1. Elaiosomes are lipid-rich appendages on the seeds of some plant species that promote dispersal of the seeds by ants. In this study we investigated the hypothesis that elaiosomes on seeds have converged in composition and attractiveness with the insect prey of ants. 2. The fatty acid compositions of 12 species of elaiosomes were compared to seeds of the same species and to seven orders of insects. The fatty acid compositions of the elaiosomes were more like those of insects than seeds; the levels of palmitic, palmitoleic, stearic and oleic acids in elaiosomes and insects were particularly similar. 3. The response of six ant species to the diglyceride 1,2-diolein was compared. This lipid is purported to be the principal attractant in elaiosomes and a review of the literature revealed that it is also an important component of insect haemolymph