由于缺少氧氣,，翼展達(dá)75厘米的蜻蜓等巨型昆蟲(chóng)在數(shù)百萬(wàn)年前滅亡，正是充足的氧氣讓它們長(zhǎng)得這么大
研究人員主要著眼于石蠅,，這種昆蟲(chóng)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(shí)間11月30日消息，一項(xiàng)最新研究稱(chēng),，數(shù)百萬(wàn)年前巨型飛行昆蟲(chóng)的出現(xiàn)和衰落,，可能與在水中呼吸的它們的幼蟲(chóng)能夠獲得的氧氣量有關(guān)。
科學(xué)家研究了巨型昆蟲(chóng)（其中包括翼展將近1米的蜻蜓）體型更小的現(xiàn)代后裔,，認(rèn)為他們通過(guò)觀(guān)察生活在水里的這些昆蟲(chóng)的幼蟲(chóng),，已經(jīng)揭開(kāi)困擾專(zhuān)家100多年的一個(gè)謎團(tuán)。他們相信,，3億年前的巨型昆蟲(chóng)幼蟲(chóng)就是憑借比現(xiàn)在更高水平的氧氣,，長(zhǎng)到像古生物學(xué)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(chóng)的幼蟲(chóng)因無(wú)法獲得充足氧氣，而大量死亡,，這導(dǎo)致這些生物最終走向滅亡,，只剩下體型相對(duì)較小的近親。
英國(guó)普利茅斯大學(xué)海洋學(xué)與工程學(xué)院的大衛(wèi)-比爾頓博士參與了這項(xiàng)研究,，他說(shuō)：“史前時(shí)期,，更高水平的氧氣通過(guò)對(duì)它們的幼蟲(chóng)產(chǎn)生影響,，可能助長(zhǎng)了巨型昆蟲(chóng)的進(jìn)化,，很多已經(jīng)滅絕的龐然大物都要經(jīng)歷水棲幼蟲(chóng)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(chóng)幼蟲(chóng)對(duì)氧氣水平的起伏波動(dòng),，比在空中呼吸的陸棲成蟲(chóng)更敏感,。
盡管以前科學(xué)家也曾提出氧氣水平同巨型昆蟲(chóng)之間有聯(lián)系，但是并沒(méi)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(chēng),，蜻蜓、石蠅和蜉蝣等水棲幼蟲(chóng)直接從水里獲得氧氣,，而水體里的氧氣遠(yuǎn)比空氣里的少,。而且幼蟲(chóng)從水里獲取氧氣的效率也遠(yuǎn)比在空中呼吸的成蟲(chóng)更低?？茖W(xué)家稱(chēng),，因此它們可能對(duì)可用氧氣的變化更敏感，氧氣塑造昆蟲(chóng)體型大小的作用,，或許對(duì)水棲幼蟲(chóng)尤為重要,，它決定了昆蟲(chóng)身體生長(zhǎng)的上限。
巨型昆蟲(chóng)是涉及到遙遠(yuǎn)過(guò)去的科幻故事里的一大特色,，以巨型蜻蜓為特寫(xiě)的邁克爾-克瑞奇頓的小說(shuō)《侏羅紀(jì)公園》,，產(chǎn)生了票房收入高達(dá)數(shù)百萬(wàn)美元的電影巨制?？茖W(xué)家認(rèn)為,，記錄顯示翼展長(zhǎng)達(dá)75厘米的這種巨型昆蟲(chóng)，生活在大約3.54億到2.9億年前的石炭紀(jì)時(shí)期,。威爾伯克說(shuō)：“迄今為止,，了解生活在過(guò)去的巨型昆蟲(chóng)的嘗試，主要是通過(guò)觀(guān)察（化石）陸棲成蟲(chóng)來(lái)實(shí)現(xiàn),。而我們的工作表明,，通過(guò)幼蟲(chóng)解決史前巨人癥的問(wèn)題，或許有助于我們更好地了解氧氣是如何限制昆蟲(chóng)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.
