Estimates of the total number of eukaryotes range up to about 2,000,000 species. The invertebrates constitute more than 95% of these and include a vast diversity of organisms, ranging from unicellular protozoans to the more complex echinoderms and protochordates. Insects are by far the largest group of animals within the invertebrates and the great interest in research on these animals arises from their roles as vectors for many human and animal diseases, such as malaria, and the serious harm done by insects to crops and food.

Thus an interest in the diseases, pathogens and immune responses of insects has been a longstanding research interest. Even though one might think that since most insects are short-lived they do not need a sophisticated immune response, this is not the case, and instead insects and other invertebrates have been found to possess very complex and efficient immune systems. To mention just one aspect of insect immunity; the so-called proph-enoloxidase activating system and the coagulation system are activated by minute amounts (picogram) of carbohydrates from bacteria or fungi, and hence these systems are in practical use to detect bacteria and fungi as contaminants or as pathogens. It is also noteworthy that these two systems are much more sensitive in detecting products from microorganisms than is the vertebrate complement system, meaning that invertebrate animals are definitely much more efficient in detecting and responding to pathogens than vertebrates.

Professor Hans G. Boman and colleagues in 1972 were the first to clearly demonstrate that an insect, namely Drosophila, could respond to a challenge with dead or live bacteria, with the specific induced synthesis of antimicrobial substances. Later in 1981, the same researchers were for the first time able to sequence an antimicrobial peptide (AMP), which was named cecropin, since it was isolated from the moth Hyalophora cecropia. From 1981 and onwards, there was an intense interest in deciphering the pathways leading to the production of AMPs. This led to the breakthrough by Bruno Lemaitre and Jules Hoffmann and colleagues in 1996, whereby the pathway to the production of the antifun-gal peptide drosomycin was characterized. This opened the possibility of detailed genetic studies of the signaling pathways involved in the production of AMPs. Subsequently, the completion of the Drosophila genomic sequence in 2000 made it possible for an even more powerful molecular genetic analysis of the immune system in this insect.

Basically two lines of research on insect immunity have co-existed. One mainly focused on gen-omic, transcriptomic and proteomic analysis of the immune responses; the other line of studies being concentrated on ecological immune studies. Paul Schmid-Hempel, Joachim Kurtz, Sophie Armitage and Jens Rolff initiated several meetings between 2004-2007, where molecular biologists and ecolo-gists met and presented their research with the main aim to bring researchers representing these lines to come together to promote discussions and collaboration. This book edited by Stuart Reynolds and Jens Rolff follows this tradition and is a mix where several chapters are devoted to molecular immune studies and others to different aspects of ecoimmunity. Recent studies have shown that insects and other invertebrates may have the capacity for immune priming and also show specificity in their immune responses. It has also been shown that there seem to be specific immune defences to different bacterial species and that the immune response can vary depending on feed, time, presence of symbionts and the natural flora of the gut. Thus it seems as if these lower animals may have a very complex network to combat different pathogens and to avoid being killed by these pathogens. In order to discover how these networks operate, and to reveal whether some sort of adaptive immunity is at work in invertebrates, collaboration between molecular immunologists and more ecology-oriented entomologists will be necessary.

With this exceptionally broad panel of international authors and by the diversity of subjects, this book is likely to be a new and refreshing outlook on the exciting field of insects as well as invertebrate immunity.This book consequently provides an excellent opportunity to get an overview of insect immunity and its implications for the well being of insects in their natural habitats and for their natural behavior. It is well suited for scientists, post-doctoral fellows and post-graduates who wish to get a stringent overview of several aspects of insect immunity.

Uppsala March 2009 Kenneth Sôderhall Uppsala University Sweden

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