The Archaeognatha (formerly Microcoryphia) are an order of apterygotes belonging to the Superclass Hexapoda, Class Insecta, Subphylum Labiata, and Phylum Arthropoda. They are known by various common names, such as jumping bristletails. The order is cosmopolitan and includes about 500 species (thirty-three species within twelve genera are Nearctic) in two families (Machilidae and Meinertellidae). None are currently evaluated as being a conservation risk. Little is known about the archaeognaths of Arkansas, as only Machiloides banksi and Pedetontus gershneri have been reported from the state, both from Mount Magazine (Logan County).
Among extant arthropod taxa, they are some of the most evolutionarily primitive insects. The fossil record of Archaeognatha is sparse and often represented by fragmentary material. They first appeared in the Middle Devonian period (419.2 to 358.9 million years ago) during the same period as the class Arachnida. Specimens of archeognaths that closely resemble extant species have been found as both body and trace fossils (the latter including body imprints and trackways) in strata from the remainder of the Paleozoic Era and more recent periods.
Until the late twentieth century, the suborders Zygentoma (silverfish or fishmoths, and the firebrats) and Archaeognatha made up the order Thysanura. At the same time, it was recognized that the Thysanura was paraphyletic. Thus, the two suborders were each raised to the level of an independent monophyletic order, with Archaeognatha as sister taxon. Both orders possess small primitive insects with three-pronged tails comprising two lateral cerci and a medial filament called the epiproct (or appendix dorsalis). Of these three organs, the appendix dorsalis is considerably longer than the two cerci. This character helps differentiate the Archaeognatha from the Zygentoma, in which the three organs are subequal in length.
Further morphological characters of the Archaeognatha include elongated bodies and arched backs, especially over the thorax. Their antennae are flexible, and the two large medially contiguous compound eyes meet at the top of the head, and there are three ocelli. The mouthparts are partly retractable, with simple chewing mandibles and long maxillary palps. Their mandibles are monocondylic (the joint or socket-like attachment point to the head capsule) with only one condyle. The Archaeognatha also have a relatively small head, with bodies being compressed from side to side (laterally). Their abdominal segments possess styles, or bristles, which are small appendages moveable by muscles.
When disturbed, these insects are capable of using their tails to spring up to 30 cm (12 in.) into the air. They accomplish this unusual behavior by thrusting their abdominal muscles rapidly and forcefully against the ground, providing them lift-off. They also are unique among insects in possessing small, articulated styles on the hind (and sometimes middle) coxae and on sternites two to nine, which some entomologists consider to be vestigial appendages. They have paired eversible membranous vesicles through which they absorb water. Other unusual morphological features are that the abdominal sternites are each composed of three sclerites, and they cement themselves to the substrate before molting. The body is covered with readily detached scales that make specimens difficult to grasp by predators and also may protect their exoskeleton from abrasion. However, their thin exoskeleton offers little or no protection against dehydration. Accordingly, the insects seek areas with humid air, such as in cool, damp situations under bark or stones.
In terms of habitats, the Archaeognatha occur in a wide range of environs. While most species live in moist soil, others have adapted to chaparral, and even arid sandy deserts. Here, they hide under bark, in litter, and in rock crevices, and feed on algae, lichens, mosses, and decaying organic detritus.
Jumping bristletails undergo simple metamorphosis, progressing from egg to nymph to adult. Males of some species perform courtship rituals to ensure that females find their spermatophore. During reproductive activities, male courtship involves spinning a thread from the abdomen, attaching one end to the substrate, and stringing packages of sperm packets (spermatophores) along it. After a series of courtship dances, the female picks up the spermatophores and places them on her ovipositor. She then lays a complement of around thirty eggs in a suitable microhabitat such as a crevice. The eggs can remain in a dormant state for a year before hatching. Juvenile bristletails resemble the adults, and depending on the species and conditions such as temperature and available food, take up to two years to reach sexual maturity. Unlike most other insects, the adults continue to molt after reaching adulthood, and typically mate once at each instar. Archaeognaths may live for up to four years, which is longer than even many larger insects.
Little is known about the archaeognaths of Arkansas. Jumping bristletails (family Machilidae) reported from the state include: rock bristletail, Machiloides banksi and Pedetontus gershneri, both from Mount Magazine. The latter species was found in leaf litter in dense deciduous forest, while the former inhabited drier (xeric) habitats under rocks on the south-facing slope of the mountain.
For additional information:
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Chris T. McAllister
Eastern Oklahoma State College
Last Updated: 02/25/2021