Apterygota

Apterygota Fossil range: Devonian–Recent PreЄ Є O S D C P T J K Pg N [1]
a machilid
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Subclass:	Apterygota†
Orders
Archaeognatha Thysanura Monura (extinct)

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. 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.

Characteristics

The primary characteristic of the apterygotes is that they are primitively wingless. While some other insects, such as fleas, also lack wings, these are descended from winged insects, and have lost them during the course of evolution. By contrast, the aptyergotes represent 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 phenomena from which insects were later to evolve their winged flight capacities.^[2]

Apterygotes also have a number of other primitive features that are not shared with other insects. Males deposit sperm packages, or spermatophores, rather than fertilising the female internally. The young are born closely resembling the adults, and undergo no significant metamorphosis, lacking even an identifiable nymphal stage. They continue to moult throughout life, with multiple instars 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.^[3]

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 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, 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 crustaceans. For this reason, the whole group is often termed the Amyocerata, meaning "lacking antennal muscles".

Moreover, some entomologists^[who?] claim 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