S4.1  Monday, Jan. 5  Main Episodes in Insect Evolution and the Importance of Stem Groups GRIMALDI, D.; American Museum of Natural History, New York grimaldi@amnh.org

Stem groups are paraphyletic grades of basal or extinct species possessing some but not all of the specialized features of their crown group. Probably every feature of all species has evolved through a series of steps, and so stem groups possess nascent transformation of one or more features. Examples of stem groups are taken from four main episodes in the 400-million-year history of insects, each representing major adaptive features of hexapods that have contributed to their evolutionary and/or ecological success: 1. the hexapod colonization of land; 2. the origin of flight; 3. holometabolous development; 4. and advanced sociality (eusociality). The evidence for a Crustacean ancestry of hexapods provides novel views on the transition to land, but an undisputed sister group to hexapods remains elusive. Perhaps the single largest problem of insect evolution regards the origins of wings, which appeared suddenly and in full form in the Late Carboniferous, as an Early Carboniferous record is essentially lacking for insects. Holometaboly apparently evolved in the Permian, when the stem groups to modern holometabolous orders first appeared. Sociality appeared last, in the Cretaceous, and independently in four lineages. Implications of stem groups are discussed regarding evolutionary interpretations.