Apterygote Insecta former Thysanura sensu lato

The two extant orders of primitively wingless insects, Archaeognatha and Zygentoma, almost certainly are not sister groups, based on most analyses of morphological and molecular data. Thus the traditional grouping of Thysanura sensu lato (meaning in the broad sense, in which the name was first used for apterous insects with "bristle tails") should not be used. The taxonomic placement of the few very old aptery-gote fossils (from the Devonian) is uncertain.

Order Archaeognatha (Microcoryphia; archaeognathans, bristletails) (see also Taxobox 2)

Archaeognathans are medium-sized, elongate cylindrical, and primitively wingless ("apterygotes"). The head bears three ocelli and large compound eyes that are in contact medially. The antennae are multi-segmented. The mouthparts project ventrally, can be partially retracted into the head, and include elongate mandibles each with a single condyle (articulation point), and elongate seven-segmented maxillary palps. Often a coxal style occurs on coxae of legs 2 and 3, or legs 3 alone. Tarsi are two- or three-segmented. The abdomen continues in an even contour from the humped thorax, and bears ventral muscle-containing styles (representing reduced limbs) on segments 2-9, and generally one or two pairs of eversible vesicles medial to the styles on segments 1-7. Cerci are multisegmented and shorter than the median caudal appendage. Development occurs without change in body form.

The two extant families of Archaeognatha, Machi-lidae and Meinertellidae, form an undoubted mono-phyletic group. The order is the oldest of the extant Insecta, with putative fossils from at least the Carboniferous and perhaps earlier, and is sister group

Fig. 7.2 (opposite) Cladogram of postulated relationships of extant hexapods, based on combined morphological and nucleotide sequence data. Broken lines indicate uncertain relationships or alternative hypotheses. Thysanura sensu lato (s.l.) refers to Thysanura in the broad sense. An expanded concept is depicted for each of two orders - Blattodea (including termites) and Psocodea (former Psocoptera and Phthiraptera) - but intra-ordinal relationships are shown simplified (see Figs 7.4 and 7.5 for full details). (Data from several sources.)

Fig. 7.3 Cladogram of postulated relationships of extant hexapod orders, based on morphological and molecular data. There are alternate views concerning internal relationships and monophyly of Entognatha. (Data from several sources.)

to Zygentoma + Pterygota (Fig. 7.3). The fossil taxon Monura used to be considered an order of apterygotes related to Archaeognatha, but these monuran fossils (genus Dasyleptus) appear to be immature bristletails and the group usually is considered to be sister to all other archaeognathans.

Order Zygentoma (silverfish) (see also Taxobox 3) Zygentomans are medium-sized, dorsoventrally flattened, and primitively wingless ("apterygotes"). Eyes and ocelli are present, reduced or absent, the antennae are multisegmented. The mouthparts are ventrally to slightly forward projecting and include a special form of double-articulated (dicondylous) mandibles, and five-segmented maxillary palps. The abdomen continues the even contour of the thorax, and includes ventral muscle-containing styles (representing reduced limbs) on at least segments 7-9, sometimes on 2-9, and with eversible vesicles medial to the styles on some segments. Cerci are multisegmented and subequal to the length of the median caudal appendage. Development occurs without change in body form.

There are five extant families and the order probably dates from at least the Carboniferous. Zygentoma is the sister group of the Pterygota (Fig. 7.3) in a group called Dicondylia due to the presence of two articulation points on the base of the mandibles.

and metathorax variably united to form a pterothorax. The lateral regions of the thorax are well developed. Abdominal segments number 11 or fewer, and lack styles and vesicular appendages like those of apterygotes. Most Ephemeroptera have a median terminal filament. The spiracles primarily have a muscular closing apparatus. Mating is by copulation. Metamorphosis is hemi- to holometabolous, with no adult ecdysis, except for the subimago (subadult) stage in Ephemeroptera.

Informal grouping "Palaeoptera"

Insect wings that cannot be folded against the body at rest, because articulation is via axillary plates that are fused with veins, have been termed "palaeopteran" (old wings). Living orders with such wings typically have triadic veins (paired main veins with intercalated longitudinal veins of opposite convexity/concavity to the adjacent main veins) and a network of cross-veins (figured in Taxoboxes 4 & 5). This wing venation and articulation, together with paleontological studies of similar features, was taken to imply that Odonata and Ephemeroptera form a monophyletic group, termed Palaeoptera. The group was argued to be sister to Neoptera, which comprises all remaining extant and primarily winged orders. However, reassessment of morphology of extant early-branching lineages and some recent nucleotide sequence evidence fails to provide strong support for monophyly of Palaeoptera. Nevertheless, it is most likely that a foldable wing was the ancestral condition for pterygotes (see section 8.4) and that the non-foldable wing bases of Ephemeroptera and Odonata were derived secondarily and possibly independently. Here we treat Ephemeroptera as sister group to Odonata + Neoptera, giving a higher classification of Pterygota into three divisions. Some authors name the group consisting of the Odonata and Pterygota as the Metapterygota, which is characterized morphologically by features such as the loss of molting in the adult stage, absence of the caudal filament of the abdomen, and possession of a strong anterior mandibular articulation.

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