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The mechanism of plasticization of the abdominal cuticle in Rhodnius.

1. The mechanism of plasticization of the abdominal cuticle in Rhodnius larvae has been investigated, using the properties of loops of cuticle under varying test conditions as a model for the behaviour of the cuticle in vivo. 2. It is supposed that plasticization is effected by a change in the intracuticular environment. A number of model mechanisms for plasticization may be proposed, which suppose that the epidermis is capable of regulating (a) pH, (b) ionic strength,(c) Ca and/or Mg, (d) urea, within the cuticle. 3. Analyses of cuticle ash show that models(b) and (c) are not responsible for plasticization in vivo. The levels of inorganic ions within the unplasticized cuticle are not sufficiently high to allow plasticization upon their removal. 4. No evidence for model (d) has been found; urea does not occur in the cuticle in detectable quantities. 5. Exact measurements of the intracuticular pH have not been achieved but straining experiments strongly suggest that a change in pH occurs within the cuticle on plasticization. This pH change is probably large enough to account for the increased extensibility shown by plasticized cuticle.