Some bugs secrete a substance characteristically pungent, penetrating, and often having a disagreeable smell. Members of the shield-bug families have a particularly bad odor. Shield bugs are also characterized by their often-bright colors, and the shield-like development of their scutellum, a hard, triangular plate that covers their backs.
Shield Bugs are quite possibly the most defensive unit in the game, ability-wise. Their Stink Cloud and Deflection Armor abilities make them nearly invulnerable against ranged attacks, and their Immunity protects them from a number of special abilities. Add an above average defense score, and you have a very hard to kill bug. Their offense, though, is terrible. Make up for this by combining them with something vicious.