Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, Michigan, USA.
J Morphol. 2021 Jan;282(1):66-79. doi: 10.1002/jmor.21282. Epub 2020 Oct 19.
From the elongated neck of the giraffe to the elaborate train of the peacock, extreme traits can result from natural or sexual selection (or both). The extreme chelicerae of the long-jawed spiders (Tetragnatha) present a puzzle: do these exaggerated chelicerae function as weapons or genitalia? Bristowe first proposed that Tetragnatha chelicerae function as a holdfast because these spiders embrace chelicerae during mating. This hypothesis has remained untested until now. Here, we use functional allometry to examine how extreme chelicerae develop and perform in the long-jawed spider Tetragnatha elongata. Similar to other Tetragnatha species, chelicerae were longer in adult males than in adult females. Overall, we confirm Bristowe's hypothesis: elongation only occurred in the adult stage. However, we propose that chelicerae function as more than a holdfast in T. elongata. Male chelicerae exhibited positive allometry, which suggests scaling as weapons rather than genitalia. However, fieldwork revealed that the operational sex ratio is female-biased and both adult male-male competition and sexual cannibalism were rarely observed. Consequently, we propose that the positive allometry of male chelicerae may result from sexual selection to mechanically mesh with larger and more fecund females. Evidence for mechanical mesh includes multiple traits ranging from apophyses and grooves to guide teeth on the basal portion of the chelicerae. In contrast, we propose that chelicerae of females are analogous to the female peacock's tail: shortened by natural selection limiting the exaggeration of sexually selected traits. Indeed, females had increased foraging efficiency compared to males and exhibited negative cheliceral allometry. We discuss the implications for the evolution of elongated chelicerae in Tetragnatha.
从长颈鹿的长脖子到孔雀华丽的尾巴，极端特征可能是由自然选择或性选择（或两者兼而有之）产生的。长颚蛛（Tetragnatha）的极端螯肢是一个谜：这些夸张的螯肢是作为武器还是生殖器发挥作用？布里斯特韦首先提出，Tetragnatha 的螯肢作为固定器，因为这些蜘蛛在交配时会抱住螯肢。这个假设至今仍未得到验证。在这里，我们使用功能形态学方法来研究长颚蛛 Tetragnatha elongata 的极端螯肢是如何发育和发挥作用的。与其他 Tetragnatha 物种类似，成年雄性的螯肢比成年雌性的螯肢更长。总的来说，我们证实了布里斯特韦的假设：螯肢的伸长只发生在成年阶段。然而，我们提出，螯肢在 T. elongata 中不仅作为固定器发挥作用。雄性螯肢表现出正异速生长，这表明它们作为武器而不是生殖器进行缩放。然而，野外工作表明，实际的性别比例偏向雌性，而且成年雄性之间的竞争和性食同类行为很少观察到。因此，我们提出，雄性螯肢的正异速生长可能是由于性选择，以机械方式与更大、更具生殖力的雌性匹配。机械匹配的证据包括从基部分叉和凹槽到引导牙齿等多个特征。相比之下，我们提出，雌性的螯肢类似于雌性孔雀的尾巴：由于自然选择限制了性选择特征的夸张程度，所以雌性的螯肢被缩短了。事实上，与雄性相比，雌性的觅食效率更高，并且表现出负异速生长的螯肢。我们讨论了 Tetragnatha 中螯肢伸长的进化意义。
