The cornicle (or siphuncule) is one of a pair of small upright backward-pointing tubes found on the dorsal side of the last segment of the bodies of aphids. They are sometimes mistaken for cerci.

Observations on Aphid Cornicle Secretions
Cornicle length in Macrosiphini aphids: a comparison of ecological traits
Aphid Cornicle Secretions Ineffective Against Attack by Parasitoid Wasps
Defence by Smear: Supercooling in the Cornicle Wax of Aphids
A comparison of triglycerides from aphids and their cornicle secretions
Pea aphid, Acyrthosiphon pisum, cornicle ontogeny as an adaptation to differential predation risk
Kairomonal effect of an aphid cornicle secretion on Lysiphlebus testaceipes (Cresson) (Hymenoptera: Aphidiidae).
Cornicle secretion of Acyrthosiphon pisum (Homoptera: Aphididae) as a contact kairomone for the parasitoid Aphidius ervi (Hymenopter...
Has the Attraction of Predatory Coccinellids to Cornicle Droplets Constrained Aphid Alarm Signaling Behavior?
Superparasitism limitation in an aphid parasitoid: cornicle secretion avoidance and host discrimination ability.