Chemical composition of the slippery epicuticular wax blooms on Macaranga (Euphorbiaceae) ant-plants

The stems of many Macaranga ant-plants (Euphorbiaceae) are covered by epicu ticular wax crystals rendering the surface very slippery for most insects. These wax blooms act as selective barriers protecting the symbiotic ant par tners, which are specialized "wax-runners", against the competition of othe r ants. Glaucous stems occur almost exclusively among the ant-plants of the genus Macaranga (Federle et al. 1997). We analyzed the cuticular lipids of 16 Macaranga species by GC-MS and investigated the wax crystal morphology using SEM. Presence of crystalline wax blooms was strongly correlated with high concentrations (52%-88%) of triterpenoids. In contrast epicuticular wa xes of glossy Macaranga surfaces contained only 0% to 36% of these dominant components. Therefore we conclude that triterpenoids are responsible for t he formation of the thread-like Macaranga wax crystals. In all Macaranga an t-plants investigated, the principal components were epitaraxerol and tarax erone accompanied by smaller portions of taraxerol, beta-amyrin and friedel in. Only in the case of the non-myrmecophytic M. tanarius did beta-amyrin p redominate. Moreover, we found that only in M. tanarius, the dense wax crys tal lacework is torn into large mosaic-like pieces in the course of seconda ry stem diameter growth. Both chemical and macroscopic differences may cont ribute to a reduced slipperiness of M. tanarius stems and appear to be func tionally important. The distribution of wax crystals and their composition amongst different sections of the genus suggests that glaucousness is a pol yphyletic character within Macaranga.