Iridoid monoterpenes are special monoterpenes, mostly found in plants, predominantly as glycosides. They also occur in aphids and ants to some degree. The term, "iridoid" is based on iridomyrmecin and iridodial, two monoterpenes that occur in the defensive secretions of ants of the genus, Iridomyrmex. The terpene, 10-hydroxygeraniol is converted to iridodial and iridotrial. Iridotrial is biotransformed to a series of iridoid compounds. Iridoids are widely distributed secondary metabolites found in medicinal plants; they express various pharmacologic effects.

Thanks Sir Chithan C Kandaswami.

The majority of iridoid glycosides gives a colour reaction when treated with a solution of vanillin/HCl (3%) in methanol as spray reagent followed by heating at 120° C. This reaction is well visible on filter paper and it is very useful to visualize the spots on paper chromatography of extracts containing iridoids.