Summary of results and evolution of terminal system and expression of tailless . A. Comparison of terminal system architecture between Drosophila and the pea aphid in its oviparous mode. Gray and yellow shading indicate events occurring during oogenesis and embryogenesis, respectively, with border representing fertilization. Note that the aphid terminal system only operates in the posterior and that the signal transduction mechanism of Torso-like activity and the targets of dpERK remain unknown. Also note that these aspects of the aphid terminal system do not operate during viviparous oogenesis. B. Proposed steps in the evolution of the terminal system in Drosophila and aphids, with asexual and sexual developmental programs of aphids shown as separate evolutionary trajectories. Although steps shown in green are depicted as gains in the lineage leading to Drosophila and Tribolium, they could just as parsimoniously be represented as losses in the lineage leading to aphids. The order of steps between nodes is arbitrary with the exception of the acquisition of viviparity in asexual aphids, which we suggest either pre-dated or coincided with the loss of terminal patterning. Neither A nor B makes reference to expression of aphid tslr1 or early homogeneous dpERK in the oocyte.