Centrifugation increases the frequency of twin formation during embryogenesis in firebrat, Thermobia domestica (Packard). Newly hatched eggs of T. domestica were separated into control and experimental centrifugal groups. Double embryos were found among both groups at ≈0.1% naturally and ≈4.5% after centrifugation. Embryos with a double head and a third eye were also observed among the centrifuged eggs. The development of morphological and anatomical structures of twin embryos was compared with those of single embryos. The analysis was carried out by using light and electron (scanning and transmission) microscopy. Embryonic twins may help explain the mechanisms that lead to formation of germ bands in short germ-band insects, a group to which T. domestica belongs.