7.2. 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 the head, thorax, and abdomen. The head is composed of a pregnathal region (usually considered to be three segments) and three gnathal segments bearing mandibles, maxillae, and labium, respectively; the eyes are variously developed, and may be lacking. The thorax comprises three segments, each of which bears one pair of legs, and each thoracic leg has a maximum of six segments in extant forms, but was primitively 11-segmented with up to five exites (outer appendages of the leg), a coxal endite (an inner appendage of the leg) and two terminal claws. The abdomen originally had 11 segments plus a telson or some homologous structure; if abdominal limbs are present, they are smaller and weaker than those on the thorax, and primitively were present on all except the tenth segment.
The earliest branches in the hexapod phylogeny undoubtedly involve organisms whose ancestors were terrestrial (non-aquatic) and wingless. However, any combined grouping of these taxa is not monophyletic, being based on evident symplesiomorphies or otherwise doubtfully derived characters. Included orders are Protura, Collembola, Diplura, Archaeognatha, and Zygentoma (= Thysanura). The Insecta proper comprise Archaeognatha, Zygentoma, and the huge radiation of Pterygota (the primarily winged hexapods). As a consequence of the Insecta being ranked as a class, the successively more distant sister groups Diplura, Collembola, and Protura, which are considered to be of equal rank, are treated as classes.
Some relationships among the component taxa of Hexapoda are uncertain, although the cladograms shown in Figs. 7.2 and 7.3, and the classification presented in the following sections reflect our current synthetic view. Previously, Collembola, Protura, and Diplura were grouped as “Entognatha”, based on resemblance in mouthpart morphology. Entognathan mouthparts are enclosed in folds of the head, in contrast to mouthparts of the Insecta (Archaeognatha + Zygentoma + Pterygota) which are exposed (ectognathous). However, two different types of entognathy have been recognized, one type apparently shared by Collembola and Protura, and the second seemingly unique to Diplura. Other morphological evidence and some molecular data analyses indicate that Diplura may be closer to Insecta than to the other entognathans, rendering Entognatha paraphyletic (as indicated by broken lines in Fig. 7.3). Some highly controversial studies indicate derivation of Collembola (and perhaps Protura) from within the Crustacea, independently from other hexapods.
Broken lines indicate uncertain relationships. Thysanura sensu lato refers to Thysanura in the broad sense. (Data from several sources)
Italicized names indicate likely paraphyletic taxa. Broken lines indicate uncertain relationships. (Data from several sources)