Amazing Arachnids: Fishing Spiders

Adapted from pages 296-297 of Amazing Arachnids:

Resting its front feet on the water’s surface, a Dolomedes fishing spider waits along the edge of a small, slow-moving stream. It reads every disturbance, however subtle, on the water’s surface much the way that an orb weaver spider reads the vibrations within its web. In addition to detecting motion with its feet (specifically with the metatarsal lyriform organ), it can also see quite well; its large eyes are not very different from those of its cousin the wolf spider. The fishing spider’s patience is rewarded when an immature grasshopper attempts to leap across the stream and falls onto the surface of the water. Faster than the eye can follow, the fishing spider gallops across the water’s surface and grasps the hapless grasshopper between its two impressive fangs. The spider then returns to the edge of the stream to eat the grasshopper on land, where it efficiently masticates its food and sucks down the liquefied portion until all that is left of the grasshopper is an unrecognizable crumb and a few fragments. 

Dolomedes belongs to the family Pisauridae, also known as nursery web spiders and fishing spiders. In some ways, the fishing spider is the aquatic analogue to the terrestrial wolf spider. This family includes characteristically large, handsome spiders with good eyesight that depend on their speed and strength in order to capture prey. Many of the family frequent moist habitats, but it is the genus Dolomedes that has mastered a lifestyle connected to the water. Despite the fact that some of the species in this genus reach an impressive size (Dolomedes okefenokensis has a leg span of 4 to 5 inches, or 10 to 12.7 cm), they can “row” or even rest their bodies on the water’s surface without breaking the surface tension. The water simply indents or dimples where their legs and body contact the surface. While the spider is on the surface of the water, it

An impressive predator, this mature female Tinus peregrinus fishing spider has captured a fish as large as herself.
She has carried it up into vegetation, where she will masticate and predigest the fish. She must feed out of water or the enzymes needed for predigestion will be diluted out.

is vulnerable to attack from below by underwater predators such as frogs. In this situation, the spider literally levitates by rapidly pushing all its legs downward against the water’s surface to generate the force needed to jump straight up. It then gallops to safety. If the fishing spider becomes startled or frightened by a bird or a wasp, it scrambles underwater, clinging to vegetation so it doesn’t pop back up to the surface. A thin layer of air clings to the hydrophobic cuticle and hairs on the spider’s body, giving it a lovely silvery appearance. It can remain underwater for a good 40 minutes while waiting for the danger to pass. 

Dolomedes spiders must remain vigilant while hunting, because they themselves are hunted. A spider wasp in the pompilid family, Anoplius depressipes, preys exclusively on female Dolomedes spiders. If a fishing spider sees one of these wasps nearby, it takes evasive action, fleeing from the wasp and diving under water in an attempt to escape. But the wasp does something really extraordinary. It actually dives and then swims underwater in pursuit of the unfortunate spider. Once it finds its prey, the wasp stings and paralyzes the spider. The wasp then surfaces with the paralyzed spider and drags it across the water as it skims across in a low flight trajectory. The spider is installed in the nest burrow of the wasp, and a single egg is laid on it. The wasp larva feeds on the still-living, paralyzed Dolomedes until it finally kills the spider. Then the wasp larva pupates.

Unlike the wasp, Dolomedes hunts on the surface of the water. Some authors have written that it hunts underwater, but this has yet to be clearly documented. Instead, it captures its prey primarily either at the surface of the water or on land. Despite this limitation, it can readily catch fish as they swim very close to the water’s surface. The fangs and venom appear to be highly effective in killing the captured fish almost instantly, making it easier for the spider to carry its prey across the water and onto land or up into vegetation growing at the edge of the water. Because spiders ingest only liquefied, predigested food, the fishing spider must eat its prey above the water or else its digestive fluids will be diluted or lost.

Amazing Arachnids coverAmazing Arachnids
By Jillian Cowles

The American Southwest is home to an extraordinary diversity of arachnids, from spitting spiders that squirt silk over their prey to scorpions that court one another with kissing and dancing. Amazing Arachnids presents these enigmatic creatures as you have never seen them before. Featuring a wealth of color photos of more than 300 different kinds of arachnids from eleven taxonomic orders–both rare and common species—this stunningly illustrated book reveals the secret lives of arachnids in breathtaking detail, including never-before-seen images of their underground behavior.

Amazing Arachnids covers all aspects of arachnid biology, such as anatomy, sociality, mimicry, camouflage, and venoms. You will meet bolas spiders that lure their victims with fake moth pheromones, fishing spiders that woo their mates with silk-wrapped gifts, chivalrous cellar spiders, tiny mites, and massive tarantulas, as well as many others. Along the way, you will learn why arachnids are living fossils in some respects and nimble opportunists in others, and how natural selection has perfected their sensory structures, defense mechanisms, reproductive strategies, and hunting methods.

  • Covers more than 300 different kinds of arachnids, including ones new to science
  • Features more than 750 stunning color photos
  • Describes every aspect of arachnid biology, from physiology to biogeography
  • Illustrates courtship and mating, birth, maternal care, hunting, and defense
  • Includes first-ever photos of the underground lives of schizomids and vinegaroons
  • Provides the first organized guide to macroscopic mites, including photos of living mites for easy reference