The title has the essence of it. Spider corpses that remain can be harvested for their poison glands, but unfortunately the poisons that one obtains have a "nothing happens" effect when applied to a weapon. This is the case for all types of poison labeled as "Spider Venom (X)", where X is "weak", "superior", etc.
I was going to report this but you beat me to it!
But I got screenshots!
(http://i.imgur.com/6lTCWfd.jpg)
Curiously, the harvested poison glands also have a different name conflicting with what's listed in the combat log:
(http://i.imgur.com/bG1Rv3v.jpg)
Confirming all these things.
If you do not have use-poison a very random dex check is taken into account on the application of poison, which can make you fail nearly everytime. Something to keep in mind.
This happens even with the Use Poison feat.