Trophic eggs, which are inviable and usually function as a food supply for offspring, have been regarded as extended parental investment or the outcome of parent–offspring conflict in sibling oophagy. Adomerus triguttulus (Heteroptera: Cydnidae) is a sub-social bug showing a complex pattern of maternal care, including progressive provisioning of host seeds and trophic-egg production. To investigate the functions of trophic eggs, we removed trophic eggs from clutches under different resource conditions. The longevity of nymphs was greatly extended by feeding upon trophic eggs when seeds were excluded. When seeds were provided, trophic-egg feeding by nymphs enhanced their development, but there were no significant effects on brood survival. Some viable eggs were also fed upon by sibling nymphs. However, there was no difference in the proportion of viable eggs consumed between clutches with and without trophic eggs. Females lay viable eggs within the first oviposition day. The synchronous hatching resulting from this oviposition mode seems to prevent sib-cannibalism. The body size of females affected their relative investment in trophic eggs; larger females produced more viable eggs with relatively fewer trophic eggs. The functions and adaptive allocation of trophic eggs are discussed in light of the two hypotheses mentioned above.