The role of melanization in the immune reaction of larvae ofDrosophila algonquinagainstPseudeucoila bochei

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Abstract
The immune reaction of larvae ofD. algonquinagainstP. bocheiwas characterized either by the melanization and encapsulation of the parasites or by the production of melanized particles. In successful host reactions the eggs of the parasite were killed before they were completely melanized and encapsulated by haemocytes. When host larvae were transferred during the early stages of infection to a diet that contained phenylthiourea (PTU), the incidence of melanization and encapsulation of the parasites was significantly reduced. However, if the hosts were transferred to the diet just before the eggs of the parasite were normally melanized, PTU had virtually no inhibitory effect, and the immune reaction rate remained very high. These results suggest that the biochemical reactions associated with melanization constitute an important part of the immune reaction ofD. algonquin. The data provide additional evidence to support the idea advanced by other workers that the phenoloxidase system plays an important role in insect immunity against some internal metazoan parasites. It is suggested that, during infection, the phenoloxidase system of the haemocytes is activated by hormonal imbalance.