Dark fishing spider (Dolomedes tenebrosus), Porter, Texas

Report and Photographs Courtesy of Greg S., Porter, Texas, 3 July 2007

Greg Wrote:

Jerry--I found this spider in Porter Texas, on my back patio on the wall, at night, on July 3rd 2007. It didn't move or try to escape. Its body is about 1 1/2 inches long, and when including the legs the the whole spider is 4 1/2 inches in diameter. My camera had trouble focusing close enough for the eyes, but it has two rows of eight eyes, the top (posterior) row being larger, and the bottom (anterior row) small and bunched together. I Identified it using the information posted at http://entomology.uark.edu/museum/dolomede.html.  I can't tell whether it is tenebrosus or scriptus. Anyway, thought you might be interested--Thanks, Greg.

Editor's Notes: The dark fishing spider (Dolomedes tenebrosus) is a large spider, found in the eastern half of the U.S. and Canada, from New England and adjacent portions of southeastern Canada, south to Florida, and west to South Dakota and Texas.  It and others in the genus Dolomedes generally live near water, and feed primarily on aquatic insects.  However, they will also prey on terrestrial insects and will also capture and feed on small fishes and other small aquatic organisms.

This spider is a member of the Pisauridae family, known as nursery web spiders (they do not build a silk snare, but hunt as in the Lycosidae; the female carries an egg sac under the sternum until just prior to spiderling emergence, whereupon she builds a nursery web around the sac and guards it until the spiderlings have emerged). At least 52 genera, and 328 species are recognized worldwide.

At least five genera are recognized in North America, with Dolomedes occupying a prominent position in the extant literature.  This genus includes nine species in North America, including D. scriptus (no recognized common name, but perhaps the light colored fishing spider is most appropriate), D. tenebrosus (the dark fishing spider, being darker than D. scriptus and lacking its prominent white markings, having similar, but subdued markings instead), D. triton, previously known as D. sexpunctatus (the six spotted fishing spider, so named for the two rows of six spots on the lateral margins of the abdomen), and D. vittatus (a fishing spider with no common name and little uniqueness to its markings to commend one, though the pattern of the triangular dark spots forward of the thoracic groove is distinctive, being larger and more prominent than in most Dolomedes) . A close cousin to the Dolomedes is the genus Tinus, which contains one species (Tinus peregrinus).  Slightly more removed are the four species of the genus Pisaurina.

As Greg pointed out, the differences between Dolomees scriptus and D. tenebrosus are not easy to distinguish, and my skills are certainly no better than his.  I base my suspicion that this is D. tenebrosus on the absence of large white areas on the dorsum, and on the absence of white "W" markings on the dorsal abdomen. In D. scriptus, the white "W" markings are of a uniform albedo throughout, from right to left across the dorsal abdomen, while in D. tenebrosus they are replaced by light-gray chevrons flanked with white spots of a lighter shade of gray.  It is possible that additional photos of this same spider, from other angles under different lighting, would lead to another conclusion.

All the fishing spiders are considered beneficial, as they keep insect populations down within their spheres of influence.  Their bites are not particularly troublesome.

My Reply:

Greg--Good photos.  Thanks.  If you still have this guy around, consider taking additional photos, particularly of the dorsal abdomen, and of the eyes.  Consider using a flash, to bring out subtle features of the dorsofrontal head---Jerry