"Fieldnotes," July 28th & 29, 2013: "Aphids," by John McCutcheon (read by Allison de Jong). http://www.montananaturalist.org/
"Phloem sap is under positive pressure, so all an aphid has to do is stick its piercing mouthpart into the right part of the plant, and the sap will start flowing. This process is similar to tapping a pressurized keg of beer. Cicadas, on the other hand, eat only xylem sap, which is the sap that comes up from the roots... under negative pressure. This process is similar to sucking a thick milkshake through a straw."