The branches in this photo are very different from the usual kind. They are associated with the circular objects which almost never happens with the branches, they are mostly single branches, and towards the bottom they resemble ravines perhaps.

http://barsoom.msss.com/moc_gallery/m13_m18/images/M14/M1401694.html

In the next one around the middle of the photo the spider features are on the edge of a smoother area which may be of lower elevation. This smooth area seems inhospitable for some reason excep for a few branches.

http://barsoom.msss.com/moc_gallery/m13_m18/images/M14/M1401682.html

While near the equator these features may be related to the spiders in some way, perhaps when it was colder here. There are many features like these so perhaps the spiders were a near global phenomenon at some stage.

http://barsoom.msss.com/moc_gallery/m13_m18/images/M14/M1401676.html

This photo has many large mounds that branches seem to come out of, also it shows some spiders seem to manifest better in depressions.

http://barsoom.msss.com/moc_gallery/m13_m18/images/M14/M1401631.html

A good example of spiders associated with the terrace like features and also some depressions. Perhaps either these features manifest spiders at some stage, or more interestingly perhaps the branches may assist in forming them often disappearing later the bare terraces, circles, depressions, etc. The process that looks spider like on top may make these shapes underneath.

http://barsoom.msss.com/moc_gallery/m13_m18/images/M14/M1401621.html