Palomena prasina. Common Green Shieldbug

The Green Shieldbug is well named: it is a shield-shaped bug and – at least in the summer – it is mostly bright green (though it becomes darker and browner towards the autumn). It grows to a length of about 15 mm. Like all similar bugs, it has a distinctive triangular plate on its back between the hardened sections of the folded forewings. The entire upper surface is green except for the overlapping membranous sections of the wings at the rear of the body, which appear brownish. Their antennae have five segments (as opposed to the shorter four-segmented antennae of most other groups of bug).
Shieldbugs can give off a nasty smell to deter predators, which is why they are sometimes called stinkbugs.