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Aphids feed with the head lowered, therefore the main function of the cauda would be to prevent the ²honeydew, a sticky fluid ejected by the anus, from flowing over the insect’s body (see the video). The aphids visited by ants generally have a short or weakly developed cauda as little by little the ants can collect the honeydew there.
The cauda does not become differentiated until the adult stage. Its form, colour and length are highly variable. It is much used as a criterion for identifying adult forms of the different species.The form of the anal plate is also a criterion for identification. Generally it is a single, complete rounded piece but in some subfamilies (such as Myzocallidinae, Saltusaphidinae) it is bilobed.

Acyrthosiphon pisum (Type A)	Aphis nerii (Type B)	Takecallis arundicolens (Type E)