To

Genitalia Andreniformis

1.5. Relationships among species of Recent honey bees, genus Apis, showing important variations in tarsomeres and male genitalia. Relationships from Engel and Schultz (1997).

entirety of their ranges, distinct from regional morphotypes or ethotypes, and that may be reproductively isolated at fine geographical scales?

Perhaps the most dramatic development of variation is seen in the Cape honey bee, Apis mellifera capensis. This subspecies is facultatively parthenogenetic and a social parasite on colonies of other honey bee subspecies. While A. mellifera capensis is still reproductively compatible with other subspecies of A. mellifera, gene flow is asymmetrical and the Cape bee dominates during introgressions (Johannsmeier,

1983; Hepburn and Radloff, 1998). This may be a rare example of incipient speciation. Similar cases, but not involving the evolution of parasitic behavior, occur in the widely distributed A. cerana and A. dorsata, in which great variation is related to local differences in habitat such as elevation. Apis cerana nuluensis is often considered specifically distinct because it is found only in the mountains of Sabah above 1800 m, with mating flights temporally separated from the overall A. cerana population occurring at lower elevations (Otis, 1996). Workers forage together, and aside from remarkably variable differences in coloration correlated with latitude or elevation, there are no derived traits to support species status of A. cerana nuluensis. These morphs are all derivatives of the larger, ancestral A. cerana, thereby leaving the mother species paraphyletic if the isolates themselves are recognized as species (e.g., Tanaka et al., 2001).

Species of Apis, however, unlike higher taxonomic levels, are almost never monophyletic. Indeed, based on DNA evidence, Apis nigrocincta (which lacks fixed, morphologically diagnostic traits) is a derivative of A. cerana (Smith et al., 2000), much the way Drosophila sechellia and D. mauritiana appear to be derivatives of D. simulans. Differences in nest architecture occur between A. nigrocincta and A. cerana across their ranges (Hadisoesilo and Otis, 1998), which are congruent with differences in drone flights and morphomet-ric clusters (Hadisoesilo et al., 1995; Hadisoesilo and Otis, 1996). Apis koschevnikovi of Malaysia, Indonesia, and Borneo is reproductively isolated and genetically and morphologically distinct from other Apis species. This species is perhaps a more ancient example of peripheral specialization, being restricted to wet primary forests (Otis, 1996), while having derived from an ancestral cerana-group stock and becoming secondarily sympatric with A. cerana.

Teasing out subtle details that distinguish cryptic insect species is not an academic exercise, but it is a practical necessity in cases involving vectors of serious diseases or major crop pests. Controlling the diseases carried by cryptic species in the Anopheles gambiae complex or the Simulium damno-sum complex, for example, was completely confounded until the species were accurately defined, and slight differences in their biology were deciphered. Also, if most individuals can be grouped into discrete and diagnosable species, as in Drosophila and Apis (why should other insects be different?), this would have profound implications for evolutionary biology and systematics. Traditionally, species are believed to have formed gradually, through the steady accretion of small genetic changes, called phyletic gradualism. Studies on Drosophila have found good correlation between genetic distance and degree of reproductive isolation (Ayala et al., 1974; Coyne and Orr, 1989), supporting the view of phyletic gradualism. But if this were the standard mode for speciation, one would expect many examples of intermediates, individuals with features of D. melanogaster, D. simulans, or other species, or at least many species with great ranges of variation. The extensive genetic and phenotypic evidence from Drosophila and Apis indicates that there exist discrete groupings of individuals - species - though in some cases to define the groups this may require extensive data on mtDNA, courtship songs, swarming behavior, and other evidence. Perhaps species actually are "typological," contrary to Mayr (1942). Moreover, discrete groups would suggest that the time for the formation of a species is quick relative to its entire lifespan, which is consistent with the concept of punctuated equilibrium, but this is an area that still needs considerable exploration.

A great deal more, in fact, could be discussed about the exact nature of species, but to explore 400 my in the evolution of insects, we need to consider how many species of them presently exist.

HOW MANY SPECIES OF INSECTS?

Scientists know far more about (and spend vastly more money studying) the systematics of stars than the systematics of earthly organisms. Consequently, they have as good a knowledge of the number of atoms in the universe - an unimaginable abstraction - as they do of the number of species of plants and animals. -Robert May, 1992

Numerus specierum in entomologia fere infinitus et nisi in ordinen redigantur, chaos semper erit entomologia. [The number of species in entomology is almost infinite, and if they are not brought in order entomology will always be in chaos.]

-J. C. Fabricius, 1778, Philosophia entomologicaVI, section VI, para. 3 [translation by Tuxen, 1967a]

I have heard it stated upon good authority that 40,000 species of insects are already known, as preserved in collections. How great, then, must be the number existing in this whole globe! -W. Kirby and W. Spence, 1826

Insects are so diverse that their numbers are impressive even in the most parochial of places. Cockroaches, of course, are expected in New York City dwellings, but a quick entomological survey of a typical apartment can yield 20 or more species of arthropods (Volk, 1995). New species of midges, an ant, and various other insects are known throughout the eastern United States, some of which even occur in New York's Central Park, the most visited green space on earth. A new dwarf genus of arrupine millipedes, in fact, was discovered in Central Park in 2000. It is clearly introduced, probably from eastern Asia or western North America, but so far the genus is known only from Central Park (Foddai et al., 2003). In a forgotten study done in the 1920s, Frank Lutz of the American Museum of Natural History surveyed the insect species in a typical one-acre yard in the suburbs of northern New Jersey: he found 1,250 species. That was before we had refined concepts of species among the myriad tiny acalyptrate flies, parasitoid wasps, and staphylinoid beetles, so the number is probably at least 1,500 species. Another surprise about these kinds of studies is that there have been very few intensive surveys of the insects or terrestrial arthropods of natural areas (e.g., Proctor, 1946; Woodley and Hilburn, 1994), even though that type of study is so important to estimating how many species of insects exist.

One million species is commonly recited for the diversity of named living insects, but even this figure is ambiguous. Estimates range from 750,000 (Wilson, 1992) to approximately 1.4 million (Hammond, 1992), but the number appears close to 925,000 named species based on recent figures for the "big four" orders (Hymenoptera, Lepidoptera, Coleoptera, and Diptera) (Gaston, 1991; Resh and Cardé, 2003) (Table 1.1; Figure 1.6 ). Diptera is the only major group of insects where the world species have been catalogued within the last few decades, and it will be necessary for similar catalogues to be produced before accurate tallies of all described species are made. Proper species catalogues require tedious checking and verifying of old literature (names, dates, types, etc.) so it has attracted little effort, even though these are the very scaffold for other work in systematics.

What has engendered most of the discussion about insect diversity, though, are the estimates of total numbers of insect species, described and mostly undescribed. These estimates differ wildly, from approximately 2 million species (Hodkinson and Casson, 1991), to 8.5 million (Stork, 1988, 1996; Hammond, 1992) to 30 million or more (Erwin, 1982, 1983a). Other recent estimates place the number at approximately 5 million insect species globally (Gaston, 1991), which is within a much earlier estimate (Brues et al., 1954) of 3.75 million to 7.5 million species. In a time when advances in technology allow measurements of drifting continents (an average of 2.5 cm per year), the mean diameter of the earth (7,913 miles), or the mass of an electron (9.1 X 10-28 grams), one would expect more precision on species numbers. The discrepancies lie in how the estimates are made.

Erwin's (1982) estimate of 30 million species of insects is widely criticized, but in all fairness it was the first study to bring attention to the nebulous problem of total numbers of insect species. This work also exposed a whole new biota in the canopies of tropical forests (Erwin, 1983a,b, 1990), and led to similar studies by others in forests of southeast Asia (e.g., Allison et al., 1993; Stork, 1987, 1991, 1997) and elsewhere (reviewed by Basset, 2001). The basic technique for all these studies uses a fog of insecticide that is blasted into the canopy, which degrades quickly, and the insects that rain down into basins are then collected, preserved, and sorted later back in the laboratory. The original study by Erwin (1982) extracted arthropods out of the canopies of trees in Panama, and one particular tree, Luehea seemannii, was used

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