Scenting Ketones in the Defense Glands of Two Julids From the Caucasus (Arthropoda, Myriapoda, Diplopoda, Julida).

Millipedes have developed specialized chemical defenses against predators and parasites. Here we investigated the strong repellent odour emitted by two julids from the Caucasus, Syrioiulus continentalis (Attems, 1903) and Pachyiulus krivolutskyi Golovatch, 1977. By NMR and GC-MS, as well as compound synthesis, we identified the major compound in the secretions of both species as 4-ethylhex-1-en-3-one. A second compound was tentatively identified as the dimer of the main compound as 2-ethyl-1-(6-(pentan-3-yl)-3,4-dihydro-2H-pyran-2-yl)butan-1-one. Both compounds amounting for more than 95% of the whole secretion. 4-Ethylhex-1-en-3-one was found for the first time as a naturally occurring compound, and due to its identification in millipedes of the order Julida we propose here the trivial name "julidone". Considering current knowledge, the replacement of the juliform quinones by novel components such as ketones appears to be highly derived conditions in the chemistry of Julida, possibly triggered by different environmental challenges arising from the epigeic life-style of the two species.