## 3 Results and Discussion  
The scorpion _H. petersii_ has a slit-based mechanical sensilla which is located at the basitarsus of each walking leg and consists of 12 curved slits (Figure 1a-c). The length of slits decreases from about 320 \(\upmu\)m to 50 \(\upmu\)m. Transversal cross-section of the BCSS was imaged through histological section and toluidine blue staining. The results indicate that each slit connected with the dendrite of sensory neurons through a coupling cylinder at the slit tip (Figure 1d). Longitudinal cross-sections through the slit were imaged by semithin section and SEM, which revealed that both sides of the slit wake were almost completely disconnected in the direction of exoskeleton thickness (Figure 1e). In addition, magnified views of the longitudinal cross-section further show that there is special surface membrane tissue covering the wake of the slit (Figure 1f). Figure 1f clearly indicates that there is obvious bending deformation of the surface membrane. The periodic closure and opening of the microscale slit under cyclic loads caused by walking and vibration signals was discovered in a previous study [9]. Hence, the surface membrane covering the slit wake may fold periodically under alternating loads. The deformation mode as well as the function of the surface membrane will be further discussed in detail in the following theoretical and simulation analysis. The statistical data of detailed structural parameters indicates that the initial width (\(w_{0}\)) of the slit is about 8 \(\upmu\)m, the thickness (\(t_{e}\)) of hard exoskeleton is about 50 \(\upmu\)m, the thickness (\(t_{m}\)) of the membrane is about 2.5 \(\upmu\)m, and the interval (\(d_{0}\)) between adjacent slits increases from 24 \(\upmu\)m to 120 \(\upmu\)m. The symbols for sensilla geometrical dimensions are shown in Figure 2a. In summary, the above study on structural characterization clearly indicates that mechanosensory slits are local micro-cracks embedded in the hard exoskeleton, and the wake of the slit is covered by a special surface membrane. From the perspective of engineering, the slits are risky cracks embedded in the stress body. Figure 1: **(a)** The adult scorpion _H. petersii_ collected from Hainan Province, China; **(b)** the distribution of the BCSS on eight walking legs; **(c)** the morphology of Basitarsal Compound Slit Sensilla (BCSS) with slits arranged in fan shape; **(d)** transversal cross-section with toluidine blue staining; **(e)** longitudinal cross-section image; **(f)** local enlarged image taken from **(e)**. The unique mechanical behavior of biological materials is an important factor for revealing the role of the surface membrane in balancing perception accuracy and structural safety of slit-based mechanosensory structure. Hence, we further analyzed the elastic modulus of fresh biological tissue around the slit wake through the AFM and nano-indenter. Figure 3a illustrates that, under fresh conditions, the elastic modulus of the surface membrane is strikingly lower than the exoskeleton. In contrast, the difference on the elastic modulus between them is not obvious in a dehydrated state (Figure 3b). In fact, this difference on tissue elastic modulus for different physiological states is mainly caused by water, which has also been reported for other biological materials [26]. The difference on the elastic modulus of membrane and exoskeleton further indicates that the flexible membrane with relatively low stiffness is more prone to deformation than the hard exoskeleton with the opening and closure of a slit. For additional measurements of an accurate value of elastic modulus on these two kinds of tissue, the nano-indentation experiment was carried out (Figure 3c). The result indicates that the elastic modulus of surface membrane (\(E_{m}\)) and exoskeleton (\(E_{s}\)) were 0.562 GPa and 5.829 GPa, respectively (Figure 3d).