High Quality Content by WIKIPEDIA articles! Plants and herbivores have co-evolved together for 350 million years. Plants have evolved many defence mechanisms against insect herbivory. Such defences can be broadly classified into two categories: (1) permanent, constitutive defences, and (2) temporary, inducible defences. Both types are achieved through similar means but differ in that constitutive defences are present before a herbivore attacks, while induced defences are activated only when attacks occur. In addition to constitutive defences, initiation of specific defence responses to herbivory is an important strategy for plant persistence and survival.