Introduction

Numerous light microscopic observations concerning the classification of insect hemocytes have been published (Yeager, 1945; Wigglesworth, 1959; Nittono, I960; Jones, 1962, 1964; Gupta, 1969; Arnold, 1972). However, there are considerable differences of opinion concerning hemocyte classifications and terminologies. There are also some remarkable differences between the hemocyte types in various insect orders. For example, in the larva of Prodenia eridania, Yeager (1945) recognized 10 classes, containing 32 types of hemocytes; in Bombyx larvae, Nittono (1960) reported five classes of hemocytes: prohemocytes (PRs), plasmatocytes (PLs), granulocytes (GRs), spherulocytes (SPs), and oenocytoids (OEs). Gupta (1969) classified insect hemocytes into eight types: PLs, GRs, OEs, SPs, adipohemocytes (ADs), podocytes (POs), vermicytes (VEs), and cystocytes (= coagulocytes, COs). Some authors recognize only three basic classes: PRs, PLs, and GRs. It also appears that there is greater differentiation of hemocytes in higher insect orders than in lower ones (Jones, 1962, 1964) (see also Chapter 4). Some authors consider amoebocytes, lamellocytes, POs, and VEs as various forms of PLs; COs and VEs as various forms of PLs; and COs and ADs as belonging to the category of GRs.

Recent electron microscopic studies have proved helpful in distinguishing various hemocyte types. For example, in Bombyx mori, the larval PLs and GRs, which were frequently indistinguishable under the light microscope, were clearly identifiable by their ultrastructures (Akai and Sato, 1973; Akai, 1976). Scanning electron microscopic (SEM) studies of the insect hemocytes also are necessary to elucidate their characteristic surface structures. Such studies are likely to supplement the information obtained by light and transmission electron microscopic (TEM) observations.