MECHANICAL-PROPERTIES OF THE CUTICLE OF MANDUCA-SEXTA LARVAE TREATED WITH CYROMAZINE

Cyromazine administered in artificial diet to fifth instar tobacco hornworm larvae caused marked changes in the mechanical properties of the larval body wall. Cuticle from treated insects was stiffer and reached its yield point at lower strains than cuticle from controls. In addition, treated cuticle showed increases in the maximum force sustainable before yield was reached compared to controls. These changes were apparent when testing the body wall with the epidermis intact or removed, indicating that the changes are intrinsic to the cuticle itself. The mechanical properties were changed to approximately equal degrees in circumferential and axial directions within the cuticle. The manner of failure was also affected; treated cuticle failed abruptly at the yield point while control cuticle showed a region of ductile extension between the yield point and ultimate fracture. These changes in mechanical properties occurred very rapidly (within 3 hr) upon exposure to cyromazine. It is suggested that this rapid change in cuticle mechanical properties may be the primary effect of cyromazine. © 1990.