After treatment with  Pyriproxyfen, the nymphal cuticle appeared as an amorphous layer and the epidermis was irregularly distributed underneath the cuticle. A loss of large amounts of the epidermal cells was  recorded. Pyriproxyfen treatments affected the ultrastructural configuration of thoracic muscles such as disrupted organization of A, I and H  bands and the degeneration of Z disc varied from one half of the disc to small central or peripheral degenerated areas. Generally, Pyriproxyfen caused swelling and enlargement of microvilli and disappearance of the normal size and shape of the epithelial cells. Regarding to the action of Pyriproxyfen on the ultrastructure of the intracellular organelles, some mitochondria partially or totally lost their cristae while other mitochondria were enlarged with prominent cristae. Lysosomes and Golgi bodies disappeared. The nucleus had an irregular shape and large amounts of nuclear chromatin were aggregated into numerous masses.