Archaeognatha

Archaeognatha Temporal range: Devonian–Recent PreЄ Є O S D C P T J K Pg N
Rock bristletail
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Hexapoda
Class:	Insecta
Subclass:	Monocondylia Haeckel, 1866
Order:	Archaeognatha Börner, 1904
Families
Machilidae Meinertellidae

The Archaeognatha are an order of wingless insects, also known as jumping bristletails. They are among the least evolutionarily changed insects, appearing in the Middle Devonian period along with the arachnids. They are known from both body and trace fossils (the latter including body imprints and trackways) throughout the remainder of the Paleozoic Era. The name Archaeognatha is derived from Greek, archaeos meaning "ancient" and gnatha meaning "jaw". This refers to the articulation of the mandibles, which has a single condyle, where all higher insects have two. An alternative name, Microcoryphia, comes from the Greek micro, meaning "small", and coryphia, meaning head.

The order Archaeognatha has previously been combined with the order Thysanura; both orders possess three-pronged tails comprising two cerci and an epiproct.

The approximately 350 species in the two families are distributed worldwide. No species is currently at conservation risk.

Description

Archaeognaths are small insects with elongated bodies bent into an arch shape and three long tail-like structures, of which the median filament is longest. They have flexible antennae, large compound eyes that meet at the top of the head, and three ocelli. Their mouthparts are partially retractable, with simple chewing mandibles and long maxillary palps.

Archaeognatha differ from Thysanura in being able to use their tails to spring up to 30 cm (12 in) into the air. They are also unique in possessing small, articulated "styli" on the hind (and sometimes middle) coxae and sternites 2 to 9, which are sometimes considered to be rudimentary appendages, as well as paired eversible membranous vesicles through which they absorb water.

Example of compound eyes (Trigoniophthalmus alternatus)

Further unusual features are that the abdominal sternites are each composed of three sclerites, and they cement themselves to the substrate before molting. Like Thysanura, the body is covered with scales, with a thin exoskeleton that is susceptible to dehydration.

Taxonomy

Families	No. of Species	Defining Notes	Image
Machilidae	250	Most are restricted to rocky shorelines	Need specific image
Meinertellidae	Unknown	Lack scales at base of hind legs and antennae	Need specific image

Biology

Archaeognaths are found in a wide range of habitats, and are unusual among insects in that they can even be found in the Arctic, where they live in leaf litter and rock crevices. While most species are found in moist soil, others have adapted to chaparral, and even sandy deserts. They feed primarily on algae, but also lichens, mosses, or decaying organic materials.

During courtship, the males spin a thread from their abdomens, attach one end to the substrate, and string packages of sperm (spermatophores) along it. After a series of courtship dances, the female picks up the spermatophores and places them on her ovipositor. The female then lays a batch of around 30 eggs in a suitable crevice. The young resemble the adults, and take up to two years to reach sexual maturity. Unlike most insects, the adults continue to moult after reaching adulthood, and typically mate once at each instar. Archaeognaths may have a total lifespan of up to four years, longer than many larger insects.

External links

• Archaeognatha - Tree of Life Web Project
• Microcoryphia Virginia Polytechnic Institute and State College Department of Entomology