Keywords: biomechanics, sensors, spiders, arachnids, finite element analyses.

The present paper is intended to show how computational mechanics can contribute to exploring and understanding the impressively sophisticated "design" and functionality of biological sensors [1]. Two types of sensory systems are considered that are found on the legs of arachnids: tactile hairs and, in more detail, slit sensilla [2].

Spiders have a large number of mechanosensitive cuticular hairs. So-called "tactile hairs" are stimulated by contact forces. Encoding of the stimulus leads to the appropriate action of the spider. It was found by computational simulations that these hairs are "optimally designed" both from the structural mechanics point of view and with respect to the neurobiological sensitivity [3]. Mathematical models were developed to study the force and directional sensitivity of the joint of such tactile hairs [4].

Another type of sophisticated sensory system of spiders and other arachnids are strain sensitive slit sensilla, which are elongated openings in the cuticle with aspect ratios of up to 100, appearing as single slits or as complex arrangements of a number of mechanically interacting slits that form so-called lyriform organs. The mechanical stimulus is encoded by the slit compression (i.e., the reduction of the slit's gap width) close to the location of the sensory cell ending in the slit. In order to obtain basic information about the mechanical behavior of interacting slits and in view of their high aspect ratios a first approach used Kachanov's method [5] for interacting cracks. Because of the limitations of this method with respect to cracks positioned at small distances to each other and in order to get deeper insight into the deformation patterns leading to sensory cell stimulation, discretization methods were applied [6]. By using finite element models with realistic slit arrangements and taking the layered micro-structure of the cuticle into account it was possible to study the amplitude and directional sensitivity of the slits' deformation responses to mechanical loads for different slit configurations in the lyriform organs. Even minor morphological variations in the arrangements of the slits can substantially influence the stimulus transformation [7].

[1]

F.G. Barth, "Sensors and Sensing: A Biologist's View", in F.G. Barth, J.A.C. Humphrey, T.W. Secomb, (Editors), Sensors and Sensing in Biology and Engineering, Springer, Wien, pp. 3-15, 2003.

[2]

F.G. Barth, "A Spider's World: Senses and Behavior", Springer, Wien, 2002.

[3]

H.-E. Dechant, F.G. Rammerstorfer, F.G. Barth, "Arthropod Touch Recep-tion: Stimulus Transformation and Finite Element Model of Spider Tactile Hairs", J. Comp..Physiol. A, 187, 313-322, 2001. doi:10.1007/s003590100203

[4]

H.-E. Dechant, B. Höß, F.G. Rammerstorfer, F.G. Barth, "Arthropod Mech-anoreceptive Hairs: Modeling the Directionality of the Joint", J. Comp. Physiol. A, 192, 1271-1278, 2006. doi:10.1007/s00359-006-0155-0

[5]

M. Kachanov, "Elastic Solids With Interacting Cracks and Related Prob-lems" in J. Hutchinson, T. Wu, (Editors), Advances in Applied Mechanics, Academic Press, San Diego, pp. 259-426, 1994.

[6]

B. Hößl, H.J. Böhm, F.G. Rammerstorfer, R. Müllan, F.G. Barth, "Studying Arachnid Slit Sensilla by a Fracture Mechanical Approach: Kachanov's Method and Slit Face Displacements", J. Biomech., 39, 1761-1768, 2006. doi:10.1016/j.jbiomech.2005.05.031

[7]

B. Hößl, H.J. Böhm, F.G. Rammerstorfer, F.G. Barth, "Finite Element Modeling of Arachnid Slit Sensilla - I. The Mechanical Significance of Different Slit Arrays", J. Comp. Physiol. A, 193, 445-459, 2007. doi:10.1007/s00359-006-0201-y

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