True stress (σ) and strain (ε) were derived from the following equations: σ=FA and ε=logeLL0 where F is the force applied to the specimen, A is the cross-sectional area of the thread calculated from the thread diameter assuming a constant thread volume, L is the instantaneous length of the fiber at a given extension value and L0 is the original gage length of the fiber. Stress vs strain curves were determined for each silk tested by a standard trapezoidal method from which we calculated the following mechanical properties using both Bluehill 3.0 (Instron Machines, Melbourne, Australia) and Microsoft Excel 2010: (1) ultimate strength; or the stress at rupture, (2) extensibility; or the strain at rupture, (3) toughness; the area under the stress strain curve, and (4) Young’s modulus (stiffness); the slope of the stress-strain curve during its initial elastic phase.