|Treehopper helmet- shortsharpscience|
The excess sap is concentrated as honeydew which attracts ants. Aphids also produce honeydew and some species are actually farmed by ants which bring them in at night to protect them, taking them out during the day to resume farming. Like some other insects including Lycaenidae butterfly caterpillars, some leafhopper species are protected from predators by the ants who know a "sweet deal" when they see it.
Treehoppers have so-called helmets which are appendages attached to the the back of the pronotum or first thoracic segment. Think of it as just above and just between your shoulder blades if you were suddenly morphed into an insect. These helmets come in a fantastic variety of shapes as you can see here at Bugguide.net.
Treehopper anatomy rewrites the standard description of insects. It includes a pair of legs on the three thoracic segments and a pair of wings on the second and third segment. Wings on the first thoracic segment were present on some ancient treehopper fossils but are thought to have disappeared around 100 million years ago by the loss or repression of a functioning Hox gene.
The New Science blog shortsharpscience describes recent research on these helmets. The new report has found that the helmets of treehoppers are actually a jointed appendage on the first thoracic segment (pronotum). This is thought to be due to a reactivation of the Hox gene, revising what would have been a wing. Since it isn't functional for flight, bizarre shapes are possible.
The science behind this is as fantastic as some of the helmet pictures.