Secondary metabolism in fungal-insect interactions: Short Title: Metabolites in insect symbioses.

Natural products are central to the ecology and evolution of many insect-microbe symbioses, where they underpin parasitism, host manipulation, and cooperative interactions. In this review, we examine recent discoveries of secondary metabolites produced by fungal symbionts of insects with key ecological roles, focusing on compounds with demonstrated impacts on insect health, behaviour, or survival. We consider systems ranging from well-characterised models to largely unexplored associations where modern genomic and metabolomic approaches will uncover substantial new chemistry. We emphasize how the integration of these tools with functional bioassays offers a powerful means not only to uncover novel metabolites and their biosynthetic pathways but also to yield new insights into the ecology and evolution of insect-fungus relationships.