"This creature is basically a bear-sized wasp monster that can paralyze its prey for weeks, lays eggs inside of them, and is supernaturally resistant to all movement-impeding effects."
"What are you going to name it after?"
"Probably its diet. It eats spiders: how crazy is that?"
4 magical beast RHD, large size without reach, a decent land speed and good fly speed, some natural armor, and okay ability scores all around, but animal intelligence and no opposable thumbs.
A spider eater's other traits aren't that important to its LA. Implant is merely a flavor thing, and the poison is very unique but unlikely to ever see much use in combat (and even then, there's easier ways to get save-or-lose poison).
Freedom of Movement, however, is essentially a free slotless 40000 GP item that gives a highly desirable effect at quite a high caster level. It's awesome, and it's going to ruin a lot of terrain effects and monster abilities.
I think the spider eater is worthy of +1 LA, just like the similar hippogriff and giant eagle/owl. Arguably, +2 might be in order, but let's stick with the lower value for now.