A scanning electron micrograph of a freshly moulted bird cherry-oat aphid, Rhopalosiphon padi, on a barley leaf, Hordeum vulgare. The picture shows (left) the insect that has outgrown and shed its previous exoskeleton (right). The exoskeleton is a rigid structure comprised of chitin. It functions to protect the insect from desiccation, physical damage, and to provide anchorage for internal musculature. Here, the stylet used for sap feeding is visible below the head region of the exoskeleton. In order to grow the insect must develop a new flexible covering before splitting open the old rigid one. It must then wait for its new exoskeleton to harden. R. padi is a major pest of cereal crops, living on the sap of barley, oats, wheat and maize. It is a vector of several diseases, including barley yellow dwarf virus, as well as virus diseases of maize and rice. Mag x 50 at 10x8