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Meganeura
''Meganeura'' is a genus of extinct insects from the Carboniferous period (approximately 300 million years ago), which resembled and are related to the present-day dragonflies. With wingspans of up to , ''M. monyi'' is one of the largest known flying insect species; the Permian ''Meganeuropsis permiana'' is another. ''Meganeura'' were predatory, and fed on other insects.
Fossils were discovered in the French Stephanian Coal Measures of Commentry in 1880. In 1885, French paleontologist Charles Brongniart described and named the fossil "''Meganeura''" (large-nerved), which refers to the network of veins on the insect's wings. Another fine fossil specimen was found in 1979 at Bolsover in Derbyshire. The holotype is housed in the Muséum national d'histoire naturelle, Paris.
==Size==
Controversy has prevailed as to how insects of the Carboniferous period were able to grow so large. The way oxygen is diffused through the insect's body via its tracheal breathing system puts an upper limit on body size, which prehistoric insects seem to have well exceeded. It was originally proposed (Harlé & Harlé, 1911) that ''Meganeura'' was able to fly only because the atmosphere at that time contained more oxygen than the present 20%. This hypothesis was dismissed by fellow scientists, but has found approval more recently through further study into the relationship between gigantism and oxygen availability. If this hypothesis is correct, these insects would have been susceptible to falling oxygen levels and certainly could not survive in our modern atmosphere. Other research indicates that insects really do breathe, with "rapid cycles of tracheal compression and expansion". Recent analysis of the flight energetics of modern insects and birds suggests that both the oxygen levels and air density provide an upper bound on size.
The presence of very large Meganeuridae with wing spans rivaling those of ''Meganeura'' during the Permian, when the oxygen content of the atmosphere was already much lower than in the Carboniferous, presented a problem to the oxygen-related explanations in the case of the giant dragonflies. However, despite the fact that meganeurids had the largest known wing spans, their bodies were not very large, being smaller than those of several living Coleoptera; therefore they were not true giant insects, only being giant in comparison with their living relatives. Other explanations for the large size of meganeurids compared to living relatives are warranted.〔Nel A.N., Fleck G., Garrouste R. and Gand, G. (2008): The Odonatoptera of the Late Permian Lodève Basin (Insecta). ''Journal of Iberian Geology'' 34(1): 115-122 (PDF )〕 Bechly (2004) suggested that the lack of aerial vertebrate predators allowed pterygote insects to evolve to maximum sizes during the Carboniferous and Permian periods, perhaps accelerated by an evolutionary "arms race" for increase in body size between plant-feeding Palaeodictyoptera and Meganisoptera as their predators.〔Bechly G. (2004): Evolution and systematics. pp. 7-16 in: Hutchins M., Evans A.V., Garrison R.W. and Schlager N. (eds): ''Grzimek's Animal Life Encyclopedia. 2nd Edition. Volume 3, Insects''. 472 pp. Gale Group, Farmington Hills, MI (PDF )〕 Another theory suggests that insects that developed in water before becoming terrestrial as adults grew bigger as a way to protect themselves against the high levels of oxygen.〔(Why Giant Bugs Once Roamed the Earth )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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