The evolution of wetness perception: A comparison of arachnid, insect and human models

Abstract

Hygroreceptors are a type of humidity sensor that have been identified in several invertebrate classes including Insecta and Arachnida. While their structure has been well researched, the nature of the mechanisms behind their function is debated as being either mechanical, evaporative, or psychrometric in insects and potentially also olfactory in arachnids. There is evidence that can be used to support or oppose each of these concepts, which also invites the possibility of multiple unified mechanisms occurring together. The integration of multiple sensory modalities has also formed the foundation of wetness perception in humans, led by thermal and tactile cues with supplementary information from vision and sound. These inputs are integrated by a vast neural network in the brain, which also occurs on a smaller scale in insects and arachnids. It is possible that as cerebral capacity increased throughout human evolution, this facilitated a preferable system of wetness perception via multisensory integration and rendered hygroreceptors obsolete. While this cerebral development hypothesis is only speculative, it gives a framework for further investigation. Additional research needs to be conducted to correctly classify hygroreceptor types in invertebrates and their relative prevalence before evolutionary associations can be made with vertebrate species. This integratory premise also applies to the human system, as knowing the relative contribution and compounding effects of each sensory modality on wetness perception will aid the overall understanding of the system and help to uncover the evolutionary development pathways underpinning each sense.

Influence of air temperature and humidity on Stratiolaelaps scimitus (Acari, Mesostigmata) locomotor activity in a laboratory experiment

“…In our experiment, in humidity below 30%, mites successfully left the circle to explore the field. Their organs of senses are definitely able to trace relative humidity (Merrick & Filingeri, 2019). However, increase in humidity above 80% altered the regular behaviour of mites: in the humidity range of 80-84%, 33.3% of mites left the migration circle, and in the range of 85-89% humidity, only 19.3% of mites left the circle.…”

Hearing water temperature: Characterizing the development of nuanced perception of sound sources

“…To what extent are these crossmodal mappings present from early infancy or birth, versus a product of protracted maturation and learning over childhood? For auditory perception of liquid materials, previous work has repeatedly raised the question of development, and the relative contributions of innate factors, maturation, and learning (Merrick & Filingeri, 2019; Spence, 2020). However, developmental data have not previously been brought to bear on this question.…”

“…The ability to hear water temperature may also depend on multisensory integration, the ability to combine inputs from two or more senses (Stein et al., 2014). Human perception of another liquid quality, wetness, similarly occurs through multisensory integration of information from thermal and tactile input together with information from sound and vision (Merrick & Filingeri, 2019). Many aspects of multisensory integration develop in infancy (Bahrick & Lickliter, 2000; Kopp, 2014; Lewkowicz, 1992), and experience with crossmodal input in the first months of life is needed for typical development (Putzar et al., 2007; Wallace et al., 2004).…”

“…The perception of liquids may hold specific adaptive value: For example, the ability to perceive wetness is thought to have evolved repeatedly in multiple species, due to its value when selecting appropriate habitats and ensuring physical health (Merrick & Filingeri, 2019). As the accurate manipulation of liquids at particular temperatures is crucial to fundamental human activities (e.g., cooking, washing), accurate perception of other liquid features may have adaptive value as well.…”

Skin wetness detection thresholds and wetness magnitude estimations of the human index fingerpad and their modulation by moisture temperature

“…Instead, our brains must form a comprehensive picture of the outside world by integrating a range of external stimuli such as tactile and thermal stimuli ( 2 ). The ability to sense wetness has been a critical factor in the evolution of humans and is an intrinsic component of both survival and comfort ( 3 ). This applies on a fundamental level surrounding wider homeostatic processes such as thermoregulation ( 4 ) and maintaining ion concentrations in fluid systems as part of body water balance ( 5 ).…”