In much of this volume, the focus has been on interactions between antagonists—viruses, bacteria, and parasitoids—whose presence is not welcome within the host, and where death of the host is often required for the propagation of the virus, bacterium, or parasitoid. However, the central importance of pathology in the life history of microparasites that interact with insects is by no means universal. For microbes that benefit from vertical transmission—that is, through the gamete to the next generation—the maintenance of a healthy female host is a pre-requisite for the survival of the microbe. In these cases, the microbes are termed symbionts to reflect that their whole existence is within the host. Symbiosis, in this context, does not judge whether the interaction is net beneficial or deleterious to the parties. Rather, it is a value-free term, in the sense of de Bary (1879), that reflects 'living together'.

Why should a book on insect infection and immunity carry a chapter on symbionts? There are two main reasons. First, recent work has indicated that some of these symbionts are themselves involved in defence of the host against pathogens and parasites. Whereas these bacteria are not part of the classical immune armoury as discussed elsewhere in this volume, recent studies have led to their emergence as important components of resistance to pathogens in aphids and elsewhere.

As such, they will have important effects on the ecological and evolutionary dynamics of host-pathogen interactions. Second, because bacterial symbionts live within an insect with a 'fully loaded' immune system, they represent an interesting area of interaction that may be important in the design and evolution of insect immune systems. For inherited symbionts that are parasitic, they represent antagonists that the immune system is selected to detect and remove. However, for symbiotic bacteria that are beneficial (indeed, necessary) to the host, selection must act on the host to maintain a fully functional immune system while not killing off its partner. We will discuss how this occurs, and how possession of a necessary symbiont might affect the evolution of the immune system which runs alongside it. We will raise the hypothesis that immune-system evolution may be best understood not solely as a product of antagonistic coevolution with parasites, but is additionally subject by the need to accommodate a changing array of beneficial partners. In this chapter, we first review briefly the diversity of insect-symbiont interactions, before examining these two topics in turn.

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