 It turns out that many species of insects are toxic or distasteful when they have fed on plants that contain chemicals of particular classes, but not when they have fed on plants that lack those chemicals.For instance, some milkweed butterflies feed on milkweeds ("Asclepias") which contain the cardiac glycoside oleandrin; this makes them poisonous to most predators.These insects are often aposematically coloured and patterned.