Two aroid congeners, Alocasia odora and Alocasia cucullata, grow on Okinawa Island, Japan. Two floricolous species of Colocasiomyia alocasiae and Colocasiomyia xenalocasiae (Diptera: Drosophilidae), previously known to be specific pollinators for A. odora, were found pollinating A. cucullata. We collected the floral volatiles of A. odora and A. cucullata and compared them using gas chromatography and mass spectrometry because floral volatiles act as attractive signals for these pollinators. The volatile compositions detected were similar and dominated by methyl salicylate, 4,8-dimethyl-1,3,7-nonatriene, β-caryophyllene, bicyclogermacrene, methyl benzoate and α-humulene, which means it is likely that the flies cannot discriminate between the two plant species when they search for hosts. Interspecific hybridization did not occur when A. odora was hand pollinated with A. cucullata pollen, suggesting that the movement of flies from one host species to another will result in wastage of pollen. Adults of both Colocasiomyia flies emerged from A. cucullata inflorescences collected in the field, suggesting that their larvae can also develop on A. cucullata. We did not find any difference in larval performance of C. alocasiae on A. cucullata or A. odora. C. alocasiae does not appear to suffer any ill effects when using A. cucullata as a host plant. The partnership between A. cucullata and the two Colocasiomyia flies may be an example of exaptation, although other possibilities cannot be excluded.