Wednesday, October 9, 2013

Spiders of the Forest Floor

Our P. whitmani- 10 mm (2/5 inch)
This bright little spider, discovered on a training field trip, is a member of the jumping spider family Salticidae, probably Phidippus whitmani.  It is a male, identified by the bright red-orange dorsal surface and white setae (hairs) on the forelegs.  They are found in habitat frequented by velvet ants, (which are actually mutillid wasps) and have a similar coloration and texture.  This provides P. whitmani with Batesian mimicry, helping to protect them from predators that are familiar with the painful velvet ant sting.

These jumping spiders live in the leaf litter and low lying plants in wooded areas like we were exploring.  They hunt by sight, their family having the best vision of all invertebrates.  Moving cautiously while hunting and stalking prey, they make quick jumps several times their body length.  Jumping spiders can be identified by their prominent head and two large eyes forward and their other six small eyes above.

"My what big (central) eyes you have" P.  johnsoni by  Kyron Basu
Jumping spiders are distinguished most easily in the field by....jumping!  Their short hops are a somewhat distinctive first clue.  Their cephalothorax (head+chest, the front section where the legs attach) tends to be large in comparison to the abdomen.  As seen above, the front of the cephalothorax is squared off rather than rounded as many other spiders.  Wikipedia describes some of their other characteristics.

Note large head- P. whitmani-   Stephen Schueman
They do not build webs but use their silk to build shelters for protection against inclement weather.  They also use them during their vulnerable molting period, for storage of egg cases and overwintering housing.  Their silk also serves as a safety line, a good strategy when jumping off into the unknown.

These four minute movie outtakes depict general behavior of our Phidippus whitmani, extracted from a hour long video posted by Community Video.  You can see facing turns, grooming, and feeding on a mosquito, then a fly, followed by an immature Pisaurina mira spider (Araneae: Pisauridae).

This is one of a series on finds on the Master Naturalist training field trip.  More pictures from the  field trip are at Finds from the Field

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