Example stress-strain curves in relation to toughness for single threads of N. pilipes (left) and N. plumipes (right) thread. Silk was pulled at 1 mm s⁻¹. Stress-strain curves have distinct regions signifying behavioural and structural change: before the yield point (*) the response is elastic and the curve is straight. This first slope is the initial elastic modulus (E). The yield point marks the transition between an elastic and rubber-like response. It is assumed that the amorphous fraction converts from a glass state to a rubber state at this point (Gosline et al., 1999). The gradient of the stress-strain curve falls at the yield point altering E, followed by an increase in slope (and E) as the strain continues to increase, known as work hardening (H). Post-yield response is due to behaviour of action between the rubber states and the crystalline fractions of the silk. Immediately following yield, the stiffness is due to the rubber fraction, but as strain increases and the polymer chains are forced together by increasing tensile strain, the rubber states convert to either glass or crystal, giving a stiffer material that ultimately breaks with a brittle response (Gosline et al., 1999). Points of decreased stress indicate fibre breakage with individual fibres fracturing at different points in time. Stress, strain and toughness were measured from the first fracture, indicated by the hatched area.