Micro- and nano-structural details of a spider's filter for substrate vibrations: relevance for low-frequency signal transmission

Abstract

The metatarsal lyriform organ of the Central American wandering spider Cupiennius salei is its most sensitive vibration detector. It is able to sense a wide range of vibration stimuli over four orders of magnitude in frequency between at least as low as 0.1 Hz and several kilohertz. Transmission of the vibrations to the slit organ is controlled by a cuticular pad in front of it. While the mechanism of high-frequency stimulus transfer (above ca 40 Hz) is well understood and related to the viscoelastic properties of the pad's epicuticle, it is not yet clear how low-frequency stimuli (less than 40 Hz) are transmitted. Here, we study how the pad material affects the pad's mechanical properties and thus its role in the transfer of the stimulus, using a variety of experimental techniques, such as X-ray micro-computed tomography for three-dimensional imaging, X-ray scattering for structural analysis, and atomic force microscopy and scanning electron microscopy for surface imaging. The mechanical properties were investigated using scanning acoustic microscopy and nanoindentation. We show that large tarsal deflections cause large deformation in the distal highly hydrated part of the pad. Beyond this region, a sclerotized region serves as a supporting frame which resists the deformation and is displaced to push against the slits, with displacement values considerably scaled down to only a few micrometres. Unravelling the structural arrangement in such specialized structures may provide conceptual ideas for the design of new materials capable of controlling a technical sensor's specificity and selectivity, which is so typical of biological sensors.

Micromechanical properties of strain-sensitive lyriform organs of a wandering spider (Cupiennius salei)

“…angles between slits, length gradients, and lateral shifts amongst the slits), orientation of the organ with respect to the leg axis, dendrite position within the slits, and surrounding auxiliary structures (e.g. the cuticular pad in front of HS10; [17,26]), a single organ can be "tuned" to respond well to different stimulus parameters like strain direction or a particularly wide range of strain amplitudes, and also be sensitive to specific frequency ranges [9,14,15,16,17,18].…”

“…Recently, we have studied in detail the time-and temperature-dependent micromechanical properties of the epicuticular layer covering the metatarsal pad which serves to enhance the response of the vibration sensitive lyriform organ HS10 to high frequency stimuli [17,25]. Additionally, we have analyzed a variety of chitin fibril orientations and sclerotization levels resulting in a wide range of mechanical properties in the subsurface structures of the pad [26]. The pad was also found to play a key role in HS10 compression (stimulation) including the transformation of large initial deformations caused by the tarsus (tens of µm) to much smaller deformations (tens of nm) [26].…”

Developmental morphology of a lyriform organ in the Western black widow (Latrodectus hesperus)

“…Unlike the distal portion of the pad which is soft and compliant, the 'appendix' adjacent to slit 1 of 175 HS10 is hard and sclerotized (Erko et al 2015). Without analysis of the material properties of the pad 176 in adults, or indeed juvenile L. hesperus, it is not possible to infer whether their pad performs the same 177 function as in Cupiennius or whether it has a different structure specific to the web substrate that 178 transmits vibrational signals in L. hesperus.…”

“…As well as acting as a filter it is thought that the pad serves to protect the HS10 188 organ from high amplitude, potentially damaging stimuli (Erko et al 2015). This function is perhaps 189 more relevant to adults than juveniles, as in juveniles a damaged structure will be replaced with a fully …”

“…The pad is pronounced in adults of both sexes, but in 140 the 5 early instars measured, it is small in size (Figure 3). properties being complex and heterogeneous to perform this task (Young et al 2014;Erko et al 2015). 174…”