Answer
To get the thorns, the leafhopper would have to have two kinds of mutations: one to turn on the formation of wings in the first segment (which is inhibted by Hox genes in most insects), and another to alter the wings into the thorns. The latter change would probably require the expression of a suite of genes to be modified to get something so very different from the original wings.
Work Step by Step
The first part, turning wings formation back on, is hinted from the clues in the question about the position of the thorns and their origin as wings in a place which has not held insect wings for a long period of evolution. The second part can be seen by using an analogy: imagine that the conversion of wings to thorns is like changing a lampshade into a hat. You would have to close the top of the lampshade, remove the finial and other hard metal parts, and make a suite of other alterations to turn a lampshade into anything like a hat. No single change could make either conversion; Multiple genetic changes would need to stack their effects in the treehopper to create their decorative thorns from what were previously wings.
