Springtail

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Entomobryomorpha
Poduromorpha
Symphypleona
Neelipleona (disputed)

Arthropleona

Springtails (Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects (the other two are the Protura and Diplura). Though the three orders are sometimes grouped together in a class called Entognatha because they have internal mouthparts, they do not appear to be more closely related to one another than they all are to insects, which have external mouthparts.

Some DNA sequence studies[2] suggest that Collembola represent a separate evolutionary line from the other Hexapoda, but others disagree;[3] this seems to be caused by widely divergent patterns of molecular evolution among the arthropods.[4] The adjustments of traditional taxonomic rank for springtails reflects the incompatibility of traditional groupings with modern cladistics: when they were included with the insects, they were ranked as an order; as part of the Entognatha, they are ranked as a subclass. If they are considered a basal lineage of Hexapoda, they are elevated to full class status.

Contents

Description

Members of Collembola are normally less than 6 mm long, have six or fewer abdominal segments and possess a tubular appendage (the collophore or ventral tube) with eversible vesicles, projecting ventrally from the first abdominal segment. The Poduromorpha and Entomobryomorpha have an elongated body, while the Symphypleona have a globular body. Collembola lack a tracheal respiration system, which forces them to respire through a porous cuticle, to the notable exception of Sminthuridae which exhibit a rudimentary, although fully functional, tracheal system.[5]

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