Hylas and the Nymphs. To find out more about Hylas, the lover of Heracles, see this Wikipedia article: link; for information about the image: image source.
Poor Hylas was a beautiful young prince, and he served Heracles as his arms bearer. When Heracles decided to accompany Jason on the voyage of the Argo with the other Argonauts, he took Hylas with him. At one of their stops, Hylas was kidnapped by the nymphs, as you can see here in the mosaic. Heracles called and called and called for Hylas, but there was no answer. Finally the Argo sailed away, leaving Hylas behind. Heracles was heartbroken. The grief of Heracles became proverbial: Hylam vocas, "You're calling for Hylas," was a proverbial expression that you meant you were calling out for something in vain, something or someone who is lost to you forever.