Shield beetle

The Scottish thistle (Onopordum Acanthium)s the meanest thistle I know. It's Scotland's national flower and some regard it as a weed, but it's a magnificent architectural feature in the garden. It's as prickly as a prickly thistle can be and stabs me on the arm or leg as I pass them. So why is there a delicate shield bug making its way up those prickly leaves?