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Abstract Insects have evolved a chemical communication system using terpenoids, a structurally diverse class of specialized metabolites, previously thought to be exclusively produced by plants and microbes. Gene discovery, bioinformatics, and biochemical characterization of multiple insect terpene synthases (TPSs) revealed that isopentenyl diphosphate synthases (IDS), enzymes from primary isoprenoid metabolism, are their likely evolutionary progenitors. However, the mutations underlying the emergence of the TPS function remain a mystery. To address this gap, we present the first structural and mechanistic model for the evolutionary emergence of TPS function in insects. Through identifying key mechanistic differences between IDS and TPS enzymes, we hypothesize that the loss of isopentenyl diphosphate (IPP) binding motifs strongly correlates with the gain of the TPS function. Based on this premise, we have elaborated the first explicit structural definition of isopentenyl diphosphate‐binding motifs (IBMs) and used the IBM definitions to examine previously characterized insect IDSs and TPSs and to predict the functions of as yet uncharacterized insect IDSs. Consistent with our hypothesis, we observed a clear pattern of disruptive substitutions to IBMs in characterized insect TPSs. In contrast, insect IDSs maintain essential consensus residues for binding IPP. Extending our analysis, we constructed the most comprehensive phylogeny of insect IDS sequences (430 full length sequences from eight insect orders) and used IBMs to predict the function of TPSs. Based on our analysis, we infer multiple, independent TPS emergence events across the class of insects, paving the way for future gene discovery efforts.