Introduction

The postembryonic origin and multiplication of insect hemocytes continue to provide food for thought and material for research projects for many entomologists, comparative physiologists, and comparative pathologists. Review articles discussing insect hemocytes, total and differential cell counts, and hemopoiesis include Jones (1962, 1970), Wigglesworth (1959), Arnold (1974), and Crossley (1975). One should consult these articles for the historical development of the topic as well as for an extensive review of the literature.

Because the terms “hemopoiesis,” “hematopoiesis,” and “hemocytopoiesis” seemed to be used interchangeably in the literature, I decided to do a little checking before using one term or the other. According to Webster's Third New International Dictionary (1965) “hemopoiesis” and “hematopoiesis” are equivalent in meaning and in derivation and therefore can be used interchangeably. Both mean formation of blood or blood cells within the body. Webster does not list “hemocytopoiesis,” but does list “hemocytogenesis” as being the part of hematopoiesis concerned with the formation of blood cells. I will use “hemopoiesis” because it is the shortest of the terms and includes the formation of blood cells in its definition (see also Chapter 10).

The process of hemopoiesis includes both cell proliferation and cell differentiation. As cell differentiation proceeds, the capacity for cell proliferation decreases. Mature cells are incapable of proliferation (Best and Taylor, 1966). Even though I will be principally concerned with cell proliferation and multiplication in this chapter, I will also cover some aspects of cell differentiation because it bears an important relationship to the amount of cell proliferation in the normal animal.