Vibration detection in arthropods: Signal transfer, biomechanics and sensory adaptations

Since nearly all adult insects fly, the cuticle has to provide a very efficient and lightweight skeleton. Information is available about the mechanical properties of cuticle-Young's modulus of resilin is about 1 MPa, of soft cuticles about 1 kPa to 50 MPa, of sclerotised cuticles 1-20 GPa; Vicker's Hardness of sclerotised cuticle ranges between 25 and 80 kgf mm(-2); density is 1-1.3 kg m(-3)-and one of its components, chitin nanofibres, the Young's modulus of which is more than 150 GPa. Experiments based on fracture mechanics have not been performed although the layered structure probably provides some toughening. The structural performance of wings and legs has been measured, but our understanding of the importance of buckling is lacking: it can stiffen the structure (by elastic postbuckling in wings, for example) or be a failure mode. We know nothing of fatigue properties (yet, for instance, the insect wing must undergo millions of cycles, flexing or buckling on each cycle). The remarkable mechanical performance and efficiency of cuticle can be analysed and compared with those of other materials using material property charts and material indices. Presented in this paper are four: Young's modulus-density (stiffness per unit weight), specific Young's modulus-specific strength (elastic hinges, elastic energy storage per unit weight), toughness-Young's modulus (fracture resistance under various loading conditions), and hardness (wear resistance). In conjunction with a structural analysis of cuticle these charts help to understand the relevance of microstructure (fibre orientation effects in tendons, joints and sense organs, for example) and shape (including surface structure) of this fibrous composite for a given function. With modern techniques for analysis of structure and material, and emphasis on nanocomposites and self-assembly, insect cuticle should be the archetype for composites at all levels of scale.

This paper reviews the structure and function of insect mechanoreceptors with respect to their cellular, subcellular, and cuticular organization. Four types are described and their function is discussed: 1, the bristles; 2, the trichobothria; 3, the campaniform sensilla; and 4, the scolopidia. Usually, bristles respond to touch, trichobothria to air currents and sound, campaniform sensilla to deformation of the cuticle, and scolopidia to stretch. Mechanoreceptors are composed of four cells: a bipolar sensory neuron, which is enveloped by the thecogen, the trichogen, and the tormogen cells. Apically, the neuron gives off a ciliary dendrite which is attached to the stimulus-transmitting cuticular structures. In types 1-3, the tip of the dendrite contains a highly organized cytoskeletal complex of microtubules, the "tubular body," which is connected to the dendritic membrane via short rods, the "membrane-integrated cones" (MICs). The dendritic membrane is attached to the cuticle via fine attachment fibers. The hair-type sensilla (types 1, 2) are constructed as first-order levers, which transmit deflection of the hair directly to the dendrite tip. In campaniform sensilla (type 3), there is a cuticular dome instead of a hair and the dendrite is stimulated by deformation of the cuticle. In these three types, a slight lateral compression of the dendrite tip is most probably the effective stimulus. In scolopidia, the dendritic membrane is most probably stimulated by stretch. On the subcellular level, connectors between the cytoskeleton, the dendritic membrane, and extracellular (cuticular) structures are present in all four types and are thought to be engaged in membrane depolarization.

Animal communication is a dynamic field that promotes cross-disciplinary study of the complex mechanisms of sending and receiving signals, the neurobiology of signal detection and processing, and the behaviors of animals creating and responding to encoded messages. Alongside visual signals, songs, or pheromones exists another major communication channel that has been rather neglected until recent decades: substrate-borne vibration. Vibrations carried in the substrate are considered to provide a very old and apparently ubiquitous communication channel that is used alone or in combination with other information channels in multimodal signaling. The substrate could be 'the ground', or a plant leaf or stem, or the surface of water, or a spider's web, or a honeybee's honeycomb. Animals moving on these substrates typically create incidental vibrations that can alert others to their presence. They also may use behaviors to create vibrational waves that are employed in the contexts of mate location and identification, courtship and mating, maternal care and sibling interactions, predation, predator avoidance, foraging, and general recruitment of family members to work. In fact, animals use substrate-borne vibrations to signal in the same contexts that they use vision, hearing, touch, taste, or smell. Study of vibrational communication across animal taxa provides more than just a more complete story. Communication through substrate-borne vibration has its own constraints and opportunities not found in other signaling modalities. Here, I review the state of our understanding of information acquisition via substrate-borne vibrations with special attention to the most recent literature.