Monday, January 26, 2015

Fishing Spider

I use this flat rock to monitor the progress of breeding Streamside Salamanders.  This section of the upper reaches of the creek typically has a moderate flow of clean water through the winter and spring seasons.  The salamanders visit here in good numbers and have used the underside of this rock for the past several years as a repository for their eggs.  The rock is not embedded in the stream gravel and is easy to lift for a peek beneath.  No salamanders were found during my last check beneath the rock.  Instead, as I tipped the rock up on its side, a large spider fell from beneath and began floating away in the current.

I quickly scooped the creature out of the water and found it to be a female Fishing Spider, Dolomedes vittatus.  This species is found near small, running streams and is covered by water repellent hairs that allow them to walk on the water’s surface.  They can also submerge, with the hairs maintaining a surface film of air around their bodies.  I couldn’t tell if this individual had actually been utilizing an air pocket beneath the rock or if it was just utilizing the film of air covering its body.  The bottom of the rock is smooth and fully submerged, so if an air pocket was present, it would have to be small.  Using the warmth of the water is a neat way to survive subfreezing temperatures.  Even at its coldest, the temperature of flowing water in the creek doesn’t drop below 32 degrees F.  When nighttime temperatures hover near zero, taking advantage of the warmer water temperature is a distinct advantage.  The surface of the air film covering the spider would also allow for oxygen and carbon dioxide exchange with the flowing water.

The spider was cold and its first movements caused it to roll down into my hand.  As the spider gained warmth from my hand, its movements became more coordinated.  It wasn’t long before it wouldn’t hold still for more photos.  I released it into a cavity beneath a large rock that set half in and half out of the water.  I figured the spider could choose whether or not to return to a submerged resting spot.

Talking about spider size can sometimes be confusing.  This is considered to be a large spider for this area.  If I described it by using the common measure of body length, I would say that it was just a little over half an inch in size.  That might seem small to many people.  If I used the length from the tip of one leg, on through the body and out to the tip of the opposite leg, this spider would be described as 3½ inches across.  Some might use those dimensions to describe it as palm sized.  Either way, it makes it sound like one big spider, but most of that size is just open space between the legs.

This is an attractive species.  The two dark markings near the center of the carapace and the rows of white spots along the abdomen are diagnostic.  Notice that this individual is missing one of its hind legs. 

Good eyesight is a must for these hunting spiders.  A row of four small eyes sits below a row of four larger, more widely spaced eyes.  This lady shouldn’t have any trouble keeping track of her prey.

From the front, the markings of the spider seem to form a face.  The abdomen is decorated with two dark spots with white edging that look like eyes.  The twin dark markings on the carapace suggest a nose and the curved rows of eyes designate the mouth.  I don’t know that this configuration of characters actually serves any defensive functions, but it’s still interesting.  The shape and markings of the abdomen as seen from this angle remind me of the 1978 version IL series Cylon.  By your command.

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