Abstract
Many arthropods possess filiform hair sensilla (termed trichobothria in arachnids), which are extremely sensitive detectors of medium particle displacement. Electrophysiological evidence in some taxa suggests that these sensilla can detect air particle displacements resulting from intraspecific communication signals. However, it has not yet been shown for any species that the air particle displacements detected by the filiform hairs are themselves perceived as a ‘signal’ (i.e. that individuals make behavioural decisions based upon the responses of these organs to the displays of conspecifics). We investigate the agonistic behaviour of the whip spider Phrynus marginemaculatus and the role of its trichobothria in receiving agonistic signals. Whip spiders have extremely elongated ‘antenniform’ first legs, which they vibrate close to their opponents during agonistic interactions, inducing air movements that excite their opponents' trichobothria. We find that ablation of the trichobothria causes significant increases in: (I) contest duration, and (II) the probability of contest escalation past aggressive displays to physical fighting. Therefore, in the absence of air movement-sensitive sensilla, contest assessment is impaired. This suggests that whip spiders exploit true air movement signals during agonistic interactions, and that these are received by the trichobothria. Furthermore, these results indicate that, in whip spiders, such signals help mitigate the cost of agonistic interaction.
Highlights
Many insects, arachnids and crustaceans possess arrays of filiform hair sensilla which are sensory structures that detect air particle displacements [1,2]
The African cave cricket Phaeophilacris spectrum lacks stridulatory organs, but males make silent wing flicks during courtship and aggression that induce females to be less sensitive to disturbing tactile or vibratory stimuli [10], and induce air currents that excite the filiform hairs of females (e.g. [8])
We predict that if air particle displacements from antenniform leg vibration (ALV) or other antenniform leg movements truly operate as signals: (I) animals in the trichobothria ablated (TA) groups should take longer to gather information and assess their opponents, leading to an increase in the time necessary to make a behavioural decision to retreat or escalate that would be evident as an increase in overall contest duration; and (II) contests between animals in the TA groups would be more likely to escalate past the un-assessable air movement signals to a phase of physical fighting using the pedipalps, because the air movement signals could not motivate a decision to retreat
Summary
Arachnids and crustaceans possess arrays of filiform hair sensilla (termed trichobothria in arachnids) which are sensory structures that detect air (or water) particle displacements [1,2]. We predict that if air particle displacements from ALV or other antenniform leg movements truly operate as signals: (I) animals in the TA groups should take longer to gather information and assess their opponents, leading to an increase in the time necessary to make a behavioural decision to retreat or escalate that would be evident as an increase in overall contest duration; and (II) contests between animals in the TA groups would be more likely to escalate past the un-assessable air movement signals to a phase of physical fighting using the pedipalps, because the air movement signals could not motivate a decision to retreat Since such contest parameters are proxies for the cost of agonistic competition, such findings would provide direct evidence that agonistic signals mitigate the costs of agonistic interaction in whip spiders Since such contest parameters are proxies for the cost of agonistic competition, such findings would provide direct evidence that agonistic signals mitigate the costs of agonistic interaction in whip spiders (e.g. [16])
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