School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501, USA.
Proc Biol Sci. 2010 Jul 7;277(1690):1937-46. doi: 10.1098/rspb.2010.0001. Epub 2010 Mar 10.
Insects are small relative to vertebrates, possibly owing to limitations or costs associated with their blind-ended tracheal respiratory system. The giant insects of the late Palaeozoic occurred when atmospheric PO(2) (aPO(2)) was hyperoxic, supporting a role for oxygen in the evolution of insect body size. The paucity of the insect fossil record and the complex interactions between atmospheric oxygen level, organisms and their communities makes it impossible to definitively accept or reject the historical oxygen-size link, and multiple alternative hypotheses exist. However, a variety of recent empirical findings support a link between oxygen and insect size, including: (i) most insects develop smaller body sizes in hypoxia, and some develop and evolve larger sizes in hyperoxia; (ii) insects developmentally and evolutionarily reduce their proportional investment in the tracheal system when living in higher aPO(2), suggesting that there are significant costs associated with tracheal system structure and function; and (iii) larger insects invest more of their body in the tracheal system, potentially leading to greater effects of aPO(2) on larger insects. Together, these provide a wealth of plausible mechanisms by which tracheal oxygen delivery may be centrally involved in setting the relatively small size of insects and for hyperoxia-enabled Palaeozoic gigantism.
昆虫与脊椎动物相比体型较小，这可能是由于它们盲端的气管呼吸系统的限制或成本所致。古生代晚期的巨型昆虫出现在大气 PO(2)（aPO(2)) 过氧时，支持了氧气在昆虫体型进化中的作用。昆虫化石记录的稀缺性以及大气氧水平、生物及其群落之间的复杂相互作用，使得无法明确接受或拒绝历史上氧气与体型的联系，并且存在多种替代假说。然而，最近的多种实证发现支持了氧气与昆虫体型之间的联系，包括：(i) 大多数昆虫在缺氧条件下发育出较小的体型，而有些昆虫在过氧条件下发育和进化出更大的体型；(ii) 昆虫在发育和进化过程中减少了在气管系统上的比例投资，当生活在更高的 aPO(2) 时，这表明气管系统结构和功能存在显著的成本；(iii) 体型较大的昆虫在气管系统上投入更多的身体，这可能导致 aPO(2) 对大型昆虫的影响更大。综上所述，这些为气管氧输送可能在昆虫体型相对较小和古生代巨型化中起核心作用提供了丰富的合理机制。
