In 1925, German entomologist Walther Horn announced the development of rust-proof insect pins, culminating decades of trial and error. The invention was spurred on through the correspondence with Horn’s colleague Heinrich Hugo Karny, who struggled with specimen preservation in the Dutch East Indies’ humid climate. The Horn-Karny exchange revealed how tropical humidity actively shaped European scientific practices while highlighting ongoing scientific cooperation between German and Dutch colonial networks, even after Germany’s loss of colonies. While rust-proof pins marked an adaptation to humidity’s requirements, the persistent presence of mold demonstrated humidity’s ongoing role as a co-creator of entomological knowledge and practice.