The author of a compilation like this would not get far without outside help. I am indebted to many people, and I would like to thank them all: Dr. Zsolt Bálint (Budapest) who kindly read a draft of one of the earlier typescripts and sent me invaluable advice on many points touched upon, especially on Lycaenidae; Professor Konstantin A. Efetov in Simferopol (Crimea) who shared some of his expertise in Russian Lepidoptera and lepidopterists with me; Kurt Johnson (New York) who generously supplied information on the revision of the South American Blues; D. Barton Johnson without whose confidence that the butterfly of The Eye can be identified I would not even have attempted to; Robert Dirig (Cornell University) who helped me with plentiful material on the Karner Blue; Ms. Ursula Frerichs and Ms. Sabine Toussaint of the University of Hamburg who unlocked for me the strong boxes that hold the more valuable books at the Entomological Institute; Dr. Wolfram Mey and Ms. Viola Richter at the Museum für Naturkunde in Berlin who let me settle some of the trickier questions in their library's invaluable holdings; Dr. Klaus Rohlfien and Ms. Jutta Moebert at the German Entomological Institute in Eberswalde who helped me to material from that institution's library and archives; Ron Leuschner of the Lepidopterists' Society who supplied me with some information I had almost despaired of ever finding; Julian P. Donahue (Los Angeles) who sent me biographical material and looked into the Inyo Blue problem; Dr. Peying Fong (Baltimore) who took the trouble to scrutinize my alien English; Roger C. Clark (London) who kindly drew my attention to a number of butterfly references in Nabokov's fiction that I had missed. Last but not least, I am grateful to Professor Pierre Goeldlin, Dr. Daniel Cherix and Dr. Michel Sartori of the Musée cantonal de Zoologie in Lausanne for the interest they took in my ever expanding manuscript and for a most pleasant association which consisted chiefly in a steady flow of faxes. Since 1998, the search for information on Lepidoptera has been greatly facilitated by a number of Web sites, especially by Markku Savela's magnificent Finnish database on butterflies and moths that in 2001 had about 21,000 taxa, with referenced taxonomic and distributional information and frequently also with images and links to images at other sites http://www.nic.funet.fi/pub/sci/bio/life/warp/lepidoptera-index-a.html).
A very special thank-you must go to Brian Boyd. He not only drew my attention to a number butterflies and entomologists I had overlooked, passed on Nabokov's butterfly glosses in his copy of Ada and pointed out a number of solecisms and contradictions in the manuscript. I was very fortunate to have such a close and exacting reader. Even more importantly, I probably would never have started out on the revision of the Guide without his encouragement and even insistence, and I certainly would not have finished it. I don't think I have succeeded in allaying his misgivings about my discussion of the species concept; in any case they helped me to voice and to substantiate my opinions much more clearly.
Unlike in other fields of study, in entomology there is no single institutional library where one could hope to find all or nearly all the pertinent literature. There are important monographs, manuals and journals that you either will not find anywhere or where you didn't expect them at all, so one has to try one's luck in many places. I did so at the Senckenberg Library in Frankfurt, the Museum König in Bonn, the Zoological Institute of the Freie Universität Berlin, the Naturwissenschaftliche Sammlungen of the Stiftung Stadtmuseum Berlin, the Naturkundemuseum in Berlin, the German Entomological Institute in Eberswalde, the Zoological Museum in Kiel, and abroad at the BioSciences Library of the University of California at Berkeley, the Comstock Entomological Library at Cornell, the Harvard Museum of Comparative Zoology and the Biology Library of Stanford University. Also I would like to thank the documentarists at the Natural History Museum in London and the city archives of Geneva who kindly answered my queries.