It is obviously impossible for one person to be familiar with all the taxa ofbees. I have examined specimens ofvir-tually all genera and most subgenera, but there are without doubt species that do not agree with the characters that I have chosen to use in the keys or descriptive comments. I have often depended upon publications by others for characters, even though I realize that this can be dangerous. I hope that the classification, keys, comments, and literature citations in Sections 36 to 121 will provide a useful summary of the world's bee taxa and an introduction to melittology, even though I am aware that keys will fail for some species and that frustrations will be numerous, e.g., when keys require both sexes or present other difficulties. When one has a single specimen, and the key requires characters from both sexes, what does one do? At least the key indicates some characters that need to be examined. Used along with other sources of information, such as the descriptive comments, even a very imperfect key can often be found useful.
The synonymy for each subgenus, or genus if it is not divided into subgenera, are thought to be complete, in the sense that all synonymized genus-group names are cited, with the type species indicated for each. For some names, the details of nomenclatural problems summarized here were presented more fully in a list of genus-group names (Michener, 1997b).
To satisfy taxonomic practices, indications of significant nomenclatural changes are appended to the synonymies. New synonyms are indicated as such, as are changes in status (subgenus to genus, genus to subgenus). It is presumptuous to label as new synonyms those sub-generic names that for purely subjective reasons are not accepted (not different enough, too few species, etc.). I have indicated such new synonymy merely as part of the necessary program of keeping track of names.
Sometimes, comments following a synonymy deal with nomenclatural matters not fully explained in the formal context of a synonymy.
For brevity, the descriptive comments offered for a taxon often do not duplicate characters indicated in the keys, except as their discussion may be needed. Thus to obtain all the descriptive material on the bees constituting a given subgenus, for example, one must read the comments (if any) in the subgenus text, the key to subgenera, the descriptive material on the genus, the key to genera, and so on to the higher taxa.
It is impossible and probably not even desirable in an account of a large group like all the bees to attempt to describe the same features for all taxa, as would normally be done in a revisional study. For all bees, such an approach would require impossibly long descriptions that would tend to obscure the crucial diagnostic characters. Instead, I have tried to note a few particularly distinctive characters in each case. In addition, because it is often useful, I indicate size (body length). Because male genitalia, S7 and S8 (the hidden sterna), and often other sterna are usu ally complex, difficult to describe, and highly diagnostic, I have included references to works where these structures are illustrated. For this purpose I frequently do not refer to papers containing one or a few species descriptions that are illustrated, but in other cases, especially if there are few good illustrations for a genus, I do cite such papers. I frequently mention variation among species, hoping to call characters to the attention of those who will study at the species level. In both keys and descriptive comments there are sometimes species mentioned as exceptions to a cited character, or as having a particular unusual feature. I have not always listed all such species; I usually mention just one or two to give an idea ofvariations among species.
The descriptive material for each terminal taxon (genus, or if it is divided into subgenera, then subgenus) is followed by a paragraph marked with a ■, starting with statement of range, then proceeding to number of species and references to revisions or keys to species. For the range I usually list the countries, provinces, or states that margin the range, not those that fall in the middle. When a city has the same name as a province or state, the meaning is the province or state. For simplicity, words like "palearctic" or "neotropical," representing faunal regions, are often used in place of or in addition to names of countries. Such regions are indicated in Section 26, in the text associated with Table 26-1. Important points are that nearctic includes the whole temperate part of North America, including the Mexican plateau, and that oriental is used for the tropical orient, whereas temperate parts of China and Japan are palearctic.
The number of species in a taxon is not always precise, because of variations in the state of the literature for different taxa. Sometimes I have merely used the number of species names proposed up to the time of a published listing. If there has been a recent revision, I use the number of species recognized in the revision. Among larger bees, and among all bees in some areas, the number of species names proposed is usually greater, sometimes much greater, than the number of species recognized by a reviser. But especially among small bees in some areas, a revision may include new species that greatly increase the number of species previously recognized. Examples are Toro and Moldenke's (1979) account of Chilean Xe-romelissinae, which increased the number of known species from 10 to 49; and Timberlake's revisional papers (1954-1980) on North American Perdita (including Macrotera), which increased the number of species in the USA from 113 to 498. Whether a revisional study will increase or decrease the number of species recognized in a taxon cannot always be predicted. I believe that full knowledge of bee species worldwide would increase the number of species recognized, partly by the discovery of entirely new ones and partly by subdividing currently recognized species. But because much synonymy remains to be discovered, the increase in total number of species will be less than the number of new species. Given this unsat isfactory state of affairs, I have not felt it worthwhile to be meticulous about adding the numbers of new species described in the years following a revision or listing, except for small genera. My totals are good enough to give an idea of the numbers of named or recognized species. Palearctic genera revised by Warncke present a special problem because, at least in the few groups with which I am familiar, his "species" often included several similar but distinct species. The result is much false synonymy. The palearctic fauna is the most difficult in the world to study because of the lack of catalogues and continent-wide revisions combined with the great number of species, many of them named long ago with minimal descriptions in diverse languages and no illustrations. I am certain that my estimates of numbers of species often suffer from these problems.
Following the paragraph on range, number of species, and revisions, if any, are sometimes paragraphs on species groups within the genus or subgenus, or on floral biology and nesting biology if there is anything interesting to say. At least references to accounts of these matters are included when information is available. References to original papers are provided in most cases, but for taxa whose biology has been much studied, such as Halictinae and Meliponinae, I often refer to review papers.
The references to revisions or biology appear under the highest relevant taxon. For example, if a family has been revised, this fact is noted in the account of that family but is not necessarily repeated under each included subfamily, tribe, and genus. Likewise, the keys and other useful material in major faunal works and catalogues on bees as a whole (Table 32-1) are not referenced under each included taxon.
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