Dispersal-fecundity trade-offs in wild insect populations

Wing reduction is a common feature of upland insect communities. This phenomenon is thought to be primarily driven by selection against flight, which is typically unfavorable in upland environments due to high winds and cold temperatures. In some insect taxa, wing-reduction has been directly linked to increased fecundity. However, few studies have directly tested for shifts in fecundity linked to flight musculature. Here we test for dispersal-fecundity trade-offs in the widespread subalpine stonefly Zelandoperla fenestrata. Our analysis of 450 stoneflies across 81 localities reveals significant dispersal-fecundity tradeoffs. Specifically, we identify a positive association between the size of their flight muscles and the length of their wings, and a negative association between wing length and ovarian mass. Furthermore, we found a significant negative relationship between flight musculature and ovary mass. These results represent a rare example of a dispersal-fecundity tradeoff in the wild, and illustrate that such tradeoffs can potentially involve corresponding reductions in both flight musculature and wing development. Our findings suggest that widespread taxa subject to variable environmental conditions may benefit from flexible allocation of energetic resources.

Methods
We sampled Zelandoperla fenestrata from 2017-2021. Where possible, sampling was conducted from sites where both full-winged and reduced-winged Z. fenestrata ecotypes occur in sympatry. Nymphs were collected by hand from underneath stones or wood in riffle zones, and raised in the laboratory at 11°C in water from their natal stream, until emergence. A total of 450 Z. fenestrata specimens collected across 81 localities (223 females, 227 males) were reared to adulthood in the laboratory, and preserved within 24 hours of emergence.

The forewing and hind tibia lengths of all adult individuals were measured prior to dissection, using stereo microscope images calibrated with a micrometer scale in ImageJ . We calculated the relative wing length of all individuals by dividing wing length by hind tibia length. Meso- and meta-thoracic segments (the locations of flight musculature) of both males and female stoneflies were detached from legs, wings, and the remainder of the body. For the majority of female specimens (n = 220), individual ovaries (including maturing eggs) were dissected and stored separately. All isolated segments were then desiccated by heating, and weighed on an analytical balance.

翅退化是高地昆虫群落的常见特征。该现象被认为主要由针对飞行的选择压力所驱动——在高地环境中，由于强风与低温，飞行通常并非有利性状。在部分昆虫类群中，翅退化已被直接证实与繁殖力提升相关。然而，鲜有研究直接检验与飞行肌相关的繁殖力变化。本研究针对广布的亚高山石蝇（Zelandoperla fenestrata）开展扩散-繁殖权衡的检验。我们对81个采样点的450头石蝇进行分析，发现了显著的扩散-繁殖权衡关系。具体而言，我们观察到飞行肌体积与翅长呈正相关，而翅长与卵巢质量呈负相关。此外，飞行肌体积与卵巢质量间也存在显著负相关关系。这些结果为野外环境中存在扩散-繁殖权衡提供了罕见例证，并表明这类权衡可能同时涉及飞行肌与翅发育的相应缩减。本研究结果提示，面临可变环境条件的广布类群，或可通过灵活分配能量资源获益。

研究方法
我们于2017-2021年采集泽兰石蝇（Zelandoperla fenestrata）。在可行情况下，采样位点同时存在长翅型与短翅型Z. fenestrata生态型的同域分布种群。若虫通过手采方式从急流区的石块或木材下方收集，并在实验室中用其原生溪流的水于11℃条件下饲养至羽化。最终共饲养81个采样点的450头Z. fenestrata标本（雌虫223头，雄虫227头）至成虫阶段，并于羽化后24小时内进行保存。

所有成虫个体的前翅与后足胫节长度均在解剖前通过体视显微镜拍摄图像，使用ImageJ软件结合测微标尺校准后进行测量。我们通过翅长除以后足胫节长度，计算得到所有个体的相对翅长。将雌雄石蝇的中胸与后胸节（飞行肌所在部位）从足、翅及躯体其余部分分离。对于大部分雌虫样本（n=220），我们将其卵巢（包括成熟卵）单独解剖并保存。随后将所有分离得到的组织块加热干燥，并用分析天平称重。