Hanging suspended on a silk thread seems to be a common strategy for both sexes and all developmental stages (including newly emerged spiderlings) in Evarcha arcuata, yet these spiders are also capable of building typical salticid silk retreats, which we observed both in the field and in the laboratory (Fig. 1a, supplementary methods S1: figure S3). Thus, these spiders are clearly engaging in different strategies, even at the individual level. This behavioural plasticity in resting strategy is interesting, inviting questions such as why and how resting options are chosen and what factors inform this behavioural choice.
Perhaps the most noteworthy feature of this behaviour is that it appears to provide little, if any, protection from unfavourable abiotic environmental conditions—even less than if a spider were to simply remain stationary on a surface where it could receive thermal benefits or reduced exposure to air-movements due to boundary-layer effects. Thus, biotic factors seem likely to play a more important role than abiotic ones, particularly that hanging from an overhead line might offer some form of protection from predators. Building typical silk retreats and using them for resting offers thermal benefits as well as a physical boundary from potential predators.
Our first consideration is the necessary swap in sensory modality between diurnal and nocturnal sensing. Being highly visual animals, but with no active mechanism to keep light from entering their eyes, jumping spiders showed a strong reaction towards light by terminating the hanging behavior quickly after light disturbance. Equally, in the laboratory, initiation of the behavior was observed shortly after overhead lights were turned off. These observations suggest that, not surprisingly, at greater light levels, the preferred engagement with the environment is visual. At night, visual sensory information available to them is reduced or even unavailable. Consequently, the predominant sensory modality at night is likely vibration, the other main sensory modality used by spiders [35]. This is in line with findings that even courtship displays, usually thought of as predominantly visual, can switch with males performing tactile, vibratory courtship on the silk of female retreats that are located in low light conditions (e.g., under rocks) [20]. Vibrations can thus either be sensed directly via substrate-borne cues or silk-borne cues directly at the retreat, or, in the case of suspended resting, via a single silk thread held by the tarsal claws of the spider.
Spiders reacted differently towards shaking of the vegetation (habitat disturbance) versus silk disturbance. This may be intuitive—if there is a non-direct disturbance nearby, climb up and wait, but if there is a direct disturbance of the silk line, the only path to safety is to drop rather than to climb up. However, consideration of the sensory information available to the spider in these circumstances suggests that this differential response is non-trivial. Specifically, silk-transferred vibrational cues seem to be the most likely sensory modality involved in detecting both types of events, although we cannot completely rule out that spiders perceived additional visual cues posed by the observer’s hand via their lateral and posterior eyes. We also observed spiders hanging in the field even in windy conditions (> 1.5 m/s), clearly undisturbed by the wind-induced oscillations, suggesting that they can disregard wind-borne vibrations. Thus, these differential responses suggest that these spiders can discriminate between similar stimuli within this modality. The question of how spiders discriminate between silk-borne vibrational cues is particularly interesting because we also have evidence that the response to disturbance is sex-specific, as: 50 % of male spiders dropped in response to the habitat disturbance, while all females climbed up. Substrate-borne and silk-borne vibration-based signal differentiation is described among salticids, particularly in the context of sexual selection [36, 37] and is comparable to evidence of silk-based vibration discrimination in spider lineages that use silk in prey capture (e.g., Mortimer et al. [38]). We should note that the disturbance tests performed in our study were intended as preliminary.
Behaviourally, dropping to the ground in response to vegetation vibration seems rather costly given the disadvantage of reduced sight and the potentially increased probability of encountering predators. Dropping, however, is, a common anti-predator strategy in various non-salticid spiders [39, 40] as well as in other groups such as aphids, lepidopterans or ladybirds [41], underlining its potential anti-predator function. In order to establish an anti-predator function of suspended resting, a combination of the following would need to be demonstrated: increased survival by hanging overhead compared to fixed retreats and resting exposed on vegetation; increased use of overhead silk lines when the risk of predation is higher; a temporal overlap of overhead hanging with the peak activity of nocturnal predators.
Hanging suspended from the vegetation has been studied in parasitoid wasps, where cocoon suspension was shown to be an efficient strategy against foraging ants, however not against hyperparasitoids [42]. Hyperparasitoids track the silk on the vegetation and follow down the thread to the cocoon [42]. Based on our results, this type of behaviour would probably induce spiders to drop. However, we lack information on nocturnal parasitoid wasps at this field site. We did, however, repeatedly observe ants foraging at night at the field site, including attacks on crab spiders, thus, suspension against foraging ants is plausible. We also observed other potential nocturnal predators during our field observations, including various larger spider species, such as nursery web spiders (Pisaura mirabilis), crab spiders (Xysticus sp.) and yellow sac spiders (Cheiracanthium punctorium). Another study showed that caterpillars initiate suspending themselves on a silk thread in response to invertebrate predators while adjusting silk length to predator type [43]. Caterpillars can discriminate different predators based on substrate-borne vibrational cues, thus, a similar discrimination ability in jumping spiders is not far-fetched. A suspended position is likely to be an efficient anti-predator strategy, potentially acting as an early warning system or simply by bringing the spider out of reach for some predators. In his observation, Carroll (1977) further notes suspended resting during the night for two non-salticid spiders, the yellow crab spider (Misumenops lepidus) and the brown lynx spider (Oxyopes scalaris) both of which are also day-active [25]. This indeed suggests a more common strategy specially adapted for the night-time and potentially against nocturnal predators.
An alternative explanation is that suspended resting could increase dispersal distance, while reducing the temporal, energetic and cognitive costs of building, remembering, and returning to typical silk retreats. Considering that these spiders move several meters across highly structured and complex three-dimensional habitat, the ability to navigate back to an exact location (i.e., retreat), requires elaborate three-dimensional navigation, which is assumed to be cognitively demanding and costly. Consequently, spiders could maximise time investment in foraging and finding mates by dropping on a silk line at the end of the day. For that very reason, we initially hypothesised that, in search of mates, males move farther than females. Our preliminary data did not corroborate this hypothesis, but instead suggests that neither sex did show strict site fidelity (for a full description see supplementary methods S1). More data is needed, however, to test this preliminary finding.
The apparent plasticity in the nocturnal resting strategy of Evarcha arcuata, invites future investigations of resting strategies as well as resting site selection in adaptive contexts among invertebrates more broadly. We believe that this is a promising system to explore functional questions of how sensory information shapes behavioural choices. The functional link between predator-avoidance during phases in which main sensory paths are compromised is an interesting question particularly in diurnal invertebrates. Future studies in such systems are likely to gather novel insights into sensory information processing, resting site selection, and the fundamental role of resting.
Resting in the animal kingdom is associated with reduced activity (lower metabolic rates) [1], protection from unfavourable abiotic conditions as well as protection from predators [2]. Yet, most studies investigating resting so far have focused on vertebrate species. We have reason to believe that resting in invertebrate species follows similar concepts and rules, yet very little is known about the night-time activities of even common species such as E. arcuata. Gathering rigorous data on the exact sites that invertebrates choose for resting and what selective benefits these sites offer from an adaptive point of view will contribute valuable information for a more comprehensive understanding of resting in animals.