Aphids, as the superfamily Aphidoidea, belong to the Sternorrhyncha within the Hemiptera, a group they share with Aleyrodoidea (whiteflies), Psylloidea (jumping plant lice), and Coccoidea (scale insects and mealybugs). Aphidoidea has three families: Adelgidae (adelgids), Phylloxoridae (phyllox-orids), and Aphididae (aphids), although some workers place the Adelgidae and Phylloxoridae in a separate superfamily, Phylloxoroidea. Adelgids and phylloxorids are primitive "aphids" and older groups, each with about 50 species. They differ from Aphididae by having an ovipositor and by reproducing by means of ovipary. Adelgids are restricted to conifers (Pinaceae), and some form characteristic galls (e.g., Adelges piceae, balsam woolly adelgid). Phylloxorids, which may also form galls, occur on plants of the Salicaceae (willow family), Fagaceae (oak family), Juglandaceae (walnut family), and Rosaceae (rose family). An exceptional species, Daktulosphaira vitifolae, grape phylloxera, feeds on grapes (Vitaceae), damaging European grape cultivars unless they are grafted to resistant rootstocks developed from American grape species.
Aphids originally evolved on woody plants in the Northern Hemisphere and are functionally replaced by whiteflies and psyllids in the Southern Hemisphere. As a group, they evolved and began their diversification with angiosperms, over 140 mya during the lower Cretaceous. While most fossil aphid groups became extinct during the Cretaceous—Tertiary boundary, most modern aphid groups radiated during the Miocene. Aphids have siphunculi, which vary by group from being mere pores on the abdominal surface to being very elongate tubes. They also have a cauda, which varies by group from rounded and hardly noticeable to knobbed or long and fingerlike. Aphids lack an ovipositor and are viviparous, bearing young parthenogenetically.
Aphid taxonomy is difficult; their subfamily classification has been argued and confused with nearly as many classifications as aphid taxonomists. Remaudiere and Remaudiere's 1997 classification, followed here, recognizes about 25 aphid subfamilies, with tribal groupings for about 600 genera and 4700 species of aphids. Many aphid lineages coevolved with, and radiated among, their host plant groups. Often during their phylogenetic history, however, aphid groups opportunistically switched to radically unrelated host groupings, driven by developmental requirements but tempered by evolutionary constraints.
Many aphid subfamilies are small, but several are larger and important: Chaitophorinae (e.g., Sipha flava, yellow sugarcane aphid), on Salicaceae and Gramineae (grass family); the closely related Myzocallidinae (e.g., Therioaphis trifolii f. maculata, spotted alfalfa aphid), Drepanosiphinae (e.g., Drepanaphis acerifoliae, painted maple aphid), and Phyllaphidinae (e.g., Phyllaphis fagii, beech aphid), often considered to be one subfamily and usually on dicotygledonous trees, but also Fabaceae (legume family) and bamboo; Lachninae (e.g., Essigella californica, Monterey pine aphid), mostly on Pinaceae, but also Fagaceae, Rosaceae, and Asteraceae (composite family) roots; and Pemphiginae (e.g., Pemphigus bursarius, lettuce root aphid), often on roots and host alternating to dicotyledonous trees forming galls. Other noteworthy subfamilies include Pterocommatinae, on Salicaceae; Greenideinae, on Fagaceae; Mindarinae, on Pinaceae; and the host-alternating Anoeciinae and Hormaphidinae, the latter causing galls.
The largest and most evolutionarily recent subfamily, Aphidinae, has two large, diverse, and agriculturally important tribes. The first tribe, Macrosiphini (e.g., Aulacorthum solani, foxglove aphid), is diverse in genera, which often lack attendance by ants but may alternate hosts. The second tribe, Aphidini, is diverse in species but less so in genera; these are often attended by ants. Tribe Aphidini has two important subtribes. Subtribe Rhopalosiphina (e.g., Rhopalosiphum padi, bird cherry—oat aphid) host alternates between Rosaceae to Gramineae or Cyperaceae (reed family). Subtribe Aphidina (e.g., Aphis fabae, bean aphid) host alternates mostly among Rosidae and Asteridae and is home to genus Aphis, which alone contains well over 1000 species.
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