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Ant milking treehopper nymph. Meranoplus mucronatus ant (large) collecting honeydew secreted by one of a number of a treehopper (family Membracidae) nymphs (small, green). Some species of ant protect and tend to other insects, including aphids and treehoppers, and use the sweet nectar-like liquid they secrete as nourishment. (KEYSTONE/SCIENCE PHOTO LIBRARY/MELVYN YEO/SCIENCE PHOTO LIBRARY)