THE PROCESS

The wordphagocytosis will be used in this monograph to mean the process whereby particles within the range of size of micro-organisms are engulfed by a cell. This is to strain the classical use of the term a little, for it has usually referred to particles visible by optical microscope, that is, to particles more than 100 nm in diameter (Gropp, 1963). But that limit would exclude some virus particles, and we are here concerned with cellular reactions to all kinds of infections. The modern trend, in any case, is to assume that particles of different sizes are drawn into cells by a common mechanism, endocytosis, and to use particular terms (athrocytosis, pinocytosis, phagocytosis, etc.) for variants of the mechanism depending on the nature of the substance and the size of the particles engulfed (Brandt & Pappas, 1960; Holter, 1965).

That some cells of insects are able to phagocytose particles is readily established. Micro-organisms or particulate matter injected into the body cavity of an insect can soon afterwards be found in the cytoplasm of blood cells. When phagocytosis is said to have been observed in an insect, that is almost always what is meant—that phagocytosis has been observed to have taken place. Rarely, if ever, have the cells of insects been watched in the act of engulfing particles.

That is so surprising, in a group of animals intensively studied, and it so greatly affects the investigation of phagocytosis in insects, that we had better pause to consider the evidence.