The Dock Spider / by Louisa Ulrich-Verderber

During one of our annual trips to my family’s farm in Illinois, I was taking the trash out and saw this amazing and rather alarming spider on our deck railing. I rushed in and grabbed a camera and took a few pictures. It was then that I noticed the massive egg sack hanging under the spider’s abdomen!  I put a quarter in the picture to demonstrate how large the hairy lady was.  After I went inside a little shaken, and enjoyed the fried fish my uncle was making.

I didn’t get back to researching this amazing spider until some time later when we were back in Vermont – and had access to wifi - and this is what I found …

After some research, which largely consisted of comparing my photo to dozens of similar ones on Google images, I found a match. I discovered that I had taken a picture of a Fishing spider or Dock spider (Pisauridae Dolomedes). Almost all Dolomedes spiders are semi-aquatic, and hunt by waiting at the edge of a pool or stream. When the spiders detect the ripples from prey on the water, they run across the surface and catch pray with their foremost legs, which are tipped with   small claws. Fishing spiders mainly eat insects, but some larger spiders can catch small fish and swim!

You read that right!.. I did say “They run across the water!”…. because Dolomedes spiders are covered with short, velvety hairs which are hydrophobic. (water resistant)  This allows them to use surface tension to stand or run on the water, like pond skaters. They can also climb beneath the water, trapping air within their body hairs forming a thin film over their whole body which amounts to an air bubble. Dolomedes breathe with lungs beneath their abdomens, which breathe from the air bubble while submerged. The trapped air makes the spiders float so if they don’t hold onto something they will float up and pop onto the surface, completely dry.

The fishing spider has a total of eight eyes. When viewed from the front, they have two horizontal rows of four, which they use to hunt in the dark of night, since they are nocturnal. They also have quite long thickset legs, which tend to be more robust than other spiders because these are the legs that allow them to easily tackle large prey.

 he Dolomedes sense of touch is more important than their many eyes when it comes to detecting prey. They extend their legs onto the surface, feeling for vibrations given off by prey such as mayflies, other aquatic insects, and even small fish. For fishing spiders, the water surface serves just like a web does for other spiders.  When hunting the fishing spider holds onto the shore with their back legs and stretches their body across the water. Their front legs feel for vibrations on the water and are abled to distinguish the erratic vibrations of a struggling insect from the one-off vibrations caused by falling leaves. Along with identifying the source of the vibrations, the spiders are also able to discern the distance to and direction. As soon as the spider knows there is an insect within range they run across the water and grab the insect before it flies to safety. Some fishing spiders use silk draglines to prevent themselves from speeding past the prey.

Fishing spiders are nocturnal hunters, feeding when their main predators (birds and snakes) are sleeping, so that explains why we hadn’t seen one before.  There are some species of insect that act as parasites to the fishing spider, such as wasps who sting the spider to paralyze it before carrying it off and laying an egg in its abdomen. The larvae of the wasp hatch and proceed to eat the spider from the inside out. One technique the spiders use to avoid this terrible fate is to disappear beneath the surface of the water.

Dock spiders exhibit the typical arachnid characteristic of sexual dimorphism that means that the females are much larger than the male. It isn’t uncommon for the female to eat the male after mating, and in some cases, the male brings along a sacrificial fly as a gift to take the edge off her hunger. Unfortunately for the male the presentation of gifts doesn’t always save him from the potentially nasty fate that awaits him as the female may eat him anyway to provide nourishment for her brood. These lady spiders certainly give credence to the expression, “large and in charge”!

This powerful hunting spider that does not spin web for catching prey, however females do spin an egg sac to lay their eggs in. The female carries this egg sac everywhere she goes. When the eggs are ready to hatch the mother spider will spin a nursery web in a protected area to place her egg sac in it. She then guards this nest until the spiderlings hatch and venture into the world.

There can be well over a thousand spiderlings in the nursery web when all of the eggs have hatched. The hatchlings typically stay in the protective confines of their nest for some time after birth as it usually takes them more than one season to fully mature and be ready to reproduce. It is believed that these spiders can produce only two or three egg sacs in their lifetime.

THIS is a truly fascinatinganimal and I am so glad she crossed my path that summer night. What a privilege it is to have her take me on this little journey of exploration. I think it’s fascinating how animals adapt in so many ways to fit their environments. It makes me giddy when I read or hear about obscure creatures that have adapted in strange ways and, as you can see, I run around trying to find the most about that animal’s most extraordinary trait.  This one had too many to count!