Because iridoid glucosides are apparently not directed against a specific molecular target, we hypothesize that no specific selection pressure acts to limit the structural diversity of iridoids. On the contrary, in a race of arms with herbivore β-glucosidases evolving away from toxic iridoid glucoside specificity, structural diversity of the iridoid glucoside protoxin may be strongly favored. Accordingly, the usual scope of action of plant secondary metabolism, hydroxylations, acylations, and glycosylations, gives rise to chemotaxonomic variability of the iridoid scaffold down to the species level (9, 10). Additional structural diversity originates from configurational variations of the iridoid core scaffold, which has a fused five- and six-membered ring with multiple stereocenters (Fig. 1).