What is the classification of the aphid?

Insect is the classification of the aphid.

Specifically, an aphid is considered a sap-sucking insect. It is an insect because it has an outer skeleton, three body parts (head, thorax, abdomen), three jointed legs, compound eyes, and one pair of antennae. It is a sap-sucker because its piercing mouthparts break into its prey's insides in order to extract internal fluids.