The events leading to this book started a quarter of a century ago, and since then it has very much been a story of one thing leading to another. It was in about 1980 that we first conceived the idea of a crop-oriented identification and information guide to the world’s aphids.  We were motivated in that project by two thoughts. First, it seemed evident to us that it would be very useful to adopt a world scale for the book, because the same main crops are grown on all continents, and pest aphids are rather good at eventually finding them wherever they are grown. Second, relatively few aphid species are pests, and those that occur on any one crop tend not to be closely related, so that it is possible to compile relatively simple keys for their identification. Thus, Aphids on the World’s Crops came into being (1984), and was well-enough received to be followed by a CD-ROM (1998) and a second edition (2000).

            These publications included aphid pests of fruit trees, but they did not include aphids on trees grown commercially for their timber, even though the Aphidoidea include some of the most important pests of temperate softwoods and hardwoods. We had good reason to exclude tree-dwelling aphids, as we knew that compiling keys to these was a very different task, needing to distinguish between many closely-related species and to compile accounts that were essentially, if rather superficially, world revisions of major aphid genera.  Eventually, however, we produced Aphids on the World’s Trees  (1994), with aphids listed and keyed according to tree genera. The subject matter - the trees as well as the aphids feeding on them - forced us to adopt a far more comprehensive approach, as we could find no justification for including some species – both of trees and of tree-dwelling aphids - and omitting others. There was also no good reason to exclude such tree genera as Prunus and Malus, so there was some overlap with the crops book, compounded by the fact that many pests of field crops exhibit host alternation and migrate to trees for their sexual phase.

            About 40% of the world’s aphid fauna (1760 species in 355 genera) live wholly or partly on trees. So, having completed this task, we started to contemplate whether the other 55% living on herbaceous plants and shrubs could be treated similarly (the host plants of the remaining 5% are unknown). The utility of a complete host plant-oriented treatment of the world’s aphids – something as yet unavailable for any major group of plant-feeding insects – seemed undeniable, and this provided motivation as well as lending a certain inevitability to the project, but we had no illusions about the task ahead. The number of species involved was in reality about 70% of the total world aphid fauna rather than 55%, because of those that host-alternate between trees and herbaceous plants, and a significant proportion were little-known species requiring consultation of original literature. After the first year, we had not even completed keys to aphids on plant genera beginning with “A”, stuck on Artemisia which is amazingly host to 260-plus aphid species, and seriously wondering whether the task could ever be finished. But one of us kept compiling host lists and the other writing keys, and gradually through many more years the project progressed towards a conclusion.

            One factor spurring us on was the knowledge that we were in a uniquely advantageous position to do such work. To hand was a collection of about 600,000 microscope slides of aphids, which is probably the largest, certainly the most representative, and perhaps also - we like to think - the best-curated collection of this group of insects in the world. Sitting on top of the long double-decker row of cabinets containing these slides is a double row of box files containing reprints or copies of about 95% of the taxonomic papers about aphids ever written, some dating back more than 100 years, and sharing the same bulding is the world’s largest library of books and journals relating specifically to imsect taxonomy. With such a unique resource at our fingertips, should we not do our best to find a way make all this information more readily available?

            Museum collections and libraries are essential resources, but they do not of course in themselves ensure good taxonomy. Taxonomy strives to name and classify organisms in such a way as to truly reflect their phylogenetic relationships – a fundamental requirement if we are to understand how organisms have evolved to live and interact with each other. As in many branches of biology, it is mainly a matter of correctly interpreting variation. In practice this inevitably means morphological variation, because the idea that adequate molecular data will ever be available to construct molecular phylogenies and define meaningful boundaries for all the thousands of taxa at the species level is still a pipe-dream.

Of course, morphological variation has the big disadvantage of being greatly influenced by the environment in which an organism develops. Different environmental factors , e.g. host plant, stress, humidity and temperature, affect morphology in different ways, and in aphids their environmentally- conditioned polymorphism (polyphenism) adds a further complication, because under certain conditions forms intermediate between two morphs may be produced. Such is the variation within aphid species that its correct interpretation requires a collection large enough to contain many specimens of each species, including both apterous and alate morphs, and many samples from different localities and seasons.

The correct interpretation of variation also requires lots of acquired knowledge and experience, because the various ways in which morphological features interact with the environment not only have to be recognised, but also viewed and made sense of in the context of the probable biology of the species – life cycle, host plant relationships, polymorphism – based on knowledge of the genus or species group to which it belongs. Species in some groups, e.g. Hormaphidinae and Pemphiginae, and some of the host-alternating Aphidinae, have completely different morphology on primary and secondary hosts, such that different morphs of one species have often been described in different genera. Some characters such as the relative lengths of antennal segments vary according to temperature, others such as the shapes and lengths of hairs vary more according to humidity, and some characters can differ greatly between alatae and apterae, so that intermediates exhibit a wide range of variation. All these different aspects of aphid variation provide traps for the unwary.

Between us we have nearly 100 years’ experience of working with aphids, which has perhaps made us more aware of the potential pitfalls, and of ways to avoid them. We hope therefore that we have produced a work that will be a helpful and reliable tool for both the newcomer to the world of aphids and to the more practised researcher.  However, such experience also makes us very aware of our own fallibility, and we will publish this work with near certainty  that, like its predecessors, it will contain some glaring errors. We can only hope that there are not too many of them. We would be grateful for notification of errors, omissions and difficulties with the keys, especially if supported with slide mounted specimens.

            This book would not have been possible without the BMNH aphid collection, and the many people who have helped make it what it is today. A list of all those who have donated slides or assisted with curation would be a very long one, and we can only here mention major contributors over the years. The largely but by no means exclusively European collections of F. Walker, G.B. Buckton, F.V. Theobald, J.P. Doncaster and H.L.G. Stroyan, and the European and African collections of W.J. Hall, were massively enhanced by the D. Hille Ris Lambers bequest in 1984 which added much type material.  That North American aphids are so well represented is mainly due to specimens and slides donated by others who are no longer with us; E.O. Essig, G.F. Knowlton, H.G. Walker, J.O. Pepper, C.F. Smith and A.G. Robinson. Many other aphid taxonomists have donated or lent specimens, sent copies of their publications, and been always ready to provide assistance, advice and unpublished data. Specifically we would like to mention S. Barbagallo, S. Chakrabarti, C.-k. Chan, S.K. David, A.K. Ghosh, S.E. Halbert, S.H. Hodjat, J. Holman, R. Kh. Kadyrbekov, M. Miyazaki, J.M. Nieto Nafría, W.H. Paik, N.F. Pashtshenko, G.-x, Qiao, F.W. Quednau, G. Remaudière, M. Sorin, A.V. Stekol’shchikov, M.B. Stoetzel, H.L.G. Stroyan, D. J.Voegtlin and G.-x. Zhang. On the BMNH staff, J.H. Martin’s collecting trips have added valuable new specimens to the collection from four continents, and P.A. Brown has rescued much type and other unique material that would otherwise have been lost, by skilful remounting and restoration of slides.

The last two years’ work on this book was facilitated by an Emeritus Research Fellowship to R.L.B. from the Leverhulme Trust.