The mechanical properties of the abdominal cuticle of Rhodnius larvae.

1. The mechanical properties of loops of cuticle cut from the abdomens of 5th instar Rhodnius have been investigated. The cuticle shows pronounced viscoelastic behaviour. 2. Stress-relaxation tests show a continuously falling modulus over a wide range of times after the imposition of a strain. 3. Plasticized samples of cuticle show stress-relaxation curves which are shifted along the time axis towards earlier times by up to times 10-3. The modulus at any particular time after the imposition of strain is about 10 times lower than that of the unplasticized cuticle. 4. It is concluded that the mechanical properties of this cuticle are determined, at least for maintained stresses, largely by the matrix material. Chitin microfibrils may act as a reinforcing filler for short-term, rapid stresses. The cuticular macromolecules are probably not extensively cross-linked by primary bonds, though secondary interactions between them are probably important in the viscoelastic properties of the cuticle. 5. Plasticization probably involves a change in either the number or the strength of secondary interactions between the cuticle macromolecules, or both.