The evolution of arthropod limbs Geoff A. Boxshall Department of Zoology, The Natural History Museum, Cromwell Road, London SW 75BD, UK (Email : G.Boxshall@nhm.ac.uk) (Received 24 May 2002 ; revised 2 April 2003) ABSTRACT Limb morphology across the arthropods is reviewed using external morphological and internal anatomical data from both recent and fossil arthropods. Evolutionary trends in limb structure are identified primarily by reference to the more rigorous of the many existing phylogenetic schemes, but no major new phylogenetic inferences are presented. Tagmosis patterns are not considered, although the origins and patterns of heteronomy within the postantennulary limb series are analysed. The phenomenon of annulation is examined and two basic types of annuli are recognised: terminal and in- tercalary. The annulation of the apical segment of a limb results in the formation of terminal flagella, and is typical of primarily sensory appendages such as insect and malacostracan antennules and maxillary palps of some hexapods. Intercalary annulation, arising by subdivision of existing subterminal segments, is common, particu- larly in the tarsal region of arthropodan walking limbs. Differentiating between segments and annuli is discussed and is recognised as a limiting factor in the interpretation of fossils, which usually lack information on intrinsic musculature, and in the construction of groundplans. Rare examples of secondary segmentation, where the criteria for distinguishing between segments and annuli fail, are also highlighted. The basic crown-group arthropodan limb is identified as tripartite, comprising protopodite, telopodite and exopodite, and the basic segmentation patterns of each of these parts are hypothesised. Possible criteria are dis- cussed that can be used for establishing the boundary between protopodite and telopodite in limbs that are uniramous through loss of the exopodite. The subdivision of the protopodite, which is typical of the post- antennulary limbs of mandibulates, is examined. The difficulties resulting from the partial or complete failure of expression of articulations within the mandibulate protopodite and subsequent incorporation of partial proto- podal segments into the body wall, are also discussed. The development and homology between the various exites, including gills, on the postantennulary limbs of arthropods are considered in some detail, and the question of the possible homology between crustacean gills and insect wings is critically addressed. The hypothesis that there are only two basic limb types in arthropods, antennules and postantennulary limbs, is proposed and its apparent contradiction by the transformation of antennules into walking limbs by homeotic mutation is discussed with respect to the appropriate level of serial homology between these limbs. Key words : Arthropoda, skeletomusculature, limb segmentation, annulation, arthropod evolution. CONTENTS I. Introduction ................................................................................................................................................. 254 II. Material examined ...................................................................................................................................... 255 III. The antennule .............................................................................................................................................. 255 (1) The uniramous antennule of the arthropodan groundplan ............................................................ 257 (2) Biramous or polyramous antennules .................................................................................................. 261 (3) Loss of antennules ................................................................................................................................. 263 (4) Antennulary development and the origin of flagella ........................................................................ 263 (5) Function ................................................................................................................................................. 265 (6) The antennule of the arthropodan groundplan ................................................................................ 266 IV. The postantennulary limbs – homonomous or heteronomous series ................................................... 266 V. The postantennulary limb of the arthropodan groundplan .................................................................. 267 (1) Protopodite ............................................................................................................................................. 268 (a) Defining the protopodite ................................................................................................................ 268 Biol. Rev. (2004), 79, pp. 253–300. f Cambridge Philosophical Society 253 DOI : 10.1017/S1464793103006274 Printed in the United Kingdom