7.4.1. Archaeognatha and Zygentoma (Thysanura sensu lato)
Order Archaeognatha (archaeognathans, bristletails) (see also Box 9.3)
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 with two neighboring condyles each and elongate seven-segmented maxillary palps. Often a coxal style occurs on coxae of legs 2 and 3, or 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 fossil taxon Monura belongs in Thysanura sensu lato. The two families of recent Archaeognatha, Machilidae and Meinertellidae, form an undoubted monophyletic group. The order probably is placed as the earliest branch of the Insecta, and as sister group to Zygentoma + Pterygota (Fig. 7.3). Alternatively, a potentially influential recent molecular analysis revived the concept of Archaeognatha as sister to Zygentoma, in a grouping that should be called Thysanura (sensu lato — meaning in the broad sense in which the name was first used for apterous insects with “bristle tails”).
Order Zygentoma (Thysanura, silverfish)
Zygentomans (thysanurans) 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 four extant families. Zygentoma is the sister group of the Pterygota (Fig. 7.3) alone, or perhaps with Archaeognatha in Thysanura sensu lato (see above under Archaeognatha).
Italicized names indicate likely paraphyletic taxa. Broken lines indicate uncertain relationships. (Data from several sources)