Under the traits natural weapons give 1d10 + Strength Bonus damage. It may not parry, and counts as primitive.
I don't see any mention of what Improved Natural weapons give.
So for example, If Hormagaunts had Trait: Natural Weapons (Scything Talons) does this mean-
Their attack would be WS, (1d10+5 R; Pen 3) which also adds their Strength bonus for the damage, is unable to parry and counts as primitive (non-primitive armor gets double AP before pen).
I'd imagine that Improved Natural Weapons means they are no longer considered primitive? This is just my guess. Does this also mean they are allowed to parry?