Very recently, wing scales of natural Lepidopterans (butterflies and moths) manifested themselves in providing excellent three dimensional (3D) hierarchical structures for surface-enhanced Raman scattering (SERS) detection. But the origin of the observed enormous Raman enhancement of the analytes on 3D metallic replicas of butterfly wing scales has not been clarified yet, hindering a full utilization of this huge natural wealth with more than 175 000 3D morphologies. Herein, the 3D sub-micrometer Cu structures replicated from butterfly wing scales are successfully tuned by modifying the Cu deposition time. An optimized Cu plating process (10 min in Cu deposition) yields replicas with the best conformal morphologies of original wing scales and in turn the best SERS performance. Simulation results show that the so-called rib-structures in Cu butterfly wing scales present naturally piled-up hotspots where electromagnetic fields are substantially amplified, giving rise to a much higher hotspot density than in plain 2D Cu structures. Such a mechanism is further verified in several Cu replicas of scales from various butterfly species. This finding paves the way to the optimal scale candidates out of ca. 175 000 Lepidopteran species as bio-templates to replicate for SERS applications, and thus helps bring affordable SERS substrates as consumables with high sensitivity, high reproducibility, and low cost to ordinary laboratories across the world.