Chemotaxonomic potential of exocrine alkyl esters in julid millipedes (Diplopoda: Julidae: Cylindroiulini)

Recently, however, a new class of julid exocrine products, namely complex bends of alkyl esters has been reported. We here tested the chemotaxonomic potential of these compounds by performing a comprehensive study on alkyl esters across the julid tribe Cylindroiulini. Exocrine secretions of 20 species from all five cylindroiuline genera (Allajulus, Cylindroiulus, Enantiulus, Kryphioiulus, Styrioiulus) were analyzed by gas chromatography - mass spectrometry. Apart from typical benzoquinones, 33 alkyl esters and 1 alkenyl ester belonging to hexyl-, octyl-, octenyl- and nonyl-esters with saturated and mono-unsaturated acid moieties ranging from C13 - C18 were detected. Only two species (Cylindroiulus caeruleocinctus and C. luridus) exhibited no esters at all. Ester profiles appeared to be specific, indicating five major chemical groups in cylindroiulines, characterized by specific ester arrangements. We here provide evidence for i) the widespread occurrence of alkyl esters in the Julidae, ii) their high specificity, iii) their intra-species stability, iv) along with sufficient inter-species variability, rendering them promising novel tools for millipede chemotaxonomy.