Threshold picking up too many voxels. How do I segment different areas with same voxels without having to manually paint it?

Operating system: Mac
Slicer version: 4-11-0
Expected behavior:
Actual behavior:

I want to just segment the outline (cuticle) but the inner muscles are too similar in intensity and threshold keeps picking them up. How do I segment this without have to manually paint it on each z stack?

you can segment with the Islands after threshold.