Fig. 4

Measurement of viscoelasticity of the larval body. A Measurement and control of extension and tensile force of larval body with the tensile tester. The double-headed arrow indicates that the extension is changeable during the measurement. B A schematic of tensile force and extension of a larva. The left larva is not subjected to external forces while the right one is extended. The applied force to extend is the tensile force. C In the measurement of the relationship between extension and tensile force, the larvae were extended at a constant rate as shown in this panel, and the tensile force is being recorded. D An example trace of the relationship between extension and tensile force. E Plots of D in two different ranges of extension (left 0–0.4 mm, right 0–1.0 mm). Green lines indicate linear regression lines in these ranges. F The coefficient of determination of linear regression in different ranges of extension. G In the stress-relaxation test, the larvae were extended quickly, and their length was kept constant. The tensile force under the constant extension was being recorded. H Example traces of stress-relaxation tests with the constant strain of 0.4 mm (left) and 0.6 mm (right). Fitting curves with the SLS model are shown in green (left) and magenta (right). I By the SLS model, the whole larval body can be described as two springs (spring constants: \({k}_1^{\mathrm{whole}}\) and \({k}_2^{\mathrm{whole}}\)) and one damper (damping coefficient: c^whole). J–L Scatter plots of \({k}_1^{\mathrm{whole}}\) (J), \({k}_2^{\mathrm{whole}}\) (K), and c^whole (L) measured with the maximum strain of 0.4 mm (green) and 0.6 mm (magenta)