It is scarcely possible at present to compare the defence reactions of insects and vertebrates in any detail. The two fields have been investigated in isolation, and no one has yet mastered both of their separate and rather exclusive literatures. Three interesting attempts have been made by authorities on vertebrate reactions not wholly at home among insects (Good & Papermaster, 1964; Burnet, 1968; Nelson, 1969). The paragraphs that follow are likely to betray ignorance of the complexities of vertebrate reactions. They are written in the belief that there is a pressing need to build abutments from which the gap between the two fields can eventually be bridged.

The most obvious point of difference is that cellular reactions play a relatively larger part in the defence of insects against infection. This may be partly reciprocal, in the sense that the more highly developed humoral reactions of warm-blooded vertebrates leave less for the cellular reactions to do. The difference is magnified, however, because in vertebrates the distinction between cellular and humoral reactions is a good deal blurred, owing to the intervention of opsinins, cytophilic antibodies, lymphokines and the like in their cellular reactions. Among insects there is as yet no evidence for such intervention; so that cellular and humoral reactions remain distinct, and the part played by the former is evident.

Since only the cellular reactions of insects have been reviewed in this monograph, comparison must be restricted to the cellular reactions of vertebrates.

‘Encapsulation’ and ‘encystment’ were defined by Maupas (1899), who restricted the first to a reaction of the host, the second to an activity of the parasite. A capsule is a covering laid down by the host around a parasite or other foreign body; cysts are coverings formed by parasites about themselves. Unfortunately, these terms have sometimes been used as though they were interchangeable, and it is then difficult or impossible to understand what was meant. Authors writing in French sometimes use ‘capsule’ and sometimes ‘nodule leucocytaire’. Well developed nodules are indeed very similar to capsules, but a distinction can be drawn (p. 51) and is perhaps worth preserving. There has also been some confusion in the literature between capsules and tumours, but that is unnecessary in theory whatever it may be in practice: a capsule is formed and increases in size by the aggregation and addition of cells; a tumour grows by the division and consequent proliferation of cells.

Encapsulation has not been observed to take place in vitro, and the process is therefore known from the examination of hosts dissected at intervals after implantation of a parasite or other foreign body. Since the time required for complete encapsulation is relatively long, measured in hours or days, the course of events can be described with some confidence by examining hosts at frequent intervals.

So far as present evidence goes, encapsulation is carried out exclusively by blood cells.

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.

The cellular defence reactions of insects derive ultimately from the behaviour of individual blood cells. In part those cells act separately, as in phagocytosis and in the initial reaction to a foreign body. In part they act together, as in encapsulation; and when they do so, the behaviour of each may be affected by the concerted activity of all. An attempt will now be made to analyse cellular defence reactions, with a view to exposing the succession of stimuli received and responses made by the blood cells as a reaction takes its course.

It is impossible in a small book to deal with the variant reactions of different species to different objects. In order to avoid continual qualification, the analysis that follows is largely restricted to cellular capsules, and is based principally on reactions observed in caterpillars of Ephestia.

To begin with, it will be convenient to distinguish six successive phases of reaction. The first is that of contact between a blood cell and the foreign body, and the problem is to learn how that contact is brought about. The next phase is the adhesion of the two; it calls for consideration of the stimuli that cause blood cells to adhere to most surfaces but not to all. Reaction to a large foreign body involves the cohesion of cells which have no direct contact with the foreign surface, and it also involves the aggregation of a large number of cells; so that we need to understand how those cells come together so quickly and what makes them cohere.

The cellular reactions of insects have been described above as mechanisms of defence against infection. They invite further consideration as an aspect of cell biology. In particular, an attempt to understand cellular defence reactions must include attention to the stimuli that excite the cells. A first step in the investigation of those stimuli is to survey the various kinds of objects to which reactions are made.

OBJECTS PHAGOCYTOSED

Practically all micro-organisms that enter the haemocoele of an insect are phagocytosed. For analytical purposes, therefore, interest centres on those species to which the phagocytes are said not to react. Some reports of the absence of reaction are beside the mark, because the organisms to which they refer had not been in actual contact with the blood, and therefore could not have stimulated the cells (Salt, 1969). Very few micro-organisms, if any, can be accepted as being able to challenge phagocytosis with impunity (p. 20), and nothing is known about the means by which they do so. Whether they actively inhibit phagocytosis, or whether they passively avoid stimulating the blood cells, seems never to have been investigated.

Inert particles of many kinds have been injected into insects and found to be phagocytosed. Examples are listed in table 5. In considering them, it will be wise to bear in mind Rowley's caution (1962) that it is probably fallacious to regard any particle in serum as actually inert.

This means of defence comes into operation against micro-organisms and other small particles that have formed a clump in the haemocoele. Blood cells containing particles they have phagocytosed become attached to the clump, other blood cells adhere to them, and there develops an agglomeration of foreign particles, necrotic phagocytes, and living blood cells, all of which may then become encapsulated.

Because nodules are formed by circulating cells concerned with phagocytosis and encapsulation, it is evident that they must be composed mainly of plasmatocytes. However, when the clump of particles and the aggregation of blood cells is large, there is a chance of unreactive cells, oenocytoids and other types, becoming entrapped and incorporated. Moreover, loose tissues, such as lobes of fat body or tracheoles, may also become entangled; which perhaps explains how Iwasaki (1927) was led to think that fat cells and cells of the tracheal epithelium take part in the reaction.

Nodule formation is clearly a mixture, in variable measure, of phagocytosis and encapsulation. At one extreme, a few blood cells that have engulfed foreign particles adhere to form a small group having no constant structure (fig. 8, b, c). At the other extreme, a large nodule has the distinctive lamellated structure of a capsule (fig. 8, d); indeed, it is a capsule of which the inner layer does not abut on a continuous surface but merges into an agglomeration of degenerating blood cells, foreign particles, and melanized debris.

Because insects live in almost all terrestrial and freshwater habitats, and eat almost every kind of organic matter, they are exposed to a great variety of diseases and parasites. The species that burrow in soil, or inhabit the mud around ponds, or feed on carrion or dung, are constantly subject to contamination by micro-organisms. Those that live among the litter and debris on the surface of the ground, or on the leaves and twigs of growing plants, run the risk of infection by air-borne microbes and of attack by many kinds of parasites.

To counter these dangers, insects have a number of features which operate to prevent infection. An obvious example is their exoskeleton, an almost complete covering of chitin and tanned proteins, physically tough, chemically resistant, and usually impermeable to water. It serves to exclude many potential contagions. Another example is the composition of their digestive fluids, which quickly destroy most of the micro-organisms that enter the alimentary tract with their food. These and other characteristics of their structure and physiology protect insects against many infective organisms.

In spite of defences of that sort, a large number of pathogens succeed in penetrating the insect body. Many kinds of viruses, bacteria, fungi, protozoa, parasitic worms, and parasitoid insects are able to invade the haemocoele, where they can nourish themselves on blood and whence they can find direct access to particular organs and tissues.

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.