Apterygota
Encyclopedia
The name Apterygota is sometimes applied to a subclass of small, agile insect
s, distinguished from other insects by their lack of wings in the present and in their evolutionary history. Their first known occurrence in the fossil record is during the Devonian
period, 417-354 million years ago.
The nymphs
(younger stages) go through little or even no metamorphosis, hence they resemble the adult specimens. Their skin is thin, making them appear translucent.
There are no current species
at conservation risk.
s, also lack wings, these are descended from winged insects and have lost them during the course of evolution. By contrast, the apterygotes are a primitive group of insects that diverged from other orders before wings evolved.
Apterygotes, however, show directed aerial gliding descent. It has been suggested that provided a preflight phenomenon from which insects were later to evolve their winged flight capacities.
Apterygotes also have a number of other primitive features that are not shared with other insects. Males deposit sperm packages, or spermatophore
s, rather than fertilising the female internally. The young hatch closely resembling the adults and undergo no significant metamorphosis
, lacking even an identifiable nymphal
stage. They continue to moult throughout life, with multiple instar
s after reaching sexual maturity, whereas all other insects have only a single sexually mature adult instar.
Apterygotes possess small appendages, referred to as "styli" on some of their abdominal
segments, although these are not true legs and cannot be used for walking. They also have long, paired abdominal cerci
and a single median, tail-like caudal filament.
). With the advent of a more rigorous cladistic methodology, the subclass was proven paraphyletic. While the first three groups formed a monophyletic group, the Entognatha
, distinguished by having mouthparts submerged in a pocket formed by the lateral and ventral parts of the head capsule, the Thysanura appeared to be more closely related to winged insects
. The most notable synapomorphy
proving the monophyly
of Thysanura+Pterygota is the absence of intrinsic antennal
muscles, which connect the antennomeres in entognaths
, myriapods
, and crustacean
s. For this reason, the whole group is often termed the Amyocerata, meaning "lacking antennal muscles".
Moreover, it is now assumed that the Thysanura are more closely related to the Pterygota than to the Archaeognatha, thus rendering even the amyocerate apterygotes paraphyletic.
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, distinguished from other insects by their lack of wings in the present and in their evolutionary history. Their first known occurrence in the fossil record is during the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
period, 417-354 million years ago.
The nymphs
Nymph (biology)
In biology, a nymph is the immature form of some invertebrates, particularly insects, which undergoes gradual metamorphosis before reaching its adult stage. Unlike a typical larva, a nymph's overall form already resembles that of the adult. In addition, while a nymph moults it never enters a...
(younger stages) go through little or even no metamorphosis, hence they resemble the adult specimens. Their skin is thin, making them appear translucent.
There are no current species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
at conservation risk.
Characteristics
The primary characteristic of the apterygotes is that they are primitively wingless. While some other insects, such as fleaFlea
Flea is the common name for insects of the order Siphonaptera which are wingless insects with mouthparts adapted for piercing skin and sucking blood...
s, also lack wings, these are descended from winged insects and have lost them during the course of evolution. By contrast, the apterygotes are a primitive group of insects that diverged from other orders before wings evolved.
Apterygotes, however, show directed aerial gliding descent. It has been suggested that provided a preflight phenomenon from which insects were later to evolve their winged flight capacities.
Apterygotes also have a number of other primitive features that are not shared with other insects. Males deposit sperm packages, or spermatophore
Spermatophore
A spermatophore or sperm ampulla is a capsule or mass created by males of various animal species, containing spermatozoa and transferred in entirety to the female's ovipore during copulation...
s, rather than fertilising the female internally. The young hatch closely resembling the adults and undergo no significant metamorphosis
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
, lacking even an identifiable nymphal
Nymph (biology)
In biology, a nymph is the immature form of some invertebrates, particularly insects, which undergoes gradual metamorphosis before reaching its adult stage. Unlike a typical larva, a nymph's overall form already resembles that of the adult. In addition, while a nymph moults it never enters a...
stage. They continue to moult throughout life, with multiple instar
Instar
An instar is a developmental stage of arthropods, such as insects, between each molt , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, or...
s after reaching sexual maturity, whereas all other insects have only a single sexually mature adult instar.
Apterygotes possess small appendages, referred to as "styli" on some of their abdominal
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
segments, although these are not true legs and cannot be used for walking. They also have long, paired abdominal 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...
and a single median, tail-like caudal filament.
History of the concept
The composition and classification of Apterygota changed over time. By the mid 20th century the subclass included four orders (Collembola, Protura, Diplura, and ThysanuraThysanura
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....
). With the advent of a more rigorous cladistic methodology, the subclass was proven paraphyletic. While the first three groups formed a monophyletic group, the Entognatha
Entognatha
The Entognatha is a class of ametabolous arthropods, which, together with insects, makes up the subphylum Hexapoda. Their mouthparts are entognathous, meaning they are retracted within the head. Entognatha are apterous, meaning they lack wings. The class contains three orders: Collembola , Diplura...
, distinguished by having mouthparts submerged in a pocket formed by the lateral and ventral parts of the head capsule, the Thysanura appeared to be more closely related to winged insects
Pterygota
Pterygota is a subclass of insects that includes the winged insects. It also includes insect orders that are secondarily wingless ....
. The most notable synapomorphy
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
proving the monophyly
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
of Thysanura+Pterygota is the absence of intrinsic 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....
muscles, which connect the antennomeres in entognaths
Entognatha
The Entognatha is a class of ametabolous arthropods, which, together with insects, makes up the subphylum Hexapoda. Their mouthparts are entognathous, meaning they are retracted within the head. Entognatha are apterous, meaning they lack wings. The class contains three orders: Collembola , Diplura...
, myriapods
Myriapoda
Myriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains 13,000 species, all of which are terrestrial...
, and crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s. For this reason, the whole group is often termed the Amyocerata, meaning "lacking antennal muscles".
Moreover, it is now assumed that the Thysanura are more closely related to the Pterygota than to the Archaeognatha, thus rendering even the amyocerate apterygotes paraphyletic.
Sources
- Firefly Encyclopedia of Insects and Spiders, edited by Christopher O'Toole, ISBN 1-55297-612-2, 2002