Spiders are intelligent. At some basic level everyone knows this, even if they don't admit it to themselves, because admitting it invites all sorts of existential horror.
The little ones aren't much for conversation or philosophy but have a keen and innate understanding of geometry and structural engineering that dwarfs that of any other species except for perhaps the, er, dwarves.
The big ones -- the ones the size of people -- are as intelligent as people, if not more so. The phase spiders are geniuses but they're also weird, even by the standards of spiders, because they're thinking in more than one dimension at once.
Miéville! -- so when they try to interact with other sentient species they come across as detached and a bit alien at best, and AIEE! MONSTERS! at worst.
Being clever sorts the spiders realised that they needed some sort of go-between to help them deal with other species and convey their great plans without the encounter descending into stabbing, which is just the sort of inconvenient and tiresome business that upsets the order of things. No spider would be able to deal with the limited and erratic viewpoints of the non-arachnid species for long so they created the ettercaps, hybrids of spider and humanoid and the intended ambassadors of spider kind.
Ettercaps are created when a humanoid is captured -- elves and orcs are favoured because they're the most chaotic and removing them is seen as an efficient way to tidy up the universe -- and encased in a cocoon which is then subjected to a phasey-wasey process that changes the creature within. A few days later the cocoon rips open and an ettercap is released.
Alas, ettercaps are frightening creatures in their own right and despite their best attempts at peaceful discourse often invoke the AIEE! MONSTERS! response -- and that's before anyone finds out how they're made -- but the spiders are working on that.