Arachnid aloft: directed aerial descent in neotropical canopy spiders

Abstract

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.

Touchdown to take-off: at the interface of flight and surface locomotion

“…As a result, these animals typically approach a landing target at relatively higher speeds than powered flyers do. Gliders that use their limbs and body to direct their descent include many arthropods, such as some spiders and wingless hexapods [90][91][92][93][94]. Gliding vertebrates, including flying squirrels, colugos, snakes, lizards and frogs, use their extended aerodynamic surfaces to navigate in the air [71,[74][75][76][77][78][79].…”

Stag Beetle Elytra: Localized Shape Retention and Puncture/Wear Resistance

“…Earlier studies recorded the falling behavior of insects such as ants and spiders. Their results show that the insects use a gliding mechanism to change the free-falling trajectory and achieve controlled landing back on the trees without hitting the ground [48][49][50]. The velocities of these beetles are more than twice the values reported in falling ants and locusts [48][49][50][51].…”

Dropping to escape: a review of an under‐appreciated antipredator defence

“…Classical gliding in terrestrial vertebrates was likely an important precursor to the evolution of powered flight (Dudley et al, ; Dudley & Yanoviak, ). Directed aerial descent has only fairly recently been described in a variety of wingless arboreal insects too (Yanoviak, Dudley & Kaspari, ; Yanoviak, Fisher & Alonso, ; Yanoviak, Munk & Dudley, , ; Zeng et al, ). It is important to note here that, while dropping from the air and directed aerial descent fall under our definition of dropping, those species such as flying squirrels that have extensive morphological adaptation for gliding used for routine movement through the environment as well as escape from predators are best seen as a separate phenomenon.…”

“…When not being used to escape enemies, voluntarily falling is sometimes deployed by invertebrates as a shortcut to the ground or to access high-quality food patches (Haemig, 1997;Ohzora & Yano, 2011). In fact, more controlled dropping behaviour -known as 'directed falling' or directed aerial descent -has been reported in a number of wingless ant species (Yanoviak et al, 2005(Yanoviak et al, , 2008(Yanoviak et al, , 2010Yanoviak & Dudley, 2006) as well as spiders (Yanoviak et al, 2015) and stick insects (Zeng et al, 2015). Directed aerial descent is considered a form of gliding, but it occurs at steeper angles than 'classical gliding' (Dudley et al, 2007).…”