Insects and other arthropods go through a series of stages or instars as they develop towards the adult form. As they grow, their exoskeletons must be shed before they can grow a new and larger one. Sometimes there are variations is the precise shape or colour patterns of each successive instar.
Such is the case for shield bugs, including the most common around here, the common green shield bug. Seen here, the fourth and fifth instars of a green shield bug. Pictures taken a week apart, possibly of the same individual – resting, at any rate, on the same leaf of a hazel bush.