Web making seems like a fairly complex behavior built from a pretty strong material. So how exactly did it evolve? Do we have any clues about what kind of features/behaviors preceded web making and made it possible? Are there any examples of convergent evolution?
One of the comment to David's answer (which covers the history and reasons for spiderwebs in its two links) mentions a comparison between web patterns and molecular phylogeny. Not precisely that, but I found this article which opens up some interesting possibilities (I can delete this or turn it into a comment if you consider it too unrelated, please let me know).
- Fernández, Hormiga & Giribet. 2014. Phylogenomic Analysis of Spiders Reveals Nonmonophyly of Orb Weavers. Current Biology.
Spiders constitute one of the most successful clades of terrestrial predators. Their extraordinary diversity, paralleled only by some insects and mites, is often attributed to the use of silk, and, in one of the largest lineages, to stereotyped behaviors for building foraging webs of remarkable biomechanical properties. (...) Prior molecular efforts have focused on a handful of genes but have provided little resolution to key questions such as the origin of the orb weavers. We apply a next-generation sequencing approach to resolve spider phylogeny, examining the relationships among its major lineages. (...) These results imply independent origins for the two types of orb webs (cribellate and ecribellate) or a much more ancestral origin of the orb web with subsequent loss in the so-called RTA clade.
When comparing molecular family trees, orb weaving araeonoids and deinopoids seem to be more distantly related than thought.
That discovery leaves two possible explanations for the evolution of the spider web: It either emerged much earlier than previously expected and was later abandoned by some species (most believe so). Or, web-spinning and the capacity to spin silk evolved multiple times. Both theories are viable, they just need to catch more spider families to prove which one fits better.
I kind of hate using Wikipedia as a reference, but this article offers a general overview of spider evolution... https://en.wikipedia.org/wiki/Evolution_of_spiders
And this article mentions fungus gnats (and perhaps other species) as examples of convergent evolution... http://www.mapoflife.org/topics/topic_267_Silk-production-and-use-in-arthropods/
I can't remember with certainty, but I believe some jellyfish (and/or perhaps other marine invertebrates) have sticky tentacles that function somewhat similar to spider webs. In fact, some carnivorous plants have sticky structures that trap prey.