由于缺少氧氣，翼展達75厘米的蜻蜓等巨型昆蟲在數(shù)百萬年前滅亡,，正是充足的氧氣讓它們長得這么大
研究人員主要著眼于石蠅，這種昆蟲直接從水里獲取氧氣,，而水體里的氧氣比空氣里少得多
北京時間11月30日消息,，一項最新研究稱,，數(shù)百萬年前巨型飛行昆蟲的出現(xiàn)和衰落，可能與在水中呼吸的它們的幼蟲能夠獲得的氧氣量有關(guān),。
科學家研究了巨型昆蟲（其中包括翼展將近1米的蜻蜓）體型更小的現(xiàn)代后裔,，認為他們通過觀察生活在水里的這些昆蟲的幼蟲，已經(jīng)揭開困擾專家100多年的一個謎團,。他們相信,，3億年前的巨型昆蟲幼蟲就是憑借比現(xiàn)在更高水平的氧氣，長到像古生物學家發(fā)現(xiàn)的化石遺體那么大,。稍后氣候發(fā)生變化,，氧氣水平下降，巨型昆蟲的幼蟲因無法獲得充足氧氣,，而大量死亡,，這導致這些生物最終走向滅亡，只剩下體型相對較小的近親,。
英國普利茅斯大學海洋學與工程學院的大衛(wèi)-比爾頓博士參與了這項研究,，他說：“史前時期，更高水平的氧氣通過對它們的幼蟲產(chǎn)生影響,，可能助長了巨型昆蟲的進化,，很多已經(jīng)滅絕的龐然大物都要經(jīng)歷水棲幼蟲階段，這可能并非偶然,。”該研究成果發(fā)表在《PLoS綜合》上,，比爾頓及其聯(lián)合論文作者威爾克-威爾伯克在文章中指出，水棲昆蟲幼蟲對氧氣水平的起伏波動,，比在空中呼吸的陸棲成蟲更敏感,。
盡管以前科學家也曾提出氧氣水平同巨型昆蟲之間有聯(lián)系，但是并沒有人提供可以證明它們之間是如何聯(lián)系在一起的確鑿證據(jù),。該研究主要著眼于石蠅,，它稱，蜻蜓,、石蠅和蜉蝣等水棲幼蟲直接從水里獲得氧氣,，而水體里的氧氣遠比空氣里的少。而且幼蟲從水里獲取氧氣的效率也遠比在空中呼吸的成蟲更低,?？茖W家稱,，因此它們可能對可用氧氣的變化更敏感,，氧氣塑造昆蟲體型大小的作用，或許對水棲幼蟲尤為重要,，它決定了昆蟲身體生長的上限,。
巨型昆蟲是涉及到遙遠過去的科幻故事里的一大特色,，以巨型蜻蜓為特寫的邁克爾-克瑞奇頓的小說《侏羅紀公園》，產(chǎn)生了票房收入高達數(shù)百萬美元的電影巨制,?？茖W家認為，記錄顯示翼展長達75厘米的這種巨型昆蟲,，生活在大約3.54億到2.9億年前的石炭紀時期,。威爾伯克說：“迄今為止，了解生活在過去的巨型昆蟲的嘗試,，主要是通過觀察（化石）陸棲成蟲來實現(xiàn),。而我們的工作表明，通過幼蟲解決史前巨人癥的問題,，或許有助于我們更好地了解氧氣是如何限制昆蟲的身體大小的,。”（生物谷Bioon.com）
doi:10.1371/journal.pone.0022610
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Can Oxygen Set Thermal Limits in an Insect and Drive Gigantism?
Wilco C. E. P. Verberk*, David T. Bilton
Background Thermal limits may arise through a mismatch between oxygen supply and demand in a range of animal taxa. Whilst this oxygen limitation hypothesis is supported by data from a range of marine fish and invertebrates, its generality remains contentious. In particular, it is unclear whether oxygen limitation determines thermal extremes in tracheated arthropods, where oxygen limitation may be unlikely due to the efficiency and plasticity of tracheal systems in supplying oxygen directly to metabolically active tissues. Although terrestrial taxa with open tracheal systems may not be prone to oxygen limitation, species may be affected during other life-history stages, particularly if these rely on diffusion into closed tracheal systems. Furthermore, a central role for oxygen limitation in insects is envisaged within a parallel line of research focussing on insect gigantism in the late Palaeozoic. Methodology/Principal Findings Here we examine thermal maxima in the aquatic life stages of an insect at normoxia, hypoxia (14 kPa) and hyperoxia (36 kPa). We demonstrate that upper thermal limits do indeed respond to external oxygen supply in the aquatic life stages of the stonefly Dinocras cephalotes, suggesting that the critical thermal limits of such aquatic larvae are set by oxygen limitation. This could result from impeded oxygen delivery, or limited oxygen regulatory capacity, both of which have implications for our understanding of the limits to insect body size and how these are influenced by atmospheric oxygen levels. Conclusions/Significance These findings extend the generality of the hypothesis of oxygen limitation of thermal tolerance, suggest that oxygen constraints on body size may be stronger in aquatic environments, and that oxygen toxicity may have actively selected for gigantism in the aquatic stages of Carboniferous arthropods.
