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Our understanding of the world of insect is highly biased by the senses we use the most (vision, olfaction, sound), or which have potential immediate application (olfaction). This does not necessarily reflect their relative importance for insects. The role in insect life of wave propagation in solids, fluids (air, water) and granular materials (sand) is a largely neglected area which we find most interesting. It matters for prey and predator recognition, mate

Figure 3. Wave making and echolocation in water surface insects.

Figure 1. Antlion trap architecture. The graph is a 3D representation of the cone built by the antlion larvae which waits for prey to fall in the bottom of the cone.

What did we found so far?

Sand is a strange material, behaving sometimes like a fluid, sometimes like a solid. Antlions make full use of this behaviour and are able to orient themselves while buried in the sand. We chose this unique animal construction in the hope to be able to define optimality in this context, an impossible task with spider webs, too complex and with intermingled material/structure properties. We are studying how different prey can escape the antlion pit, according to their gait, weight and other characteristics, having thereby the two– the predator’s and the prey’s- points of view.

Insects communicating using vibratory signals face the problem of signal integrity. Indeed, as surface waves are of dispersive nature (i.e. frequency dependent wave velocities), signals change as function of the distance between emitter and receiver. An engineering approach using laser Doppler vibrometry, wavelets analysis of signals and nondimensionality of the wave propagation on plant stems revealed that insects can transmit signals of high integrity in using either signals of high frequencies or by choosing stems of large diameters. We also ventured into the realm of wave propagation at the water surface, using whirligig beetles, in the hope this problem would be more tractable.

Wind carries a lot of information, such as plant odours or kairomones, but its own temporal and spatial structure can also convey information. This is used by many insects equipped with filliform hairs which sense the smallest air movements. We are studying the air flow perception in the wood cricket attacked by wolf spiders. During an attack, wolf spiders displace air in front of them, to a distance of several centimetres (see Figure 2). This information can be used by crickets to escape. Of course, the environment is full of background noise, and its influence is so far unknown.

The theory of hide and seek search games are seldom used in studies of predator-prey interactions. We found, in a theoretical study, a square root rule which says that a predator should ambush more and more during its search for an elusive prey, as function of the unsearched area. This very nice if mathematically complex result can be explained however in plain words: the longer the predator hunts, the smaller is the unvisited area. Hence, in that game, ambush increasingly ensures that the predator can get its prey when it dashes out of the hiding spot.

Our current understanding of the antlion pit is based on sand made of homogenous materials. Latest experiments show that things get more complex if you let the grain size vary, in particular since other physical principles dominate once the grain size is very small: the realm of powders. Furthermore, ants are the preferred prey and we know now why: the leg kinematics shows that they move very differently on sand or hard substrates and according to slope. The antlion trap is a bad combination of these two aspects for ants.

Using a piston mimicking the attack of a spider, we have recorded electrophysiologically the response of crickets in the field. In order to study the air flow at the surface of leaves in the litter, we mapped out, in 3D, leaf litter and studied its geometrical features (Figure 4). A geometrically complex environment changes the spider-cricket interaction a lot! We are also currently producing a FEM of a model spider and computing the produced airflow ahead of it. All these results are making this case study uniquely well understood.

Echolocation in wriggling beetles is an intriguing hypothesis which has found its way in all textbooks, but we did not find any conclusive evidence for it so far. By contrast, they most likely use the meniscus produced by objects are the air-water interface. Solving this question is more difficult than anticipated.

What next?

We would like to do much more with antlions, as they seem to defy some simple physics rules about the speed of surface wave propagation in sand and their ability to orient: where are the receptors and how can they fonction? Which concept of architectural optimality should we use over the lifetime of a pit, which includes events such of avalanches, restoring of the pit etc? What is about cleaning the pit containing heterogeneous materials? From the prey point of view, can we classify prey according to their leg kinematics and relate that to their rate of capture?

Surface wave propagation in semi-aquatic insects is still very high on our priority list, as we have several cool ideas for testing the echolocation hypothesis further and extending our knowledge to other animals at the air-water interface - Gerris bugs are the next target.

We will pursue the field of research around the air flow sensing and the aerodynamics of running arthropods for many years, because air sensitive hairs are everywhere in the insect world, and the arachnid-insect prey interaction is one of the oldest. The implication of our findings in terms of MEMS design (see under Bioinspired design) is progressing at the same pace.

Hide and seek games are mathematically very different from pursuit-escape games, as the former assume incomplete information while the later assume full information to both opponents. This implies working with probabilities in the first case and with differential equations in the second. We are merging these two fields in a single framework, as both occur during a chase in the field. The implications are expected to hold for a huge number of predator-prey interactions.

Finally, we are always keen on adding well chosen examples to our growing collection - see for example the flapping flight work on butterflies take off in the chapter on Bioinspired design!

Jerome Casas

Thomas Steinmann

Antoine Humeau

Dupuy, F., Steinmann, T., Pierre, D., Christidès, J.-P., Cummins, G., Lazzari, C., Miller, J., et al. (2012). Responses of cricket cercal interneurons to realistic naturalistic stimuli in the field. The Journal of experimental biology, 215(Pt 14), 2382–9. doi:10.1242/jeb.067405

Jonathan Voise, Michael Schindler, Jérôme Casas and Elie Raphaël (2011) Capillary-based static self-assembly in higher organisms. J. R. Soc. Interface (online) doi:10.1098/rsif.2010.0681

Voise J. & Casas J. (2010). The management of fluid and wave resistances by whirligig beetles, J. R. Soc. Interface, 7, 343-352. doi: 10.1098/rsif.2009.0210

Brice Bathellier, Thomas Steinmann, Friedrich G. Barth and Jérôme Casas (2011) Air motion sensing hairs of arthropods detect high frequencies at near-maximal mechanical efficiency. J. R. Soc. Interface (online) doi: 10.1098/rsif.2011.0690

Casas, J., T. Steinmann & O. Dangles (2008)

The aerodynamics signature of running spiders.

PLoS One 3(5): e2116.

Casas, J., C. Magal & J. Sueur (2007) Dispersive and non-dispersive waves through plants: implications for arthropods vibratory communication. Proceedings Royal Soc. B., 274: 1087-1092.

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Figure 2. The wood cricket is preyed upon by the wolf spider. They both live in the leaf litter in temperate zones. The graph shows air flow produced by an attacking spider, as measured using Particle Imaging Velocimetry.

Figure 4. 3D mapping of litter surface elevation. Close investigation of litter surface geometry shows that litter surface is spatially structured, with a tight link between geometrical properties and biological (eg. size of leaves) and chemical (decomposition time) processes.

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location and many other aspects, such as avoidance of water droplets or abrasion through particles, for example. The aim of this research is to quantify the nature and role of wave propagation in the life of several insects, in order to have an understanding of the influence of the type of substrate and the kind of waves produced and perceived. The physics of the environment constrained the signals used by insects in a profound way we hardly grasp.

This quest explains the a priori disparate choices of topics ranging from the vibrations through plants in herbivorous host-parasitoid interactions, the wave propagation through sand in the antlion pit (Figure 1), the aerodynamics of hunting spiders and its importance in prey escape (Figure 2), or the wave propagation at the water surface, in relationship with echolocation in wriggling beetles (Gyrinidae) (Figure 3). Please check also the section on bionics for our latest grant on the kinematics and aerodynamics of butterfly wing movement and the application of flapping wing design in technology.