An in vitro phagocytosis assay, adjusted to as little as 1 microL of insect hemolymph, enables the microscopic determination of phagocytosis for single individuals of small insects. Even repeated determination over the lifetime of individuals is possible. This method makes it feasible to study individual variation in invertebrate phagocytic capacity. Possible sources of such variation are reviewed in this article: genetic differences, development, aging, reproduction, presence of parasites, and diverse environmental influences are natural sources of individual variation in phagocytosis. However, the methods used for phagocytosis and microscopic evaluation are also (unwelcome) sources of variation. To optimize incubation time for in vitro phagocytosis, time courses were determined. Furthermore, the reliability of visual counting and image analysis for the microscopic quantification of phagocytosis are compared. The influences of larval development and adult aging on phagocytosis by Panorpa vulgaris hemocytes are subsequently demonstrated. During development, a decrease in hemocyte numbers but a simultaneous increase in the proportion of phagocytosing hemocytes was observed when larvae reached pupation. On the other hand, adults showed a dramatic decrease in phagocytic capacity with age, while cell numbers remained fairly constant. The results show that individual variation in phagocytosis can be determined accurately in small invertebrates and related to its causes. This might be especially interesting in the context of studies relating individual immunocompetence to ecology, life history variation, and behavior. Copyright 2002 Wiley-Liss, Inc.
