Insect cuticle hardens and darkens during the processes of cuticular melanization and sclerotization, and it provides an effective physicochemical barrier against parasites and pathogens. If an invader manages to breach this barrier defense, it is attacked by immune mechanisms in the hemocoel. In this study, we set out to investigate the association between these two lines of defense in the mealworm beetle, Tenebrio molitor. The immune response of freshly emerged beetles was induced with nylon monofilament implants, and later the darkness of their cuticle was assessed. Our study shows that immune challenge has a negative effect on cuticular darkness in T. molitor, which is likely to reflect a trade-off in the allocation of resources between two important components of insect defense system — the protective cuticle and the encapsulation response. The putative trade-off was not affected by the availability of energy resources.