1. Defence reactions to metazoan parasites have been reported in fourteen orders of insects. The observations are brought together and reviewed in the first part of the paper.2. Examination of the various accounts that have been given shows that blood cells are always involved in insect defence reactions. They act by forming a cellular capsule, from which a connective-tissue envelope is usually developed, and in which melanin is often deposited.3. The reaction of the epidermal cells at perforations made by parasites is of the nature of wound-healing, and plays no part in defence against metazoan parasites after they have entered the body.4. Although several other tissues have been implicated, there is insufficient evidence to show that any of them make defence reactions, their response being limited to processes of regeneration.5. It is concluded that the blood cells of insects are their only known agents of defence to internal metazoan parasites.6. The principal groups of metazoan parasites infesting insects are considered in the third part of the paper, in order to see how the defence reactions made to them are related to their mode of attack and to the nature and consequences of their parasitism.7. Most parasites elicit a defence reaction when they are in unusual hosts.8. Some parasites, at certain stages of their life-history, are able to avoid eliciting a defence reaction in their usual hosts.9. Some parasites elicit a defence reaction in their usual hosts but are able either to endure it in a dormant state or to resist it.10. General problems of host specificity in relation to defence reactions are discussed. It is concluded that analysis of the stimuli that produce defence reactions has not yet gone far enough to explain the phenomena.11. The effects of insect defence reactions on metazoan parasites range from no perceptible effect to destruction of the parasite.12. The defence reactions of insects are influenced by the species, genetic strain, stage, instar, size, health and physiological state of the host; and by the species, genetic strain, physical and physiological activity, and health of the parasite. Environmental temperature and the presence of other parasites of the same or different species also have effects on the reactions.13. A brief survey of defence reactions made by invertebrates other than insects shows that encapsulation has been reported in Annelida, Mollusca, Crustacea, Acarina, and larval echinoderms. So far as it goes, the survey does not reveal in these other groups any reaction to metazoan parasites of a kind radically different from the reactions observed in insects.14. The historical development and present state of our knowledge of insect defence reactions is traced.15. The reactions made by insects to innocuous parasites are of theoretical interest but of little consequence to the species concerned. It is their effect on potentially dangerous parasites that determines the value of defence reactions. Consideration of the evidence suggests that the protection afforded to insects by their defence reactions is greater than has been generally supposed.16. The review makes apparent many gaps in our knowledge of the phenomena. A few of the outstanding problems are mentioned.I am indebted to Mr R. T. Hughes of the Balfour Library for helping me to obtain journals not available in Cambridge; to Mr M. J. Ashby for the photography necessary in the preparation of the figures; and to Miss G. M. Edwards for her careful typing from my manuscript. The paper would not have been completed without the goodwill and assistance of two persons: Professor D. Keilin, F.R.S., encouraged me to continue and finish it when my effort flagged; my wife not only gave me positive help in many ways but also exercised great forbearance in allowing me to devote vacations and spare time to it.
Origin of a major infectious disease in vertebrates: The timing of Cryptosporidium evolution and its hosts