Entognatha

Entognatha

Overview
The Entognatha is a class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 of ametabolous arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s, which, together with insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s, makes up the subphylum Hexapoda
Hexapoda
The subphylum Hexapoda constitutes the largest grouping of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura . The Collembola are very abundant in terrestrial environments...

. Their mouthparts are entognathous, meaning they are retracted within the head. Entognatha are apterous, meaning they lack wings
Insect wing
Insects are the only group of invertebrates known to have evolved flight. Insects possess some remarkable flight characteristics and abilities, still far superior to attempts by humans to replicate their capabilities. Even our understanding of the aerodynamics of flexible, flapping wings and how...

. The class contains three orders
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

: Collembola (springtail
Springtail
Springtails form the largest of the three lineages of modern hexapods that are no longer considered insects...

s), Diplura and Protura. These three groups were historically united with the order Thysanura
Thysanura
Thysanura is an order of insects, encompassing silverfish and firebrats, known for their three long caudal filaments.The families Machilidae and Meinertellidae of the jumping bristletails were once included with Thysanura....

 to form the class Apterygota
Apterygota
The name Apterygota is sometimes applied to a subclass of small, agile insects, distinguished from other insects by their lack of wings in the present and in their evolutionary history...

, but it has since been recognized that the hexapodous condition of these animals has evolved independently from that of insects, and independently within each order.
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Encyclopedia
The Entognatha is a class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 of ametabolous arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s, which, together with insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s, makes up the subphylum Hexapoda
Hexapoda
The subphylum Hexapoda constitutes the largest grouping of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura . The Collembola are very abundant in terrestrial environments...

. Their mouthparts are entognathous, meaning they are retracted within the head. Entognatha are apterous, meaning they lack wings
Insect wing
Insects are the only group of invertebrates known to have evolved flight. Insects possess some remarkable flight characteristics and abilities, still far superior to attempts by humans to replicate their capabilities. Even our understanding of the aerodynamics of flexible, flapping wings and how...

. The class contains three orders
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

: Collembola (springtail
Springtail
Springtails form the largest of the three lineages of modern hexapods that are no longer considered insects...

s), Diplura and Protura. These three groups were historically united with the order Thysanura
Thysanura
Thysanura is an order of insects, encompassing silverfish and firebrats, known for their three long caudal filaments.The families Machilidae and Meinertellidae of the jumping bristletails were once included with Thysanura....

 to form the class Apterygota
Apterygota
The name Apterygota is sometimes applied to a subclass of small, agile insects, distinguished from other insects by their lack of wings in the present and in their evolutionary history...

, but it has since been recognized that the hexapodous condition of these animals has evolved independently from that of insects, and independently within each order. The orders may not be closely related, in which case Entognatha would be a polyphyletic
Polyphyly
A polyphyletic group is one whose members' last common ancestor is not a member of the group.For example, the group consisting of warm-blooded animals is polyphyletic, because it contains both mammals and birds, but the most recent common ancestor of mammals and birds was cold-blooded...

 group.

Morphology


These minute arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s are apterous or wingless, unlike some other orders of insects that have lost their wings
Insect wing
Insects are the only group of invertebrates known to have evolved flight. Insects possess some remarkable flight characteristics and abilities, still far superior to attempts by humans to replicate their capabilities. Even our understanding of the aerodynamics of flexible, flapping wings and how...

 secondarily (but are derived from winged ancestors). Their mouthparts are enclosed within a pouch in the head capsule, called the gnathal pouch, so that only the tips of the mandibles and maxillae are exposed beyond the cavity. This pouch is created in the embryo by a flap or lateral head sclerite near the mouth on each side of the head which fuses with the labium. Other differences with insects are that each antennal
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

 segment is musculated; in insects, only the two basal segments are. Sperm transfer is always indirect, and there is an ovipositor
Ovipositor
The ovipositor is an organ used by some animals for oviposition, i.e., the laying of eggs. It consists of a maximum of three pairs of appendages formed to transmit the egg, to prepare a place for it, and to place it properly...

 in the females. Of the three orders, only collembolans possess eyes; nevertheless, many collembolans are blind, and even when compound eyes are present, there are no more than eight ommatidia.
  • Collembola have a ventral tube termed a collophore on the first abdominal segment. The collophore is involved in moisture absorption. On the third abdominal segment is the retinaculum that holds the furcula
    Furcula (Collembola)
    The furcula, or furca, is a tail-like appendage shaped like a fork, found ventrally on the fourth abdominal segment of springtails. Present in most species dwelling the upper soil layers, it is used for jumping when the animal is threatened...

    . The furcula is the "spring" for which the Collembola are given the name springtails.
  • Proturans, sometimes referred to as "coneheads", do not have eyes or antennae. They possess abdominal styli that are thought to be vestigial legs.
  • The Diplura have a pair of caudal cerci
    Cercus
    Cerci are paired appendages on the rear-most segments of many arthropods, including insects and arachnids but not crustaceans. Cerci often serve as sensory organs, but they may also be used as weapons or copulation aids, or they may simply be vestigial structures.Typical cerci may appear to be...

    , from which their name, meaning "two-tailed", is derived.