The "spiders" are raised regions in the polar regions of the Mars that have dendritic troughs emanating from a central point. This page has an extensive colection of them:

http://www.martianspiders.com/

Kieffer et.al. propose an explanation based on CO2 gas flows.:

Annual punctuated CO2 slab-ice and jets on Mars.
http://www.lpi.usra.edu/meetings/polar2000/pdf/4095.pdf

However, another possibility is that they are low, flat pingos:

Pingos, Prince Patrick Island.
http://iss.gsc.nrcan.gc.ca/page1/landf/north/queen/patrick/p13.htm

Pingos are usually regarded as conical hills. The low, flat pingos on Prince Patrick Island are the only examples I've found with this shape.

Some examples of the more common conical pingos:

BOAT TO THE PINGOES
http://www.auroranet.nt.ca/atc/noframe/tour12.htm

Pingos
http://sts.gsc.nrcan.gc.ca/beaufort_dcp/index_e.asp?CaID=7&PgID=37

Pingos
http://www.arctic.uoguelph.ca/cpe/environments/land/features/freeze-thaw/pingoes.htm

Bob Clark

Pingos, Prince Patrick Island.