The aphid rostrum is the feeding appendage used to access and feed on plant phloem

The rostrum is composed of five segments. It contains a sclerotized groove in which the stylets sit and extend back and forth as they access the phloem. This sclerotized groove is in most of the second rostral segment and all of segments three, four, and five. In many aphid species, the fifth segment is hard to distinguish from the fourth, so they are often treated together as the "ultimate rostral segments", or simply URS. URS is usually used in the singular.