Membracoidea (leafhoppers and treehoppers, Fig. 3), by far the most speciose of the auchenorrhynchan superfamilies, are characterized morphologically by the narrow costal space of the forewing, the large, transversely articulated metathoracic coxae, the elongate hind femora, the longitudinal rows of enlarged setae on the hind tibiae, and the presence of scutellar apodemes. The superfamily includes Cicadellidae (leafhoppers), a paraphyletic taxon that apparently gave rise to a lineage comprising the three currently recognized families of treehoppers (Melizoderidae, Aetalionidae, and Membracidae). A fifth family, Myerslopiidae, consists of two genera of small, flightless, litter-dwelling insects found only in New Zealand and Chile and thought to represent a distinct, relatively primitive lineage. Together, these groups comprise nearly 25,000 described species, currently grouped into about 3500 genera.

Membracoidea first appeared in the Jurassic, represented by the extinct family Karajassidae. These early membracoids were leafhopperlike insects with inflated faces (indicative of xylem feeding), and they retained a median ocellus and more primitive wing venation (forewing with CuAj free distally), but nevertheless had acquired the rows of enlarged setae on the hind tibia characteristic of modern leafhoppers. The first Cicadellidae appeared in the Lower Cretaceous. Treehoppers (Aetalionidae and Membracidae) make their first appearance in Tertiary age Mexican and Dominican amber.

The largest family, Cicadellidae [Fig. 3(10-13)], is characterized by the presence of four rows of enlarged, spinelike setae on the hind tibia, a peg-and-socket joint between the hind coxae, and the production of brochosomes. Membracidae [Fig. 3(15)], the next largest family, differ from Cicadellidae in having three or fewer rows of enlarged setae on the hind tibia, the male genital capsule with a lateral plate, and the pronotum enlarged, usually extended posteriorly over the scutellum and frequently bearing spines, horns, or other ornamentation. Like Membracidae, Aetalionidae [Fig. 3(14)]. have three or fewer setal rows on the hind tibia but differ in having the front femur fused to the trochanter, in having the scutellum completely exposed, and in having digitiform processes on the female genital capsule. Melizoderidae also resemble Membracidae but differ in having parapsidal clefts on the mesonotum. Myerslopiidae, thought to be the most primitive membracoid family, are bizarre, flightless insects with elytra-like forewings, vestigial ocelli, and a triangular mesocoxal meron resembling that of Cercopoidea. The phylogenetic status and relationships among the major lineages are only beginning to be understood.

Cicadellidae are unique among insects in producing bro-chosomes, which are minute proteinaceous granules synthesized in a specialized segment of the Malpighian tubules. After each molt, leafhoppers spread brochosomes over external surfaces of the body in an act known as anointing.

Rows of modified setae on the legs of leafhoppers are used to distribute the brochosomes during anointing and subsequent acts of grooming. The brochosome coating of nymphal and adult leafhoppers makes the integument extremely hydrophobic and protects leafhoppers from becoming entrapped in drops of water and their own often copious excreta.

Ant mutualism and parental care behavior are widespread among treehoppers [Membracidae and Aetalionidae, Fig. 3(15)]. Females of many species guard their eggs [Fig. 3(14)] and sometimes remain with the nymphs throughout their development. In the treehopper tribes Hoplophorionini and Aconophorini, ant mutualism was lost but parental care was retained. In these groups, females are often able to drive off invertebrate predators by buzzing the wings and/or using the hind legs to kick the intruder off the plant. Acoustic alarm signals produced by the nymphs trigger the mother's defensive response. Female Aconophora coat the stem of the host plant on either side of their egg masses with a sticky secretion that traps predators and parasitoids.

Most species of Membracoidea seem to have fairly narrow host and habitat requirements, and this has probably contributed to their remarkable diversity. Particularly notable are the large leafhopper faunas of temperate and tropical grasslands, where they are, by far, the most speciose component of the grass-feeding herbivore fauna. Many leafhopper species in deserts and dry grasslands are flightless or only occasionally produce winged individuals. This trait has presumably reduced gene flow among populations and facilitated speciation in some lineages. In temperate forests of the Northern Hemisphere, the leafhopper subfamily Typhlocybinae has diversified extensively through specialization on individual tree genera and species. In tropical forest canopies, the treehopper family Membracidae and the leafhopper subfamilies Idiocerinae and Typhlocybinae are particularly diverse. In Australia, the endemic fauna has radiated extensively on Eucalyptus. The North American treehopper tribe Smiliini has radiated extensively on oak (Quercus spp.).

Membracoidea are distributed worldwide. Among the five currently recognized families, Cicadellidae and Membracidae occur on all continents except Antarctica. Aetalionidae have a disjunct neotropical/oriental distribution, Melizoderidae are restricted to South America, and Myerslopiidae occur only in New Zealand and Chile. Most species and genera are restricted to a single continent; many tribes and subfamilies are also restricted to particular continents.

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