Arthropods (Arthropoda)

Abstract With more than 1 million described species (1), arthropods are a marvelous evolutionary success. 7is phylum of animals includes one extinct subphylum (Trilobita, 4000 described species) and four living subphyla: Myriapoda (e.g., centipedes and millipedes, 11,500 species), Chelicerata (e.g., spiders, mites, and horseshoe crabs, 70,000 species), Hexapoda (e.g., insects, 948,000 species), and Crustacea (e.g., shrimps and crabs, 68,000 species). thePhylum Arthropoda is characterized by species having an articulated chitinous exoskeleton divided into thick areas, corresponding to segments, joined by thin “rings” (2). Each segment typically carries a pair of articulated legs, and a number of anterior segments are fused into a cephalon. In most groups the cephalon carries eyes, and two or more pairs of limbs (2). Arthropods have been treated as a single animal group essentially since 1753, when Linnaeus called them Insecta in the Arst edition of the Systema Naturae (3). However, their monophyly has long been debated because of their morphological disparity (4–6). Molecular data eventually conArmed arthropod monophyly (7) and to date arthropods are classiAed into one extinct subphylum (the Trilobita) and four extant subphyla: Crustacea, Hexapoda, Myriapoda, and Chelicerata (Fig. 1). Each subphylum is further divided into classes (8). Here, I will review the relationships and divergence times among the arthropod classes and subphyla.