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Aphids are a group of about 4,400 species of small insects that feed on the phloem fluid of plants. Many species of aphids attack important agricultural crops and are therefore of major economic importance. Aphids combine three traits - thelytokous parthenogenesis (obligate parthenogenesis where females give birth only to female offspring), short generation time (about 10 days on average), and telescoping of generations (where grandaughters begin developing directly within the daughters which are themselves not yet born) - to reach very large population sizes very quickly. These reproductive characteristics allow aphids to quickly colonize ephemeral resources and quickly growing plants and make them ideal enemies of monocultured crops. Many species of aphids display complex life cycles with alternation of sexual and asexual generations and host plant alternation. Host alternation has evolved at least three times independently in the aphids: (1) in the Phylloxeroidea, (2) in the common ancestor of the Pemphigidae, Hormaphididae, and Anoeciidae (Aphidoidea), and (3) in the Aphididae (Aphidoidea).