(A) Adult female Cupiennius salei. (B) The distal end of a leg of Cupiennius with an arrow pointing to the joint between tarsus and metatarsus.
(A) Adult female Cupiennius salei. (B) The distal end of a leg of Cupiennius with an arrow pointing to the joint between tarsus and metatarsus.

A collaboration of US and EU researchers has found that the viscoelectric properties of a spider’s leg helps it to detect vibrations

Biological sensory organs help us to receive, interpret and respond to environmental stimuli. In the world of invertebrates, these sensors are remarkably complex – spiders ‘hear’ – or more accurately, sense vibrations – through strain-sensitive grooves, called lyriform organs, distributed along their legs. One species of nocturnal spider found in Central America - Cupiennius salei – optimizes its ‘hearing’ by sitting on mechanically stiff plants, ensuring that vibrations from nearby prey, predators or sexual partners can be easily sensed.

The lyriform organ is extremely sensitive to substrate vibrations – at high frequencies (> 40 Hz) deflections as small as 10-9 – 10-8 elicit a response in the leg. As well as being highly sensitive, the system can also filter out low-frequency background noise – a challenge facing those designing bio-inspired sensing systems. An international team of researchers believe that they have discovered how this ‘filter’ works, and say that their results will establish a basis for bio-inspired sensor design.

Led by the Georgia Institute of Technology [Acta Biomaterialia (2014) DOI:10.1016/j.actbio.2014.07.023], this work focused on the mechanical properties of a skin-pad close to the sensory organ. The pad is found between the metatarsus (second-last segment) and tarsus of each leg, adjacent to the lyriform organ. Earlier research suggested that this pad contributed to the filtering mechanism, but details were unclear. By using surface force spectroscopy (SFS), the team directly measured the mechanical response of the pad’s viscoelastic surface. By mapping the pad’s surface at a range of temperatures (between 15–40 °C) and frequencies (from 0.05 to 40 Hz), it was possible to define the thermomechanical behavior of the material under typical environmental conditions experienced by the spider.

The group found that the viscoelastic properties of the pad surface were highly temperature-sensitive. At around 20 °C, it became highly viscous, meaning that the spider is particularly sensitive to substrate vibrations at this temperature. This matches closely with the environment Cupiennius - the mountainous region it inhabits has an average night-time temperature of 19 °C. The viscoelastic properties of the pad also define the filtering effect at low frequencies - the mechanical contact between the pad and the tarsus displays a higher effective modulus at high frequencies than at low frequencies. This suggests that mechanical energy is more efficiently transmitted to the sensory grooves at high frequencies.

While more research is needed, the authors believe that this work will help in the design and development of efficient bio-inspired sensors.

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