Hemocytes are the primary cellular components of the innate immune system in Drosophila melanogaster and share functional and molecular characteristics with vertebrate myeloid cells. They arise in two developmental waves, an embryonic and a larval wave, and persist through metamorphosis into the adult fly, where they carry out phagocytosis, melanisation, encapsulation, and cytokine signalling. Because hemocytes are in contact with the hemolymph throughout life, they register the nutritional state of the organism through conserved nutrient-sensing pathways, including insulin-like signalling, the target of rapamycin, adipokinetic hormone, and glucose transport. Signalling through these pathways can influence hemocyte metabolism and function, and hemocyte-derived signals can in turn affect systemic metabolism, fitness, and lifespan. Growing evidence indicates that these effects are not confined to the exposed individual, since parental and in particular maternal diet can alter hemocyte number, composition, and immune capability in the progeny. In this review, we discuss the ontogeny of Drosophila hemocytes across the life cycle, their contribution to immune ageing and lifespan, and the nutrient-sensing pathways through which they translate dietary input into altered immune function. We further examine the intergenerational consequences of maternal diet for hemocyte programming, drawing parallels with the developmental programming of tissue-resident macrophages in mammals. Understanding hemocytes as nutrient-sensing regulators may clarify how diet shapes immune function and survival across generations.
