Sometimes the mechanical properties of the exoskeleton can be changed rapidly and reversibly. In bloodsucking bugs (e.g., nymphs of Rhodniusprolixus), the abdominal cuticle is stiff and inextensible before a blood meal. When a meal is initiated, the abdominal cuticle is plasticized, enabling the animal to gorge itself with a volume of blood 10 to 12 times larger than the total volume of the animal before the meal. To do this, stretch receptors send nerve impulses via the central nervous system to axons terminating in the abdominal epidermis. A neurohormone is released from these nerve endings, and the epidermal cells respond by effecting a slight decrease in intracuticular pH. The water content of the abdominal cuticle increases simultaneously, probably owing to the pH change, and the interactions between cuticular proteins decrease, resulting in increased plasticity of the cuticular material.

To facilitate emergence from the old cuticle during ecdysis, the stretchability of the new, pharate cuticle may be temporarily increased to make it easier for the animal to escape from the rather stiff exuvium and facilitate expansion of the new cuticle after emergence. In the tobacco horn worm Manduca sexta, and probably in many other insects, the plas-ticization of the pharate adult cuticle is triggered by release of eclosion hormone into the hemolymph. As in Rhodnius nymphal abdominal cuticle, the plasticization of Manduca pharate cuticle at emergence is probably due to an intracu-ticular pH decrease in combination with increased hydration.

Newly emerged blowflies, which must dig free of the soil before they can expand to their proper size, have a relatively stiff cuticle until they have reached the surface and can begin to swallow air for expansion. For a brief period, their cuticle is plasticized, from release of the neurohormone bursicon. This hormone also plays a role in initiating sclerotization and deposition of endocuticle in the blowflies and probably in other insects.

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