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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). Although the three orders are sometimes grouped together in a class called Entognatha because they have internal mouthparts, they do not appear to be any more closely related to one another than they all are to insects, which have external mouthparts. Some DNA sequence studies suggest that Collembola represent a separate evolutionary line from the other Hexapoda, but others disagree; this seems to be caused by widely divergent patterns of molecular evolution among the arthropods. The adjustments of traditional taxonomic rank for springtails reflects the occasional 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. Collembolans are omnivorous, free-living organisms that prefer moist conditions. They do not directly engage in the decomposition of organic matter, but contribute to it indirectly through the fragmentation of organic matter and the control of soil microbial communities. The word "collembola" is from the Greek ''colle'' meaning glue and ''embolon'' meaning piston or peg. == Description == Members of Collembola are normally less than 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. Most species have an abdominal, tail-like appendage, the furcula, that is folded beneath the body to be used for jumping when the animal is threatened. It is held under tension by a small structure called the retinaculum and when released, snaps against the substrate, flinging the springtail into the air. All of this takes place in as little as 18 milliseconds. Springtails also possess the ability to reduce their body size by as much as 30% through subsequent ecdyses (molting) if temperatures rise high enough. The shrinkage is genetically controlled. Since warmer conditions increase metabolic rates and energy requirements in organisms, the reduction in body size is advantageous to their survival. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「springtail」の詳細全文を読む スポンサード リンク
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