Much research and discussion have focused on the effects of key innovations on lineage diversification, whereas little has been done to investigate their role in morphological evolution using phylogenetic approaches. Here we present the first comprehensive molecular phylogeny of the Harpactorini (Insecta: Reduviidae), the largest assassin bug tribe, sampling 229 terminal taxa and using five gene segments (28S D2, D3–D5, 16S, COI, and Deformed). Employing comparative phylogenetic methods, we demonstrate the correlation of a putative key innovation, the sticky trap predation strategy, with accelerated rates of morphological evolution of the predatory fore leg in assassin bugs. We show that bugs exhibiting sticky trap predation have evolved more slender and longer fore femora than non-sticky bugs. Using phylogenetically independent contrast analyses, we document correlated evolution between femoral thickness and length. We argue that the novel sticky trap predation strategy may allow sticky bugs to alleviate functional constraints on the fore femur and thus to attain a higher rate of evolution than other Harpactorini or Reduviidae. We discuss the possibility that sticky bugs represent a case of adaptive radiation. We also test historical supra-generic groups within the Harpactorini, and show that most of them are not monophyletic. We confirm the paraphyly of Harpactorini with respect to Rhaphidosomini.