Aphid. Coloured scanning electron micrograph of an aphid, Aphis sp., feeding on leaf sap. Aphids feed off the sugary sap of plant stems and leaves by inserting their tube-like stylet into the plant's veins. This can cause the leaves to yellow, wilt and finally die. Aphids excrete sweet waste fluids (honeydew) which is sometimes collected as food by ants. Aphids are serious plant pests of certain crops, flowers and houseplants. Magnification: x100 when printed at 10 centimetres wide.