Six-spotted Fishing Spider - Dolomedes triton

Monday, June 22, 2009

Nature Notes
by Bob Thomas

There is a group of spiders in our area that live predominantly around water. They are quite capable of running about on the water's surface, and feeding on animals, such as insects, small fish, tadpoles, and the like, that live at or near the interface of water and air. These are the Fishing Spiders of the genus Dolomedes, Family Pisauridae (members of which are commonly called the Nursery Web Spiders).

Since they run and float about on the surface, the genus is called Raft Spiders in England.

Why don't they sink? They simply spread their weight over the eight points of contact with the water and thus don't break the surface tension at the surface. Wax coated limbs enhance this process. Each leg presses a dimple into the water's surface. The most common form of active movement results when they engage the combination of their lower limbs and the dimple. They may move up to about one foot per second by moving their third pair of limbs backward, an action that performs like a paddle, thus moving the spider forward. The second pair of legs/dimples follow to add to the movement. When both pair are extended as far as possible toward the rear, they are quickly picked up and moved forward for the next rowing action. The spiders' first and fourth sets of legs remain still and actually support the spider above the surface.

Should the spider want to move more quickly (such as escaping from a predator), it propels itself much as we do. It vertically lifts its limbs, then quickly pushes them into the water as it gallops forward. In this instance, surface tension is a minor, if present, component of movement. Instead, the slicing of the limbs into the water sets up an opposing force from the water and the spider is launched to its next landing place.

Fishing spiders are known to glide motionless across the surface of the water, blown by the wind. The spiders have two behaviors that assist in this motion. They may either raise their two front legs into the wind, or simply raise their bodies to increase contact with the wind. Since there is so little friction on the water's surface, an imperceptible (to humans) movement of air may propel them to their destination.

They are, at their wish, capable of going under water, but this is not typical. When they do this, a thin pocket of air forms between the body and the tip of the body hairs. This is their "scuba tank" used during the dive. They have been observed diving to catch small fish and tadpoles, which are most often caught by the spider simply reaching into the water while remaining on the surface.

As mentioned, they belong to the Nursery Web Spider family. Fishing Spiders lay their eggs in spherical egg cases (like the Wolf Spiders) and carry them in their jaws (like the Huntsman Spiders). When the eggs are ready to hatch, they find a bush and pull leaves together and construct a very dense nursery web in which they place the egg case. When the eggs hatch, the baby spiders distribute themselves about the nursery web and stay there for a week or so. The mother is normally nearby, frequently on the surface of the nursery web, standing guard. It is common to see Fishing Spider nursery webs constructed among leafless twigs over water.

Also published in Delta Journal, The Times Picayune, May 25, 2008.

                                        

Dolomedes triton (brown phase) resting                                        Dolomedes triton (black & gray phase)
on the water's surface.                                                                in the water.
Photo by James Beck.                                                                Photo by James Beck.

                                       

Dolomedes triton on land.                                                           Dolomedes triton holding an egg sac with
Photo by James Beck.                                                               her chelicerae.
                                                                                                   Photo by Bob Thomas.

Dolomedes triton with web, egg sac,
and young.
Photo by Bob Thomas.