Archaeognatha

Archaeognatha are small insects with elongated bodies and backs that are arched, especially over the thorax. Their abdomen ends in three long tail-like structures, of which the lateral two are cerci, while the medial filament, which is longest, is an epiproct. The tenth abdominal segment is reduced.{{cite book | url=https://books.google.com/books?id=xAP3BwAAQBAJ&dq=%2210th+segment+reduced+and+tergum+of+11th+forming+median%22&pg=PA117 | title=Entomology | isbn=9781461569183 | last1=Gillot | first1=Cedric | date=14 March 2013 | publisher=Springer }} The antennae are flexible. The two large compound eyes meet at the top of the head, and there are three ocelli. The mouthparts are partly retractable, with simple chewing mandibles and seven-segmented maxillary palps which are commonly longer than the legs.{{cite book | url=https://books.google.com/books?id=Jk0Hym1yF0cC&dq=seven-segmented+maxillary+palps&pg=PA49 | title=Encyclopedia of Insects | isbn=9780080920900 | last1=Resh | first1=Vincent H. | last2=Cardé | first2=Ring T. | date=22 July 2009 | publisher=Academic Press }}{{rp|341–343}}

Unlike other insect orders, they do not have olfactory receptor-coreceptors (Orco), which have either been lost or were never present in the first place.{{cite journal | pmc=7873100 | year=2021 | last1=Wicher | first1=D. | last2=Miazzi | first2=F. | title=Functional properties of insect olfactory receptors: Ionotropic receptors and odorant receptors | journal=Cell and Tissue Research | volume=383 | issue=1 | pages=7–19 | doi=10.1007/s00441-020-03363-x | pmid=33502604 }}{{cite journal | doi=10.7554/eLife.38340 | title=The origin of the odorant receptor gene family in insects | year=2018 | last1=Brand | first1=Philipp | last2=Robertson | first2=Hugh M. | last3=Lin | first3=Wei | last4=Pothula | first4=Ratnasri | last5=Klingeman | first5=William E. | last6=Jurat-Fuentes | first6=Juan Luis | last7=Johnson | first7=Brian R. | journal=eLife | volume=7 | pmid=30063003 | pmc=6080948 | doi-access=free }}

Archaeognatha differ from Zygentoma in various ways, such as their relatively small head, their bodies being compressed laterally (from side to side) instead of flattened dorsiventrally, and in their being able to use their tails to spring up to {{cvt|30|cm|in}} into the air if disturbed. They have eight pairs of short appendages called styli on abdominal segments 2 to 9. Family Machilidae is also unique among insects in possessing small muscleless styli on the second and third thoracic legs,{{cite book | url=https://books.google.com/books?id=jrdcDwAAQBAJ&dq=%22as+they+are+often+supposed+to+be%2C+since+the+true+exopodite+of+Crustacea+%22&pg=PA271 | title=Principles of Insect Morphology | isbn=9781501717918 | last1=Snodgrass | first1=R. E. | date=31 May 2018 | publisher=Cornell University Press }} but are absent on the second pair of thoracic legs in some genera.{{cite web | url=https://books.google.com/books?id=Z_9MAAAAYAAJ&dq=Machilidae+%22coxal+stylets+never+appear+in+the+P+I%22+%22and+in+some+other+genera+in+P+III+only%22&pg=PA18 | title=An Annotated List of Generic and Specific Names of Machilidae (Microcoryphia, Insecta) with Identification Keys for the Genera and Geographical Notes | last1=Mendes | first1=Luís F. | year=1990 }} Similar stylets on the legs are absent in family Meinertellidae.{{cite journal | url=https://www.jstor.org/stable/25010224 | jstor=25010224 | title=A New Subgenus and Two New Species of the Genus Machilinus (Meinertellidae, Archaeognatha = Microcoryphia, "Apterygota", Insecta) from Mexico | last1=Sturm | first1=Helmut | journal=Journal of the New York Entomological Society | year=1997 | volume=105 | issue=1/2 | pages=15–23 }} They have one or two pairs of eversible membranous vesicles on the underside of abdominal segments 1 to 7, which are used to absorb water and assisting with molting.{{cite book | url=https://books.google.com/books?id=gAamEAAAQBAJ&dq=%22in+litter%2C+soil%2C+among+coastal+rocks+and+cliffs%2C+some+in+dry+regions%22&pg=PA39 | title=Essential Entomology | isbn=978-0-19-265483-0 | last1=McGavin | first1=George C. | last2=Davranoglou | first2=Leonidas-Romanos | date=4 January 2023 | publisher=Oxford University Press }} There are nine pairs of spiracles; two pairs on the thorax, and seven pairs on abdominal segments 2 to 8.{{cite book | url=https://books.google.com/books?id=sm3uCAAAQBAJ&dq=%22archaeognatha%22+%22nine+pairs+of+spiracles%22&pg=PA124 | title=Outlines of Entomology | isbn=9789401705080 | last1=Davies | first1=R. G. | date=11 November 2013 | publisher=Springer }} The pair of spiracles on the first abdominal segment has been lost.[https://www.zobodat.at/pdf/Ent-Abhandlungen-Dresden_61_0119-0172.pdf Proceedings of the 1st Dresden Meeting on Insect Phylogeny: "Phylogenetic Relationships within the Insect Orders". (Dresden, September 19-21, 2003)]

File:Jumping Bristletails (15567941692).jpg

File:Tête de Trigoniophthalmus alternatus.jpg, showing compound eyes, prominent maxillary palps, and detachable scales.}}]]

File:Lubbock Machilis Scale p0437P.png}}]]

Further unusual features are that the abdominal sternites are each composed of three sclerites, and they cement themselves to the substrate before molting, often using their own feces as glue.{{cite book | url=https://books.google.com/books?id=IEvyDwAAQBAJ&dq=%22bristletails%22+molting+anchor+themselves&pg=PA40 | title=Common Insects of Texas and Surrounding States | isbn=9781477310359 | last1=Abbott | first1=John C. | last2=Abbott | first2=Kendra | date=13 October 2020 | publisher=University of Texas Press }} The body is covered with readily detached scales, that make the animals difficult to grip and also may protect the exoskeleton from abrasion. The thin exoskeleton offers little protection against dehydration, and they accordingly must remain in moist air, such as in cool, damp situations under stones or bark.

The name Archaeognatha is derived from the Greek {{langx|grc|ἀρχαῖος|archaios|label=none}}, meaning 'ancient', and {{langx|grc|γνάθος|gnathos|label=none}}, meaning 'jaw'. This refers to the articulation of the mandibles, which are different from those of other insects. It was originally believed that Archaeognatha possessed a single phylogenetically primitive condyle each (thus the name "Monocondylia"), where all more derived insects have two, but this has since been shown to be incorrect; all insects, including Archaeognatha, have dicondylic mandibles, but archaeognaths possess two articulations that are homologous to those in other insects, though slightly different.{{cite journal | url=https://doi.org/10.1111/syen.12107 | doi=10.1111/syen.12107 | title=Mandibles with two joints evolved much earlier in the history of insects: Dicondyly is a synapomorphy of bristletails, silverfish and winged insects | year=2015 | last1=Blanke | first1=Alexander | last2=Machida | first2=Ryuichiro | last3=Szucsich | first3=Nikolaus Urban | last4=Wilde | first4=Fabian | last5=Misof | first5=Bernhard | journal=Systematic Entomology | volume=40 | issue=2 | pages=357–364 | bibcode=2015SysEn..40..357B | s2cid=85309726 | url-access=subscription }}

An alternative name, Microcoryphia, comes from the Greek {{langx|grc|μικρός|mikros|small|label=none}}, and {{langx|grc|κορυφή|koryphē|label=none}}, which in context means 'head'.{{cite book|author=H. G. Liddell|title=An Intermediate Greek-English Lexicon: Based on the 7th Ed of Liddell & Scott's Lexicon. |url=https://books.google.com/books?id=3tKwYgEACAAJ|year=1889}}

Archaeognatha occur in a wide range of habitats. While most species live in moist soil, others have adapted to chaparral, and even sandy deserts. They feed primarily on algae, but also lichens, mosses, or decaying organic detritus.

Three types of mating behavior are known. In some species the male spins a silk thread (carrier thread) stretched out on the ground. On the thread there are droplets of sperm which the female will take up when her ovipositor makes contact. In others a packet of sperm (spermatophore) is deposited on the top of a short stalk. If a female takes up the sperm or not is often random, but in many species the male will try to lead the female's genitals over the sperm during courtship. A more direct way of fertilization occurs in species of the genus Petrobius, where the male place a droplet of sperm directly on the female's ovipositor. {{cite journal | url=https://www.sciencedirect.com/science/article/abs/pii/S096517481300057X | doi=10.1016/j.ibmb.2013.03.014 | title=Silverfish silk is formed by entanglement of randomly coiled protein chains | year=2013 | last1=Walker | first1=Andrew A. | last2=Church | first2=Jeffrey S. | last3=Woodhead | first3=Andrea L. | last4=Sutherland | first4=Tara D. | journal=Insect Biochemistry and Molecular Biology | volume=43 | issue=7 | pages=572–579 | pmid=23578395 | bibcode=2013IBMB...43..572W | url-access=subscription }}{{cite journal | url=https://www.sciencedirect.com/science/article/abs/pii/S1467803912000576 | doi=10.1016/j.asd.2012.06.003 | title=The exoskeleton of the female genitalic region in Petrobiellus takunagae (Insecta: Archaeognatha): Insect-wide terminology, homologies, and functional interpretations | year=2012 | last1=Klass | first1=Klaus-Dieter | last2=Matushkina | first2=Natalia A. | journal=Arthropod Structure & Development | volume=41 | issue=6 | pages=575–591 | pmid=22766521 | bibcode=2012ArtSD..41..575K | url-access=subscription }} One hypothesis is that the external genitals of insects started as structures specialized for water-uptake, which could reach deeper crevices than the coxal vesicles, and over time the female would use it to take up sperm from the ground instead of water.[https://www.senckenberg.de/wp-content/uploads/2019/09/abstracts_9th-dresden-meeting-on-insect-phylogeny_2019.pdf 9th Dresden Meeting on Insect Phylogeny 2019] After fertilization she 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, depending on the species and conditions such as temperature and available food.

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 most larger insects.

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{{Wikispecies|Archaeognatha}}

{{Commons category|Archaeognatha}}

• [http://tolweb.org/tree?group=Archaeognatha&contgroup=Insecta Archaeognatha] - Tree of Life Web Project
• [https://web.archive.org/web/20100615151356/http://www.idlab.ento.vt.edu/IDLab/insect_orders/microcoryphia.html Microcoryphia] Virginia Polytechnic Institute and State College Department of Entomology{{dead link|01-13-2014|date=January 2015}}

{{Orders of Insects}}

{{Archaeognatha}}

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Category:Insect orders

Category:Insects of the Arctic

Category:Middle Devonian first appearances