The wonders of evolution!

These yellow flowers and leaves belong to Jacobaea vulgaris,  a very common wild flower in the daisy family that is native to northern Eurasia. Its common names include ragwort, common ragwort, stinking willie, tansy ragwort, benweed, St. James-wort, stinking nanny/ninny/willy, staggerwort, dog standard, cankerwort and stammerwort. 

Ragwort contains copious pyrrolizidine alkaloids that make it extremely poisonous to many animals including horses and cattle. Despite the presence of these poisonous alkaloids  the leaves are the food of choice for the caterpillars of the cinnabar moth. Such a diet means that the caterpillars themselves become poisonous and their striking colouration is a warning to birds to leave them well alone.