In this enhancement chapter, written by Professor Richard Brusca of Columbia University, current views of arthropod phylogeny are assessed in light of recent research in morphological and molecular phylogenetics, developmental biology, comparative neurobiology, and paleontology. Recent fossil discoveries and molecular clock data inform us that arthropod diversification began in the Precambrian, and suggest that by the Cambrian the arthropods were already the most speciose metazoan phylum on earth (figure 43e.1). The combination of metamerism and jointed appendages (with intrinsic musculature), and the evolutionary potential of homeotic genes, has profoundly affected arthropod evolution and created many morphological homoplasies. Evidence strongly favors a monophyletic Arthropoda. Accumulating evidence supports a hypothesis that insects and modern crustaceans comprise a sister group, and that they, and perhaps also trilobites, chelicerates, and myriapods, all could have evolved out of an ancient crustacean stem line. Two implications of this hypothesis are that Crustacea is a paraphyletic taxon and insects may be viewed as "flying crustaceans."