Cannot gather venom from a T3 spider with a Steel Istarian Army Knife.

Specificly, I tried to gather strong venom from a Redbacked Spider Hatchling, with a Steel tool, and got the message: "T3/T4 Spider or Scorpion (Dead) requires a tool with a skill quality of 3. You will likely need to change your active school so that your skill quality is high enough to gather from this creature."

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To harvest from the T3/T4 Dead Spider or Scorpion you need 400 Intuition skill, too. Since the resources are grouped as two different tiers (t3 + T4), its a little bit higher requirement than things that are single tiered, Iron, Dark Iron ore, etc.

You should have be able to harvest those at level 40 adventure school

It needs Intuition (adventure) only, not Ingenuity (crafting)?

Correct. It does seem a bit odd to use Intuition, asking devs about that.

There are some resources that do use the adventure level gained skill, and not the craft level gained one.

Usually that's the case for quest items that are not meant to have a craft level requirement to gather, hence the reason for the separate skill gained from adventure not craft was put in. Things like the flowers, and some quest step in Volcano uses intuition.

This item could be set for Intuition so that adventurers can gather it. It would make some sense, since the bug needs killed before it can be harvested. Adventure levels would be the "more needed" skill not a crafting one.

Guaran is correct, some resources like Grave Dust, flowers are for adventure quests and use Intuition instead of Ingenuity.

Since the dead spiders and scorpions (venom) are also used in the Antidote and Tech Kit: Venom forms, they'll be set to both Intuition and Ingenuity for delta 279.