Insects adapt to various environmental conditions by living symbiotically with microorganisms. Some insects possess specialized cells for harboring symbiotic microbes called “bacteriocytes”, which often constitute symbiotic organs called “bacteriomes”. Such cells and tissues for symbiosis have repeatedly evolved during the insect evolution and diversification, thereby underpinning the stable host-symbiont associations and the essential biological roles of the symbiosis. Besides some old histological descriptions in the early 20^th century, recent developmental studies on the bacteriomes have been quite limited, and thus the developmental and evolutionary origins of the bacteriomes are still an enigma. In an attempt to tackle this research topic, we investigated the stored-product pest beetles with well-developed bacteriomes, Oryzaephilus surinamensis (Silvanidae) and Rhyzopertha dominica (Bostrichidae), which are experimentally tractable and easily maintainable in the laboratory. Here we report the embryogenesis of these beetles with special focus on the formation and development of the bacteriomes. Currently, for the purpose of uncovering the molecular mechanisms underlying the bacteriome differentiation, we are conducting transcriptomic analyses of the symbiotic cells and organs, and also establishing an efficient gene knock-down system using RNAi with these insects.