Phylogeny Of Hexapoda The subphylum Hexapoda from the
Phylogeny Of Hexapoda
ØThe subphylum Hexapoda (from the Greek, for six legs) constitutes the largest (in terms of number of species) grouping of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura (all of these were once considered insects). ØThe Collembola (or springtails) are very abundant in terrestrial environments. Ø Hexapods are named for their most distinctive feature: a consolidated thorax with three pairs of legs. ØMost other arthropods have more than three pairs of legs.
Hexapods have bodies divided into an anterior head, thorax, and posterior abdomen. The head is composed of a presegmental acron that usually bears eyes (absent in Protura and Diplura), followed by six segments, all closely fused together, with the following appendages: ØSegment I. None ØSegment II. Antennae (sensory), absent in Protura ØSegment III. None ØSegment IV. Mandibles (crushing jaws) ØSegment V. Maxillae (chewing jaws) ØSegment VI. Labium (lower lip)
ØThe mouth lies between the fourth and fifth segments and is covered by a projection from the sixth, called the labrum (upper lip). Ø In true insects (class Insecta) the mouthparts are exposed or ectognathous, while in other groups they are enveloped or endognathous. ØSimilar appendages are found on the heads of Myriapoda and Crustacea, although these have secondary antennae.
ØThe thorax is composed of three segments, each of which bears a single pair of legs. ØAs is typical of arthropods adapted to life on land, each leg has only a single walking branch composed of five segments, without the gill branches found in some other arthropods. ØIn most insects the second and third thoracic segments also support wings. ØIt has been suggested that these may be homologous to the gill branches of Crustaceans, or they may have developed from extensions of the segments themselves. ØThe abdomen consists of eleven segments in all true insects (often reduced in number in many insect species), but in Protura it has twelve, and in Collembola only six (sometimes reduced to only four). ØThe appendages on the abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment.
Hexapod evolution and relationships: ØThe myriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity. These were then considered subclasses of a subphylum called Uniramia or Atelocerata.
ØHexapods are very old and highly speciose group, the phylogenetic work based on the comparative study of characters of both fossils and present day species. ØThe recent day discussion based on morphological and molecular analysis of Wheelar et al. (2001). ØThe fossil record for precambrian time is scanty whereas in the Cambrian period, marine arthropods were recorded consisting of Trilobites, Crustaceans and Xiphosurans. ØThe first terrestrial arthropods (scorpions, spiders and milipedes) appeared in the Silurian period , the first hexapod appeared in the Devonian and many in Carboniferous and later periods.
ØThe Hexapods have arisen from a myriapod like ancestors that had paired leg like appendages on each body segments and thus changed to Hexapodous appearance involves the development of head , modification of three segments behind the head, and loss or reduction of most of the appendages of the remaining body segments. ØThe first Hexapods were undoubtedly wingless and therefore called apterygotes. ØThe Proturans, Diplurans and Collembolans are key distinct groups and Entognathous are traditionally classified as insects.
ØThe Protura and Collembola are recognised as a monophyletic group, the Ellipura whereas the monophyly of the Diplura is disputed and this order represents early offshoot of the hexapod line. ØThe oldest known hexapod fossil from the Devonian identified as Collembola.
ØThe other two primitive wingless orders are thr Microcoryphia and Thysanura also known as Archaeognatha and Zygentoma respectively, these creatures as well as the winged insects have exposed mouthparts called Ectognatha. ØKristensen (1981) suggested restricting the word “insects” to these ectognathous species and hexapods to include both the entognathous orders and ectognatha, thus the term “insects” and “hexapods” are not synonymous.
ØTwo major lines developed within the winged insects or pterygotes: The Paleoptera and Neoptera with a difference in their ability to flex their wings. ØThe paleoptera cannot flex their wings over the abdomen whereas Neoptera can. ØSeveral orders of Neoptera were present in the late Paleozoic but only the Ephimeroptera and Odonata have survived.
Wheeler et al (2001) separated these orders in groups: a) Monophyletic group composed of the Orthoptera, Phasmida, Grylloblatteria and Dermaptera. b) another group containing Blatteria, Mantodea, Isoptera and Zoraptera and, c) third group of Plecoptera and Embiidina.
ØThe combined analysis of groups with Dictyoptera (Mantodea, Blattaria, and Isoptera) on the basis of several morphological characters. ØHemipteroid groups classified as the Paraneoptera-Hemiptera, Thysanoptera, Psocoptera and Pthiraptera characterized by simple metamorphosis, modification of the maxillary laciniae into stylets, absence of large anal lobe, no cerci. ØComplete metamorphosis appeared in the remaining nine orders: Hymenoptera, Neuroptera, Coleoptera, Lepidoptera, Trichoptera, Mecoptera, Siphonaptera, Strepsiptera and Diptera.
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