Many kinds of aphid secrete a sweet substance called honeydew, prized as food by ants, flies, and bees. This substance consists of partially digested, highly concentrated plant sap and other wastes, and is excreted from the anus, often in copious amounts. Certain aphid species have a symbiotic relationship with various species of ants that resembles the relationship of domestic cattle to humans; hence the name "ant cows" for aphids. The ants tend the aphids, transporting them to their food plants at the appropriate stages of the aphids' life cycle and sheltering the aphid eggs in their nests during the winter. The aphids, in turn, provide honeydew for the ants.