| Apterygota | |
|---|---|
 | |
| Petrobius maritimus (Archaeognatha:Machilidae) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Insecta |
| Subclass: | Apterygota Brauer 1885[2] |
| Groups included | |
| Cladistically included but traditionally excluded taxa | |
The nameApterygota is sometimes applied to a formersubclass of small, agileinsects, distinguished from other insects by their lack of wings in the present and in their evolutionary history; notable examples are thesilverfish, thefirebrat, and thejumping bristletails. Their first known occurrence in the fossil record is during theDevonian period, 417–354 million years ago. The group Apterygota is not a clade; it isparaphyletic, and not recognized in modern classification schemes. As defined, the group contains two separateclades of wingless insects:Archaeognatha comprises jumping bristletails, whileZygentoma comprises silverfish and firebrats. The Zygentoma are in the cladeDicondylia with winged insects, a clade that includes all other insects, while Archaeognatha is sister to this lineage.[4]
Thenymphs (younger stages) go through little or even nometamorphosis, hence they resemble the adult specimens (ametabolism).Currently, nospecies are listed as being at conservation risk.
The primary characteristic of the apterygotes is they are primitively wingless. While some other insects, such asfleas, 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.[5]
Apterygotes also have a number of other primitive features not shared with other insects. Males deposit sperm packages, orspermatophores, rather than fertilizing the femaleinternally. When hatched, the young closely resemble adults and do not undergo any significantmetamorphosis, and lack even an identifiablenymphal stage. They continue to molt throughout life, undergoing multipleinstars 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 theirabdominal segments, but play no part in locomotion. They also have long, paired abdominalcerci and a single median, tail-like caudal filament, or telson.[6]
While all members of winged insects (Pterygota) has a closed amniotic cavity during embryonic development, this varies within Apterygota. In Archaeognatha, species likePetrobius brevistylis andPedetontus unimaculatus have a wide open cavity, whereasTrigoniophthalmus alternatus does not have an amniotic cavity at all. In Zygentoma, the cavity is open through a narrow canal called the amniopore in the speciesThermobia domestica andLepisma saccharina, but in other species likeCtenolepisma lineata it is completely closed.[7]
The composition and classification of Apterygota changed over time. By the mid-20th century, the subclass included four orders (Collembola,Protura,Diplura, andThysanura). With the advent of a more rigorouscladistic methodology, the subclass was provenparaphyletic. While the first three groups formed a monophyletic group, theEntognatha, distinguished by having mouthparts submerged in a pocket formed by the lateral and ventral parts of the head capsule, the Thysanura (Zygentoma plusArchaeognatha) appeared to be more closely related towinged insects. The most notablesynapomorphy proving themonophyly of Thysanura+Pterygota is the absence of intrinsicantennal muscles, which connect theantennomeres inentognaths,myriapods, andcrustaceans. For this reason, the whole group is often termed theAmyocerata, meaning "lacking antennal muscles".
However, the Zygentoma are now considered more closely related to the Pterygota than to the Archaeognatha,[4] thus rendering even the amyocerate apterygotes paraphyletic, and resulting in the dissolution of Thysanura into two separate monophyletic orders.
