Viscoelasticity

Most types of cuticle are more or less viscoelastic; when exposed to a deforming force for extended periods, they will suffer a slight, time-dependent elongation, and recovery after release of the force may not be complete. A special type of highly stretchable, viscoelastic cuticle is found in the abdominal intersegmental membranes of sexually mature female locusts. This stretchability allows elongation of the abdomen necessary for depositing eggs in the soil at a sufficient depth. The membranes in both male and female locusts are soft and pliable, but not very stretchable, as long as the animals are sexually immature. When sexual maturation is initiated in the females by resumed production of juvenile hormone, the organization of the chitin microfibrils in the intersegmental membranes changes from a helicoidal arrangement to one that is perpendicular to the long axis of the animal; at the same time, special hydrophilic proteins are deposited in the membranes. The fully mature intersegmental membranes stretch when loaded, but recover only partly when the load is released. When reloaded with the same load as before, they elongate significantly more than during the first load, and by repeated application of even small loading forces the females can elongate the membranes to about 10 to 15 times their relaxed length, corresponding to a threefold elongation of the total abdomen. Such stretching enables the female locust to deposit eggs in the soil to a depth of 10 to 12 cm.

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