The Extant Hexapoda

The Hexapoda (usually given the rank of superclass) contains all six-legged arthropods. Traditionally, the closest relatives of hexapods have been considered to be the myriapods (centipedes, millipedes, and their allies). However, as shown in Box 7.1, molecular sequence and developmental data plus some morphology (especially of the compound eye and nervous system) suggest a more recent shared ancestry for hexapods and crustaceans than for hexapods and myriapods.

Diagnostic features of the Hexapoda include the possession of a unique tagmosis (section 2.2), which is the specialization of successive body segments that more or less unite to form sections or tagmata, namely

Box 7.1 Relationships of the Hexapoda to other Arthropoda

The immense phylum Arthropoda, the joint-legged animals, includes several major lineages: the myriapods (centipedes, millipedes, and their relatives), the che-licerates (horseshoe crabs and arachnids), the crustaceans (crabs, shrimps, and relatives), and the hexapods (the six-legged arthropods - the Insecta and their relatives). The onychophorans (velvet worms, lobopods) have been included in the Arthropoda, but are considered now to lie outside, amongst probable sister groups. Traditionally, each major arthropod lineage has been considered monophyletic, but at least some investigations have revealed non-monophyly of one or more groups. Analyses of molecular data (some of which were naive in sampling and analytical methods) suggested paraphyly, possibly of myriapods and/or crustaceans. Even accepting monophyly of arthropods, estimation of inter-relationships has been contentious with almost every possible relationship proposed by someone. A once-influential view of the late Sidnie Manton proposed three groups of arthropods, namely the Uniramia (lobopods, myriapods, and insects, united by having single-branched legs), Crustacea, and Chelicerata, each derived independently from a different (but unspecified) non-arthropod group. More recent morphological and molecular studies reject this hypothesis, asserting monophyly of arthropodization, although proposed internal relationships cover a range of possibilities. Part of Manton's Uniramia group - the Atelocerata (also known as Tracheata) comprising myriapods plus hexapods - is supported by some morphology. These features include the presence (in at least some groups) of a tracheal system, Malpighian tubules, unbranched limbs, eversible coxal vesicles, postantennal organs, and anterior tentorial arms. Furthermore, there is no second antenna (or homolog) as seen in crustaceans. Proponents of this myriapod plus hexapod relationship saw Crustacea either grouping with the chelicerates and the extinct trilobites, distinct from the Atelocerata, or forming its sister group in a clade termed the Mandibulata. In all these schemes, the closest relatives of the Hexapoda always were the Myriapoda or a subordinate group within Myriapoda.

In contrast, certain shared morphological features, including ultrastructure of the nervous system (e.g. brain structure, neuroblast formation, and axon development), the visual system (e.g. fine structure of the ommatidia, optic nerves), and developmental processes, especially segmentation, argued for a closer relationship of Hexapoda to Crustacea. Such a grouping, termed the Pancrustacea, excludes myriapods. Molecular sequence data alone, or combined with morphology, tend to support Pancrustacea over Atelocerata. However, not all analyses actually recover

Pancrustacea and certain genes evidently fail to retain phylogenetic signal from what was clearly a very ancient divergence.

If the Pancrustacea hypothesis of relationship is correct, then features understood previously to support the monophyly of Atelocerata need re-consideration. Postantennal organs occur only in Collembola and Protura in Hexapoda, and may be convergent with similar organs in Myriapoda or homologous with the second antenna of Crustacea. The shared absence of features such as the second antenna provides poor evidence of relationship. Malpighian tubules of hexapods must exist convergently in arachnids and evidence for homology between their structure and development in hexapods and myriapods remains inadequately studied. Coxal vesicles are not always developed and may not be homologous in the Myriapoda and those Hexapoda (apterygotes) possessing these structures. Thus, morphological characters supporting Atelocerata may be non-homologous and may have been conver-gently acquired in association with the adoption of a terrestrial mode of life.

A major finding from molecular embryology is that the developmental expression of the homeotic (developmental regulatory) gene Dll (Distal-less) in the mandible of studied insects resembled that observed in sampled crustaceans. This finding refutes Manton's argument for arthropod polyphyly and the claim that hexapod mandibles were derived independently from those of crustaceans. Data derived from the neural, visual, and developmental systems, although sampled across few taxa, may reflect more accurately the phylogeny than did many earlier-studied morphological features. Whether the Crustacea in totality or a component thereof constitute the sister group to the Hexapoda is still debatable. Morphology generally supports a mono-phyletic Crustacea, but inferences from some molecular data imply paraphyly, including a suggestion that Malacostraca alone form the sister taxon to Hexapoda. Given that analysis of combined morphological and molecular data supports monophyly of Crustacea and Pancrustacea, a single origin of Crustacea seems most favored. Nonetheless, some data imply a quite radically different relationship of Collembola to Crustacea, implying a polyphyletic Hexapoda. In this view, aberrant collembolan morphology (entognathy, unusual abdominal segmentation, lack of Malpighian tubules, single claw, unique furcula, unique embryology) derives from an early-branching pancrustacean ancestry, with ter-restriality acquired independently of Hexapoda. Such a view deserves further study - evidently there remain many questions in the unraveling of the evolution of the Hexapoda and Insecta.

Fig. 7.2 Cladogram of postulated relationships of extant hexapods, based on combined morphological and nucleotide sequence data. Italicized names indicate paraphyletic taxa. Broken lines indicate uncertain relationships. Thysanura sensu lato refers to Thysanura in the broad sense. (Data from several sources.)

Fig. 7.2 Cladogram of postulated relationships of extant hexapods, based on combined morphological and nucleotide sequence data. Italicized names indicate paraphyletic taxa. Broken lines indicate uncertain relationships. Thysanura sensu lato refers to Thysanura in the broad sense. (Data from several sources.)

Protura (proturans), Collembola (springtails), and Diplura (diplurans)

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