Tricholepidion gertschi, copyright Samuel DeGrey.

Belongs within: Allotriocarida.
Contains: Protura, Collembola, Diplura, Archaeognatha, Zygentoma, Pterygota.

The Panhexapoda include the modern Hexapoda (insects and related taxa) together with a small number of Devonian relatives. Members of this clade are united by the presence of only a single pair of antennae, and reduction of appendages posterior to the anterior three pairs of walking legs. In crown-group hexapods, the posterior appendages have been more or less lost though some basal lineages retain small remnants that do not function as walking legs (Grimaldi & Engel 2005). Functional post-thoracic legs remain present in the Devonian marine fossils Devonohexapodus, Wingertshellicus and Cambronatus. Leverhulmia mariae is a fragmentary fossil from the earlier Devonian that preserves at least five pairs of legs but which resembles modern Insecta in the possession of paired lateral claws and a fused median claw on the leg tarsi (Fayers & Trewin 2005). Modern hexapods are divided between four basal clades, the Protura, Collembola, Diplura and Insecta. The Insecta differ from other living hexapods in that the mandibles are not recessed within a pouch on the head capsule (entognathous); other synapomorphies of the insects include loss of musculature in the antenna beyond the scape, loss of articulations between the leg coxae and sternum, and division of the tarsi into tarsomeres (Grimaldi & Engel 2005). Authors have disagreed on whether the remaining entognathous hexapods form a single clade that is sister to the insects or whether they form a series of successive sister groups. Diplura may be united with Insecta by the presence of ancestrally filiform cerci (Kukalová-Peck 1991).

One of the most speciose of all groups of organisms, the insects dominate the terrestrial environment. The majority of insects belong to the winged clade Pterygota, but some basal clades retain the primitive wingless morphology. These latter groups, the Archaeognatha and Zygentoma, are superficially similar in having a more or less elongate body form with three long terminal filaments (the cerci and median filament), and so have been combined in the past under the name Thysanura. However, the Zygentoma share a number of synapomorphies with the Pterygota, including the presence of mandibles with a secondary articulation in addition to the single articulation found in Archaeognatha, and are now recognised a forming a clade with the winged insects (Grimaldi & Engel 2005). The northern Californian Tricholepidion gertschi is a silverfish-like species retaining a number of plesiomorphic features such as ocelli, large abdominal sterna, abdominal styli and eversible vesicles, and five-segmented tarsi (Grimaldi & Engel 2005), and some analyses have suggested it may represent a relict sister group to Zygentoma and Pterygota.

The origin of hexapods
Published 12 March 2019

Insects have been described as the most evolutionarily successful group of animals in the modern world, and with good reason. Something like two-thirds of the currently known animal species are insects, and they are near-ubiquitous in the terrestrial and freshwater environments (for whatever reasons, they’ve never made that much of a go of it marine-wise). Nevertheless, the questions of how and when insects first came to be remains very much an open one.

The long-necked fungus beetle Diatelium wallacei, one of the countless weird oddballs in the insect world. Copyright Artour Anker.

Insects are usually recognised as including three main subgroups: the winged insects, silverfish and bristletails. They are readily united into a group known as the hexapods with a few less speciose assemblages: the springtails, the proturans and the diplurans. All living hexapods have the body divided into a head, thorax and abdomen, with three pairs of walking legs on the thorax and none on the abdomen. Though monophyly of the hexapods has been questioned in the past (which is why the springtails and the like are usually excluded from our concept of ‘insect’ these days despite having been included previously), the majority view is now firmly in favour of regarding them as a single, coherent lineage. How hexapods are related to other arthropods has been more vigorously debated. Earlier authors commonly associated them with the myriapods, the lineage including centipedes and millipedes. In more recent years, an increasing number of studies have instead associated insects with crustaceans. This realignment has primarily been pushed by molecular studies but there are also a number of interesting morphological features such as eye and brain structure that are more crustacean- than myriapod-like in insects. Indeed, it seems not unlikely that insects are not merely related to but are nested within crustaceans: for instance, a few recent studies have supported a relationship between hexapods and a rare group of crustaceans known as remipedes (Schwentner et al. 2017). The features previously seen as shared between insects and myriapods, such as tracheae and uniramous (unbranched) limbs, are then held to probably be convergent adaptations to a terrestrial lifestyle.

Whatever its relationships, it seems most likely that the immediate ancestor of the living hexapods was indeed terrestrial. Of the six basal hexapod lineages referred to above, five (all except winged insects) are almost exclusively terrestrial and were probably ancestrally so. The winged insects include a number of basal subgroups (such as mayflies and dragonflies) that are aquatic for at least the early part of their life cycle, but a terrestrial origin for winged insects as a whole remains credible.

Head of Rhyniella praecursor, from Dunlop & Garwood (2017).

From the perspective of the fossil record, the evidence related to hexapod origins is incredibly slight. The earliest fossil species that have been directly proposed as hexapod relatives are known from the Early Devonian and less than half a dozen such species have been mooted as such in recent years. The only named Devonian fossil whose status as a hexapod seems unimpeachable is Rhyniella praecursor, a springtail from the Rhynie chert of Scotland (Dunlop & Garwood 2017). The same deposit provided Rhyniognatha hirsti, a fragmentary fossil comprising a pair of mandibles and surrounding parts of the head capsule. Rhyniognatha has long been thought to be an insect, possibly even an early member of the winged insect lineage, but Haug & Haug (2017) recently argued that it could just as easily be the head of a centipede (a group already known from other fossils in the Rhynie chert).

Rhyniognatha hirsti, from the University of Aberdeen. Scale bar = 200 µm; m = mandible.

The Windyfield chert, a deposit of similar age and location to the Rhynie chert, has provided Leverhulmia mariae, originally described as a myriapod but reinterpreted as a hexapod relative by Fayers & Trewin (2005). Leverhulmia is a difficult beast to know what and how much to make of it. The original specimen is, speaking charitably, a bit of a mess: a flattened smear looking a bit like a sausage burst open after cooking for too long on the pan. The front and back ends of the animal both appear to be missing and the only features really distinguishable are a series of small jointed legs. Other specimens associated with this species by Fayers & Trewin (2005) are simply more legs detached from their original body. These legs, though, do preserve a reasonable amount of detail, including the presence of paired lateral claws at the ends of the tarsi like those of most insects (Leverhulmia also possesses a smaller median claw between the lateral claws, a feature not found in winged insects but present in silverfish and bristletails). In contrast, the legs of myriapods (as well as those of springtails and proturans) end in a single terminal claw.

Holotype specimen of Leverhulmia mariae, from Dunlop & Garwood (2017); the size of the scale bar was not specified but the entire specimen is about 12 mm long

The overall appearance of Leverhulmia‘s legs might therefore be seen a suggestive of a relationship specifically to insects and not just to hexapods in general, but their number provides something of a barrier to accepting Leverhulmia as a bona fide insect. The train-wreck nature of Leverhulmia‘s preservation means we can’t state confidently how many legs it had but there were at least five pairs: a couple more than the hexapods’ standard-issue three. A number of structures on the abdomens of some living hexapods are potentially derived from modified legs, such as the springing furca of springtails and the ventral styli in hexapods other than springtails and winged insects, so some parallelism in appendage reduction is not out of the question. Nevertheless, unjointed styli are one thing; fully-jointed, functional walking legs are another. Supposed early members of the bristletail and silverfish lineages with jointed abdominal legs have been described from the Carboniferous by Kukalová-Peck (1987) but (as I’ve noted before) many of the more outlandish reconstructions of early insects by Kukalová-Peck have failed to stand up to subsequent scrutiny.

Similar interpretative difficulties surround Strudiella devonica, described as an early relative of the winged insects from the Late Devonian of Belgium. Though I was not unfavourable to this specimen when it was first described, Hörnschemeyer et al. (2013) would later argue against recognising it as an insect. The latter authors professed to be simply unable to discern many of the features cited by its original describers as evidence of insect affinity, and saw Strudiella as closer to a Rorschach blot than a dragonfly. Strudiella‘s status was defended by its original authors (Garrouste et al. 2013) but a number of subsequent authors seem to have taken Hörnschemeyer et al.‘s caution to heart.

Close-up of the head of Strudiella devonica from Hörnschemeyer et al. (2013); the asterisk marks the base of a structure originally interpreted as an antenna.

The final candidate for stem-hexapod status worthy of consideration here is Wingertshellicus backesi from the Lower Devonian Hunsrück Slate of Germany. This marine fossil was interpreted as a stem-hexapod under the name Devonohexapodus bocksbergensis, with a thorax bearing three pairs of legs and an elongate abdomen with uniramous appendages. However, it was reinterpreted by Kühl & Rust (2009) who synonymised Devonohexapodus with the previously described Wingertshellicus, regarded the previously described ‘thoracic legs’ as appendages of the head, and did not accept the presence of differentiated thorax and abdomen. The appendages of the trunk (previously seen as the abdomen) were biramous rather than uniramous with a small endopod and a large flap-like exopod adapted for swimming, and the end of the body bore a pair of fluke-like appendages (comparable to the tail of a crayfish). Wingertshellicus thus lacked any resemblance to a hexapod, and Kühl & Rust doubted that it even belonged to the crown group of arthropods.

Laterally preserved specimen of Wingertshellicus backesi, from Kühl & Rust (2009); scale bar = 10 mm.

An attempt to estimate the age of divergence of hexapods from other arthropods using a molecular clock analysis by Schwentner et al. (2017) suggested that hexapods and remipedes went their separate ways in the late Cambrian or early Ordovician. This is up to 100 million years earlier than the fossils described above but we should be careful how much to read into this discrepancy. If most of the features associated with hexapods are related to adoption of a terrestrial lifestyle, then it might be difficult to recognise any early marine relatives if found. Conversely, while it is uncertain how much if any terrestrial vegetation was present prior to the Devonian, the only potential cover would have been low lichens, non-vascular plants or micro-algae. If stem-hexapods emerged onto land during this time, the environment would not be conducive to their preservation in the fossil record. Finally, not only are hexapods other than winged insects not found in the fossil record before the Devonian, they are barely found after it: after Rhyniella, none are known until the appearance of amber-producing trees during the Cretaceous. So if we can’t find any sign of them for some 300 milion years that we know that they are around, then we obviously can’t say too much about not finding them over the previous hundred million years. The stem-hexapods may have been around in this time but they remain in hiding.

Systematics of Panhexapoda
    |--Devonohexapodus bocksbergensis Haas, Waloszek & Haretenberger 2003GE05, FT05
    |--+--Wingertshellicus backsei Briggs & Bartels 2001GE05, FT05
    |  `--Cambronatus brasseli Briggs & Bartels 2001GE05, FT05
    `--Hexapoda [Apterodicera, Apterygota, Ellipura, Entognatha, Entotropha, Nonoculata, Parainsecta, Thysanura]GE05
         |  i. s.: Leverhulmia Anderson & Trewin 2003FT05
         |           `--*L. mariae Anderson & Trewin 2003FT05
         |         Praemachilis [Praemachilidae]P92
               `--Insecta [Ectognatha, Ectotropha, Lepismatona, Lepismenae, Scarabaeoda, Thysanuroidea, Triplura]LSE13
                    |  i. s.: Drosicha mangiferaeG01
                    |         Neocolochelyna itoi Takeuchi 1951I92
                    |         Paratlanticus tsushimensis Yamasaki 1986I92
                    |         Cercomantispa shirozui Nakahara 1961I92
                    |         Armatocillenus sumaoi (Morita 1981)I92
                    |         Heplochlaenius insularis Uéno 1964I92
                    |         Ogasawarazo daitoensis Voss 1971I92
                    |         Xyelecia japonica Tagashi 1972I92
                    |         Pleroneura hikosana Togashi 1972I92
                    |         NipponorhynchusI92
                    |           |--N. bimaculatus Naito 1973I92
                    |           `--N. mirabilis Takeuchi 1941I92
                    |         Caenosclerogibba japonica Yasumatsu 1958I92
                    |         Heliophobus texturatus (Alpheraky 1892)I92
                    |         Eldana saccharinaH91
                    |         Tricopria drosophilaeGJR97
                    |         Amaurocoris laticepsG89
                    |         Aspongopus viduatusG89
                    |         Nariscus spinosusG89
                    |         Ischnopeza hirticornisG89
                    |         Tephrocerus gracilisG17
                    |         Eudamus proteusG18
                    |         Melitara junctolineellaB88
                    |         Proceras sacchariphagumB88
                    |         Stomopteryx simplexellaB88
                    |         Diplosis sorghicolaB88
                    |         Adelidium cordatum Tillyard 1917F71
                    |         Agetochoristella adscita Riek 1952F71
                    |         AnomaloscytinaF71
                    |           |--A. incompleta Evans 1943F71
                    |           `--A. metapteryx Davis 1942F71
                    |         Aphryganoneura anomala Tillyard 1926F71
                    |         Archaeosmylus pectinatus Riek 1952F71
                    |         Atelophlebis culleni (Etheridge & Olliff 1890) [=Ephemera culleni]F71
                    |         Austroprosbole maculata Evans 1943F71
                    |         Austroscytina imperfecta Evans 1943F71
                    |         BeaconiellaF71
                    |           |--B. fennahi Evans 1963F71
                    |           `--B. multivenata Evans 1963F71
                    |         Belpsylla reticulata Evans 1943F71
                    |         Elateridium angustius Tillyard 1917F71
                    |         Eochiliocycla angusta Davis 1942F71
                    |         Eupincombea postiva Davis 1942F71
                    |         Homaloscytina plana Tillyard 1926F71
                    |         Mesonotoperla sinuata Riek 1954F71
                    |         Mesothyris westraliensis Riek 1968F71
                    |         Metrorhynchites grandis Tillyard 1917F71
                    |         Mitchelloneura permiana Tillyard 1921F71
                    |         Neoageta elongata Riek 1953F71
                    |         Neopetromantis australis Riek 1953F71
                    |         NotoblattitesF71
                    |           |--N. mitchelli Tillyard 1917F71
                    |           `--N. wianamattensis Tillyard 1917F71
                    |         OrthoscytinaF71
                    |           |--O. belmontensis Tillyard 1926F71
                    |           |--O. indistincta Tillyard 1926F71
                    |           |--O. mitchelli Tillyard 1926F71
                    |           |--O. obliqua Tillyard 1926F71
                    |           |--O. pincombei Tillyard 1926F71
                    |           |--O. quinquemedia Tillyard 1926F71
                    |           |--O. subcostalis Tillyard 1926F71
                    |           `--O. tetraneura Tillyard 1926F71
                    |         Palaeovicia incerta Evans 1943F71
                    |         Permagra distincta Evans 1943F71
                    |         Permoberothella perplexa Riek 1953F71
                    |         PermobrachusF71
                    |           |--P. dubia (Tillyard 1926) [=Permodiphthera dubia]F71
                    |           `--P. magnus Evans 1943F71
                    |         Permocapitus globulus Evans 1943F71
                    |         Permocephalus knighti Evans 1943F71
                    |         Permodiphthera robusta Tillyard 1926F71
                    |         PermojassusF71
                    |           |--P. australis Tillyard 1926F71
                    |           `--P. dubius Tillyard 1926F71
                    |         Permopsyllidops stanleyi Davis 1942F71
                    |         Permopsylloides insolita Evans 1943F71
                    |         Permorapisma biserialis Tillyard 1926 [incl. P. triserialis Tillyard 1926]F71
                    |         PermoscartaF71
                    |           |--P. mitchelli Tillyard 1917F71
                    |           `--P. triventulata Tillyard 1926F71
                    |         Permothea latipennis Tillyard 1926F71
                    |         Permotheela scytinopteroides Davis 1942F71
                    |         Permovicia obscura Evans 1943F71
                    |         Phipoides elegans Riek 1953F71
                    |         Protopsyllops minuta Evans 1943F71
                    |         Psocopsyllidium media Davis 1942F71
                    |         Psocoscytina bifida Davis 1942F71
                    |         Psyllidiana davisia Evans 1943F71
                    |         Stanleyana pulchra Evans 1943F71
                    |         Stenoglyphis kimblensis Evans 1947F71
                    |         Stenoscytina australiensis Tillyard 1926F71
                    |         Triassophlebis stigmata Tillyard 1922F71
                    |         Triassopsylla plecioides Tillyard 1917F71
                    |         Triassocytinopsis paranotalis Evans 1956F71
                    |         Tripsyllidium wadei Evans 1956F71
                    |         Tychticoloides belmontensis Evans 1963F71
                    |         Paleacrita vernataZ91
                    |         Itame argillaceariaDC10
                    |         Biloba subsecivelaVHK02
                    |         Lygropia quaternalisVHK02
                    |         Marasma suspicalisVHK02
                    |         Nephanteryx rhodobasalisVHK02
                    |         Anomalococcus indicusVHK02
                    |         Phidodonta modestaVHK02
                    |         ‘Cheyletus’ nigripes Mola 1907B49
                    |         Stephanostoma Danielsen 1880D56
                    |         Phrynomorphus Curtis 1831FG06
                    |         Catulus Kniphof 1759C84
                    |         Fur Jones 1940C84
                    |         Pterolamia Breuning 1942C84
                    |         Griphologus loweiK-P91
                    |         Aethiocarenus Poinar & Brown 2017 [Aethiocarenidae, Aethiocarenodea]PB17
                    |           `--*A. burmanicus Poinar & Brown 2017PB17
                    |         Ammoconia caecimaculaF92
                    |         Calocampa exoletaF92
                    |         Calymnia trapezinaF92
                    |         DianthoeciaF92
                    |           |--D. capsincolaF92
                    |           |--D. carpophagaF92
                    |           `--D. cucubaliF92
                    |         Himera pennariaF92
                    |         Lygris poputataF92
                    |         Parasemia plantaginisF92
                    |         Phragmasobis fuliginosaF92
                    |         Porthetria disparF92
                    |         Theria rubricaprariaF92
                    |         Carcharodus alceaeF92
                    |         Ochrostigma velitarisF92
                    |         Arsilonche alhovenosaF92
                    |         Brotolomia meticulosaF92
                    |         Hydroecia micaceaF92
                    |         Triphosa dubitataF92
                    |         Hipocrita jacobaeaeF92
                    |         Trichiocampus viminalisF92
                    |         Jaspidea celsiaF92
                    |         Plecoptera reflexaF92
                    |         Perinephela lancealisF92
                    |         Diacerisia obliquaF92
                    |         Pterostoma palpinaF92
                    |         Drepama harpogulaF92
                    |         Bhima idiotaF92
                    |         Ortholintha cervinataF92
                    |         Amphidasis betulariaF92
                    |         Rhodophaea suavellaF92
                    |         Psammotis hyalinalisF92
                    |         Polychrosis botranaF92
                    |         Rhopobota naevanaF92
                    |         Notocelia uddmannianaF92
                    |         Sophronia humerellaF92
                    |         Phalacropteryx praecellensF92
                    |         Plastenia retusaF92
                    |         Locastra muscosalisF92
                    |         Erocampa ovataF92
                    |         Rhyparia purpurataF92
                    |         Pyrrhidium sanguineumF92
                    |         Tephroclystia innotataF92
                    |         Thamnonoma wauariaF92
                    |         Epineuronia cespitisF92
                    |         Cyanis argiolusF92
                    |         Dasystoma salicellaF92
                    |         Demas coryliF92
                    |         Alconeura Ball & DeLong 1925DC37
                    |         Hyloidea McAtee 1926DC37
                    |         Dikraneuroidea Lawson 1929DC37
                    |         Forcipata DeLong & Caldwell 1936DC37
                    |         Halidisota argentataM38
                    |         Euschausia ingensM38
                    |         OsprinchotusK18
                    |           |--O. capensisK18
                    |           `--O. (Xenodocon) seductorK18
                    |         ‘Gervaisia’ Robineau-Desvoidy 1863 nec Bonaparte 1854 nec Waga 1858OF80
                    |         Cathilaria rigidaG05
                    |         Doryscelis calcarataB28
                    |         Scopelosoma satellitiaR13
                    |         Agrypnetes crassicornisR13
                    |         Agrypnia pagetanaR13
                    |         Cheimatobia boreataR13
                    |         Sciaphila cellariaR13
                    |         Eugaster guyoniR13
                    |         Gloveria psidiiR13
                    |         Jatrophobia brasiliensisM99
                    |         Ascidium Tode 1782 (n. d.)KC01
                    |         Caulogaster Corda 1831 (n. d.)KC01
                    |         Crateromyces Corda 1831 (n. d.)KC01
                    |         ‘Uralia’ Martynova 1976 nec Mulsant & Verreaux 1866 nec Likharev 1925EH19
                    |         Euscelis Brulié 1832DMH16
                    |         Chiroleptes Kirby 1837DMH16
                    `--Dicondylia [Cercofilata]GE05
                         |--Tricholepidion gertschi Wygodzinsky 1961TW05, KZ02

*Type species of generic name indicated


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