophobic fluid that repels water. Touched on the water surface, the foot forms a tiny depression or dimple; but it does not penetrate the water. By kicking the eight dimples with powerful strokes, the spider skims across the surface.

By scuttling down the stem of a cattail or other water plant, the spider can even escape or hunt below the surface. The hairs of its abdomen trap a silvery bubble of air so the spider can breathe as if it were wearing an aqualung. This bubble even functions like a primitive gill, absorbing dissolved oxygen and dissipating carbon dioxide. The spider can stay beneath the surface for more than half an hour.

Fishing spiders do not build webs to catch their prey, although they may use their silk to make egg cases for incubating their babies and protecting them for some time after hatching. Unlike some web-weaving spiders that are nearly blind, fishing spiders have excellent vision. Their eight eyes pick up the slightest movement, have good depth perception, and function well in both bright and dim light.

The fishing spider hunts the frontiers of two very different habitats: water and land. Its unusual adaptations for moving and breathing allow it to function efficiently in both environments, tapping into food sources and eluding enemies. The abundant insect life hatching from the water – mayflies, caddis flies, stoneflies, etc. – are vulnerable to a fishing spider skimming across the surface. It will occasionally eat a small fish or tadpole. Yet scampering up a reed (or dock support) will take the spider out of reach of a hungry bass.

We admire the fishing spider's versatility with an undercurrent of fear and repulsion, given its huge size. In our hypocrisy, we deem it is OK for vertebrates like us to eat an invertebrate. But here is an invertebrate that occasionally eats vertebrates. That's a bit chilling. Or, as Corey muttered under his breath, "Disgusting!"

Don Blegen photographs and writes about nature from Spring Valley, Wis.