Honeydew is a sugar-rich sticky liquid that is secreted by aphids, some scale insects, many other true bugs, and some other insects as they feed on plant sap. When their mouthpart penetrates the phloem, the sugary, high-pressure liquid is forced out of the anus of the insects, allowing them to rapidly process the large volume of sap required to extract essential nutrients present at low concentrations. Honeydew is particularly common as a secretion in hemipteran insects and is often the basis for trophobiosis. Some caterpillars of Lycaenidae butterflies and some moths also produce honeydew. In addition to various sugars, honeydew contains small amounts of amino acids, other organic compounds, and inorganic salts, with its precise makeup affected by factors such as insect species, host plant species, and whether a symbiotic organism is present.
Honeydew-producing insects, such as cicadas, pierce phloem ducts to access sugar rich sap; the excess fluid released by cicadas as honeydew is called "cicada rain". The sap continues to bleed after the insects have moved on, leaving a white sugar crust called manna. Ants may collect, or "milk", honeydew directly from aphids and other honeydew producers, which benefit from the ants' presence due to their driving away predators such as lady beetles or parasitic wasps—see Crematogaster peringueyi. Animals and plants in a mutually symbiotic arrangement with ants are called Myrmecophiles.
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