Treehoppers (more precisely typical treehoppers to distinguish them from the Aetalionidae) and thorn bugs are members of the family Membracidae, a group of insects related to the cicadas and the leafhoppers. There are about 3,200 known species of treehoppers in over 600 genera. They are found on all continents except Antarctica, although there are only three species in Europe.
They are best known for their enlarged and ornate pronotum, which most often resembles thorns, apparently to aid camouflage. But in some species, the pronotum grows to a horn-like extension, and even more bizarre and hard-to-describe shapes are also found (see gallery).
Thorn bugs, due to their unusual appearance, have long interested naturalists. This has even extended to popular culture; they are for example among the types of insects mentioned in the movie Master and Commander: The Far Side of the World.
They pierce plant stems with their beaks, and feed upon sap. The immatures can frequently be found on herbaceous shrubs and grasses whereas the adults more often frequent hardwood tree species. Excess sap becomes concentrated as honeydew, which often attracts ants. Some species have a well developed ant mutualism, and these species are normally gregarious as well, which attracts more ants. The ants provide protection from predators.
Eggs are laid by the female with her saw-like ovipositor in slits cut into the cambium or live tissue of stems, though some species lay eggs on top of leaves or stems. The eggs may be parasitised by wasps, such as the tiny fairyflies (Mymaridae) and Trichogrammatidae. The females of some membracid species sit over their eggs to protect them from predators and parasites, and may buzz their wings at the intruder. The females of some gregarious species work together to protect each others' eggs. In at least one species, Publilia modesta, mothers serve to attract ants when nymphs are too small to produce much honeydew. Some other species make feeding slits for the nymphs.
Like the adults, the nymphs also feed upon sap, and unlike adults, have an extensible anal tube that appears designed to deposit honeydew away from their body. The tube appears to be longer in solitary species that are rarely ant attended. It is important for sap feeding bugs to dispose of honeydew, as otherwise it can become infected with sooty moulds. Indeed, there is evidence that one of the benefits of ants for individuals of the species Publilia concava is that ants remove the honeydew and reduce such growth.
Most species are innocuous to humans, although a few are considered minor pests, such as Umbonia crassicornis (a thorn bug), the Three-cornered Alfalfa Hopper (Spissistilus festinus), or the Buffalo Treehopper (Stictocephala bisonia) which has been introduced tro Europe.
The diversity of treehoppers has been rather little researched, and their systematic arrangement is tentative. It seems that three main lineages can be distinguished; the Endoiastinae are the most ancient treehoppers, still resembling cicadas a lot. Centrotinae form the second group; they are somewhat more advanced but the pronotum still does not cover the scutellum in almost all of these. The Darninae, Heteronotinae, Membracinae and Smiliinae contain the most apomorphic treehoppers.
Several proposed subfamilies seem to be paraphyletic. Centronodinae and Nicomiinae might need to be merged into the Centrotinae to result in a monophyletic group. And while most the Stegaspidinae also appears to belong there, united by genera like Smerdalea and Smergotomia, members of the "Stegaspidinae" in fact appear to be distributed all across the treehoppers, and the taxon is probably best abolished altogether.
Images showing the range of systematic variation in treehoppers