Observations on the cytochemistry of the hemocytes of an insect, Galleria mellonella

A number of cytochemical parameters of the hemocytes of larval Galleria mellonella, an insect frequently used as a model by comparative cellular immunologists, are described. Cytochemical methods were used to quantify hemocyte granule-associated components, the results are compared to those obtained for leukocytes from higher animals. Granulocytes contained a population of nonlysosomal granules rich in mucopolysaccharide not seen in plasmatocytes. The numbers and dimensions of these granules showed a positive correlation to cell size, probably reflecting a developmental sequence in granulocyte maturation. Both granulocytes and plasmatocytes had other granules containing the typical lysosomal enzymes, acid phosphatase, beta-glucuronidase, esterase, and lysozyme. The nonlysosomal enzyme alkaline phosphatase was not found in Galleria hemocytes; it is also absent from vertebrate monocytes, macrophages, and immature polymorphonuclear leukocytes. Insect hemocytes appear to lack certain components of antibacterial systems typical of mammalian blood cells, such as H2O2-generating systems, cationic proteins, and myeloperoxidase. The bactericidal mechanisms of hemocytes probably involve lysozyme, as well as other biologically active cellular and humoral factors unique to insects.