How the Snake Lost Its Legs fly embryo. His books on evolutionary developmental biology (evo-devo) include "Imaginal Discs: The Genetic and Cellular Logic of Pattern Formation" (2002), "Quirks of Human Anatomy: An Evo-Devo Look at the Human Body" (2009), and "Deep Homology? Uncanny Similarities of Humans and Flies Uncovered by Evo-Devo" (2017). In Held's view, "Quirks", "Snake", and "Deep Homology" form a trilogy on evo-devo. "How the Snake Lost Its Legs" is ostensibly organised into six chapters, but in effect into three parts. The first chapter serves as an introduction and overview. The next four chapters provide what Held calls "the meatier aspects of evo-devo" with
Butterfly from larval feeding, which allow them to develop more quickly as caterpillars, and gives them a longer lifespan of several months as adults. The thorax of the butterfly is devoted to locomotion. Each of the three thoracic segments has two legs (among nymphalids, the first pair is reduced and the insects walk on four legs). The second and third segments of the thorax bear the wings. The leading edges of the forewings have thick veins to strengthen them, and the hindwings are smaller and more rounded and have fewer stiffening veins. The forewings and hindwings are not hooked together (as