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.