In the preceding chapters the cellular defence reactions of insects have been described in detail, and an attempt has been made to analyse them. In this chapter it is proposed to widen the scope of inquiry and to discuss the reactions more generally. In particular, my intention is to consider how cellular reactions are adapted to perform their various functions, especially that of providing insects with immunity to infection; and then to examine the degree of specificity they exhibit and how it is achieved.

ADAPTABILITY

The cellular defence reactions of insects are very adaptable in that they provide a wide range of responses automatically adjusted to the need. This is brought about at two levels. The reaction of an individual blood cell, so far as is known, is all or none—it adheres or it does not adhere—but its subsequent behaviour, the extent to which it flattens for instance, can be varied. Moreover, different kinds of cells behave differently: coagulocytes are defined as being especially sensitive; lamellocytes are prone to encapsulate (Rizki, 1962); macro- and micro-plasmatocytes are disparate in their phagocytic and encapsulating activities (Wittig, 1965 b). How far individual cells of the same type in the same insect differ in their reactions is unknown; but, since prohaemocytes are said generally not to phagocytose or to encapsulate whereas the plasmatocytes into which they develop are active in both these reactions, even individual cells must differ in their reactivity in the course of time.