Flufenoxuron caused a blockage of the production of cuticle of the last instar nymphs since its thickness remained unchanged while that of control congeners increased in thickness as the development proceeded. The formation of endocuticle was prohibited, so the epicuticle and exocuticle did not properly attached to the epidermis. The epidermis appeared with irregularly distributed cells underneath the cuticle. On the other hand, Lufenuron exhibited more dangerous effects. The epidermis was degenerated and detached from the endocuticle which could not distinguish from the exocuticle. The available electron micrographs reveal several dangerous effects of Flufenoxuron on the thoracic muscles such as distortion shape of the Z line and disorganization of A, I and H bands, appearance of gaps and vacuoles in the sarcomere. Similar effects were recorded for Lufenuron beside the complete distortion of the Z disc. Flufenoxuron affected the ultrastructure of mid-gut such as destruction of the cell vacuolization and a rupture of the epithelial walls. In respect to Lufenuron, some signs of morbidity in both nuclei and cytoplasm of the epithelial cells were observed, such as curling and rupturing of the microvilli and the formation of large area of necrosis in a vacuolated cytoplasm. Flufenoxuron caused several ultrastructural changes in the intracellular organelles of last instar nymphs. The mitochondria appeared generally in an irregular shape. Their two membranes were not demarcated with loss of cristae but increased granules. Morphology and of Golgi bodies was remarkably influenced and some of them were fragmented into small particles. The secretion granules associating the Golgi bodies disappeared. The limiting membranes of lysosomes were ruptured. The margination and nuclear chromatin seemed to be early changed leading to the cell death. Lufenuron, on the other hand, caused many serious ultrastructural changes since the mitochondrial cristae were partially or totally lost. Some mitochondria appeared swollen with irregular shape while others appeared greatly elongated with prominent cristae. The mitochondrial membranes were not demarcated, and some mitochondria were filled with dark electron dense granules. Also, the nuclear membrane was disrupted. As well as, the lysosomes and Golgi bodies were hypertrophied with different deformities.