The present study was carried out to evaluate the nutritional indices, histological (light microscopy) and ultrastructural (transmission electron microscopy) changes in the midgut of the 4^th larval instar of Spodoptera littoralis treated with the LC₃₀ of the essential oil of garlic (Allium sativum) (16.60×10⁴ mg/l) and lemon (Citrus limon) (20.09×10⁴ mg/l) based on leaf–dip method compared to untreated larvae (control). The results of the nutritional metabolism revealed a significant reduction in food consumption index (CI), relative growth rate (RGR), and approximate digestibility (AD) of treated larvae. On the contrary, the efficiency of conversion of ingested food to body mass (ECI) and the efficiency of conversion of digested food to body mass (ECD) increased significantly in treated larvae. The most prominent symptoms revealed by light microscopy on the midgut of the treated larvae were vacuolization of the epithelial layer, clumping of the nuclear chromatin material and degeneration of the peritrophic membrane and microvilli. On the other hand, transmission electron microscopy showed vacuolization and degeneration of the columnar cells, clumping of the nuclear chromatin, breaking of microvilli, loss of the cytoplasmic projections of the goblet cells, appearance of residual bodies, and disintegration of mitochondria. In particular, treatment with lemon oil resulted in the appearance of numerous lysosomes and electron-dense mitochondrial matrices. In conclusion, the reduction of the RGR would result in undersized females in the next generations; in turn this would lead to a reduction of the reproductive potential. Also, the present investigation indicates that the microvilli and/or mitochondria are the probable target site(s) of action of garlic and lemon essential oils.