This study deals with the ontogenesis of leaflets and galls induced by Euphalerus ostreoides Crawf. on the second-order veins of leaflets of Lonchocarpus muehlbergianus Hassl. Leaflets had a median submarginal developmental pattern with four layers of ground meristem. Procambial strands were differentiated from the median layers. Simple nonglandular trichomes were differentiated from the protoderm and abscised before leaflet maturation. Mature leaflets had uniseriate epidermis and isolateral mesophyll. Galls were bivalve-shaped, with a single nymphal chamber and glabrous outer epidermis. Sparse trichomes occurred on the margin of the valves and inner epidermis. Cecidogenesis led to vascular neoformation, cell hypertrophy, and conspicuous hyperplasia of the cortical layers, with cytological distinctiveness. Histochemistry of invertases indicated that sugars supplied plant cell machinery. The developmental pattern of nongalled leaf tissues of L. muehlbergianus, as well as the variations caused by the feeding activity of E. ostreoides, were in accordance with a developmental model proposed for Psyllidae galls in the Neotropics. The valves are vascularized leafy appendages that originated from the most adaxial layers of the ground meristem. Vascularization supplied additional feeding sites to E. ostreoides, as well as water uptake, which were of major importance in the determination of the final shape of the gall.