The name Apterygota is sometimes applied to a former subclass of small, agile

insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...

s, distinguished from other insects by their lack of wings in the present and in their evolutionary history; notable examples are the

silverfish The silverfish (''Lepisma saccharinum'') is a species of small, primitive, wingless insect in the order Zygentoma (formerly Thysanura). Its common name derives from the insect's silvery light grey colour, combined with the fish-like appearanc ...

, the firebrat, and the jumping bristletails. Their first known occurrence in the fossil record is during the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

period, 417–354 million years ago. The group Apterygota is not a clade; it is

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

, and not recognized in modern classification schemes. As defined, the group contains two separate

clades In biology, a clade (), also known as a monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach to taxonomy ...

of wingless insects:

Archaeognatha The Archaeognatha are an order of apterygotes, known by various common names such as jumping bristletails. Among extant insect taxa they are some of the most evolutionarily primitive; they appeared in the Middle Devonian period at about the sa ...

comprises jumping bristletails, while

Zygentoma Zygentoma are an order in the class Insecta, and consist of about 550 known species. The Zygentoma include the so-called silverfish or fishmoths, and the firebrats. A conspicuous feature of the order are the three long caudal filaments. The t ...

comprises silverfish and firebrats. The Zygentoma are in the clade

Dicondylia Dicondylia is a taxonomic group (taxon) that includes all insects except the jumping bristletails (Archaeognatha). Dicondylia species have a mandible attached with two hinges to the head capsule (dicondyl), in contrast to a hypothetical ancestra ...

with winged insects, a clade that includes all other insects, while Archaeognatha is sister to this lineage.A. Blanke, M. Koch, B. Wipfler, F. Wilde, B. Misof (2014) Head morphology of ''Tricholepidion gertschi'' indicates monophyletic Zygentoma. Frontiers in Zoology 11:16 doi:10.1186/1742-9994-11-16 The

nymphs A nymph (; ; sometimes spelled nymphe) is a minor female nature deity in ancient Greek folklore. Distinct from other Greek goddesses, nymphs are generally regarded as personifications of nature; they are typically tied to a specific place, land ...

(younger stages) go through little or even no

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and different ...

, hence they resemble the adult specimens ( ametabolism). Currently, no

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

are listed as being at conservation risk.

Characteristics

The primary characteristic of the apterygotes is they are primitively wingless. While some other insects, such as

flea Flea, the common name for the order (biology), order Siphonaptera, includes 2,500 species of small flightless insects that live as external parasites of mammals and birds. Fleas live by hematophagy, ingesting the blood of their hosts. Adult f ...

s, also lack wings, they nonetheless descended from winged insects but have lost them during the course of evolution. By contrast, the apterygotes are a primitive group of insects that diverged from other ancient orders before wings evolved. Apterygotes, however, have the demonstrated capacity for directed, aerial gliding descent from heights. It has been suggested by researchers that this evolved gliding mechanism in apterygotes might have provided an evolutionary basis from which winged insects would later evolve the capability for powered flight. Apterygotes also have a number of other primitive features not shared with other insects. Males deposit sperm packages, or

spermatophore A spermatophore, from Ancient Greek σπέρμα (''spérma''), meaning "seed", and -φόρος (''-phóros''), meaning "bearing", or sperm ampulla is a capsule or mass containing spermatozoa created by males of various animal species, especiall ...

s, rather than fertilizing the female internally. When hatched, the young closely resemble adults and do not undergo any significant

, and lack even an identifiable nymphal stage. They continue to molt throughout life, undergoing multiple

instar An instar (, from the Latin '' īnstar'' 'form, likeness') is a developmental stage of arthropods, such as insects, which occurs between each moult (''ecdysis'') until sexual maturity is reached. Arthropods must shed the exoskeleton in order to ...

s after reaching sexual maturity, whereas all other insects undergo only a single instar when sexually mature. Apterygotes possess small unsegmented appendages, referred to as "styli", on some of their

abdominal The abdomen (colloquially called the gut, belly, tummy, midriff, tucky, or stomach) is the front part of the torso between the thorax (chest) and pelvis in humans and in other vertebrates. The area occupied by the abdomen is called the abdominal ...

segments, but play no part in locomotion. They also have long, paired abdominal cerci and a single median, tail-like caudal filament, or telson. While all members of winged insects (

Pterygota Pterygota ( ) is a subclass of insects that includes all winged insects and groups who lost them secondarily. Pterygota group comprises 99.9% of all insects. The orders not included are the Archaeognatha (jumping bristletails) and the Zygent ...

) has a closed amniotic cavity during embryonic development, this varies within Apterygota. In Archaeognatha, species like '' Petrobius brevistylis'' and '' Pedetontus unimaculatus'' have a wide open cavity, whereas ''

Trigoniophthalmus alternatus ''Trigoniophthalmus alternatus'' is a species of jumping bristletail in the family Machilidae. It is found in Europe and Northern Asia (excluding China) and North America. References Further reading * * * External links

* Arc ...

'' does not have an amniotic cavity at all. In Zygentoma, the cavity is open through a narrow canal called the amniopore in the species ''

Thermobia domestica The firebrat (''Thermobia domestica'') is a small insect (typically 1–1.5 cm) in the order Zygentoma. Habitat Firebrats prefer relatively warm temperatures (36–39 °C) and require some humidity. They are commonly found ...

'' and ''

Lepisma saccharina The silverfish (''Lepisma saccharinum'') is a species of small, primitive, wingless insect in the order Zygentoma (formerly Thysanura). Its common name derives from the insect's silvery light grey colour, combined with the fish-like appearance ...

'', but in other species like '' Ctenolepisma lineata'' it is completely closed.Insect Metamorphosis: From Natural History to Regulation of Development and Evolution
/ref>

History of the concept

The composition and classification of Apterygota changed over time. By the mid-20th century, the subclass included four orders (

Collembola Springtails (class Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects. Although the three lineages are sometimes grouped together in a class called Entognatha because they have internal m ...

Protura The Protura, or proturans, and sometimes nicknamed coneheads, are very small (0.6–1.5mm long), soil-dwelling animals, so inconspicuous they were not noticed until the 20th century. The Protura constitute an order of hexapods that were previou ...

Diplura The order Diplura ("two-pronged bristletails") is one of three orders of non-insect hexapods within the class Entognatha (alongside Collembola (springtails) and Protura). The name "diplura", or "two tails", refers to the characteristic pair o ...

, and

Thysanura Thysanura is the now Deprecation, deprecated name of what was, for over a century, recognised as an Order (biology), order in the Class (biology), class Insecta. The two constituent groups within the former order, the Archaeognatha (jumping bristle ...

). With the advent of a more rigorous

cladistic Cladistics ( ; from Ancient Greek 'branch') is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is ...

methodology, the subclass was proven

. While the first three groups formed a monophyletic group, the

Entognatha The Entognatha, from Ancient Greek ἐντός (''entós''), meaning "inside", and γνάθος (''gnáthos''), meaning "jaw", are a class (biology), class of wingless and Ametabolism, ametabolous arthropods, which, together with the insects, ...

, distinguished by having mouthparts submerged in a pocket formed by the lateral and ventral parts of the head capsule, the Thysanura (

plus

) appeared to be more closely related to winged insects. The most notable

synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel Phenotypic trait, character or character state that has evolution, evolved from its ancestral form (or Plesiomorphy and symplesiomorphy, plesiomorphy). A synapomorphy is an apomorphy sh ...

proving the

monophyly In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent comm ...

of Thysanura+Pterygota is the absence of intrinsic antennal muscles, which connect the antennomeres in entognaths,

myriapods Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial. Although molecular evidence and similar fossils suggests a diversifi ...

, and

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s. For this reason, the whole group is often termed the Amyocerata, meaning "lacking antennal muscles". However, the Zygentoma are now considered more closely related to the Pterygota than to the Archaeognatha, thus rendering even the amyocerate apterygotes paraphyletic, and resulting in the dissolution of Thysanura into two separate monophyletic orders.

References

* ''Firefly Encyclopedia of Insects and Spiders'', edited by Christopher O'Toole, , 2002 {{Use dmy dates, date=April 2020 Insect taxonomy Arthropod subclasses Extant Devonian first appearances Paraphyletic groups