Data from: Arachnid aloft: directed aerial descent in neotropical canopy spiders

The behaviour of directed aerial descent has been described for numerous taxa of wingless hexapods as they fall from the tropical rainforest canopy, but is not known in other terrestrial arthropods. Here, we describe similar controlled aerial behaviours for large arboreal spiders in the genus Selenops (Selenopidae). We dropped 59 such spiders from either canopy platforms or tree crowns in Panama and Peru; the majority (93%) directed their aerial trajectories towards and then landed upon nearby tree trunks. Following initial dorsoventral righting when necessary, falling spiders oriented themselves and then translated head-first towards targets; directional changes were correlated with bilaterally asymmetric motions of the anterolaterally extended forelegs. Aerial performance (i.e. the glide index) decreased with increasing body mass and wing loading, but not with projected surface area of the spider. Along with the occurrence of directed aerial descent in ants, jumping bristletails, and other wingless hexapods, this discovery of targeted gliding in selenopid spiders further indicates strong selective pressures against uncontrolled falls into the understory for arboreal taxa.

已有研究对多种无翅六足类（wingless hexapods）从热带雨林冠层坠落时的定向空中下降行为进行了描述，但目前尚未在其他陆生节肢动物中发现此类行为。本研究针对遁蛛科（Selenopidae）赛奈蛛属（Selenops）的大型树栖蜘蛛，描述了与之类似的可控空中行为。我们在巴拿马与秘鲁的冠层平台及树冠上，对59只此类蜘蛛开展了坠落实验，其中93%的绝大多数个体能够调整空中轨迹，朝向附近树干并最终降落于其上。必要时先完成背腹翻转，坠落的蜘蛛便会调整自身姿态，随后以头部朝前的方式向目标移动；轨迹的方向变化与前足前外侧伸展时的双侧不对称运动密切相关。空中运动性能（即滑翔指数，glide index）随个体体重与翼载荷（wing loading）的增加而下降，但与蜘蛛的投影表面积无关。结合蚂蚁、跳鬃尾虫（jumping bristletails）及其他无翅六足类动物均存在定向空中下降行为的研究事实，此次在遁蛛科蜘蛛中发现的定向滑翔行为进一步证实：树栖类群面临着极强的选择压力，以避免无控坠落至林下层环境中。