Reproduction and dispersal of vascular plants in these dry conditions was achieved through the evolution of specialized seed structures. The diversification of flowering plants (angiosperms) during the Cretaceous period is associated with the sudden burst of speciation in insects. This diversification of insects represented a major selective force in plant evolution, and led to selection of plants that had defensive adaptations. Early insect herbivores bit or chewed vegetation; but the evolution of vascular plants lead to the co-evolution of other forms of herbivory, such as sap-sucking, leaf mining, gall forming and nectar-feeding.