Classic grid, loads of overlap between all the categories rather than the easier plain red herring approach. I know that's a challenge, so well done.

My only other comment would be that you've picked a few answers that are more obscure than you need to. I wasn't familiar with mouse spiders or invisible spiders - I'm guessing in the former case because they live in Australia, a long way from me, and in the latter because they're invisible. I'd also never heard of Wolf Scouts, and they seem to be just a thing in the American Cubs.

I love the ideas behind this grid, and your execution is brilliant. Maybe my knowledge just isn't general enough to appreciate your work ;-)

Fun and challenging.

I particularly liked the explanations.

I successfully backed into the 4th group .... but even then had a tough time connecting them. Nice one!

Thank you toes, toast and Sid for the positive comments:)
JamesD: Thanks for the detailed critique and your kind words about the grid. I don’t think it’s ever wise to use oneself as a yardstick for anything subjective. In the case of PuzzGrid, one person’s esoterica is almost certainly another’s general knowledge. And I’m pleased to have broadened your knowledge of Scouts and spiders; you never know when one or both might come in handy.