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Beetle Diversity

Although beetles share characters supporting their common evolutionary origin, remarkable variations have evolved from the basic beetle theme. For example, adult body size ranges from the 0.4-mm-long Nanosella fungi ptiliid feather-winged beetles of North America to the 200-mm-long Titanus giganteus cerambycid long-horned beetles of South America. A rough estimate based on maximum dimensions for adult length, breadth, and depth puts the disparity in volume at a factor of 2.8 X 107. Life cycles also can vary in extraordinary ways, depending on the larval food resources used for development. The mushroom-inhabiting aleocharine staphylinid Phanerota fasciata completes three instars in 3.2 days at room temperature. Even more impressive, Anisotoma—round fungus beetles of the family Leiodidae—can complete larval development on ephemeral slime mold fruiting bodies in as little as 2 days, making them arguably the fastest developing beetles yet recorded. Conversely, C. V Riley, the first entomologist of the U.S. Department of Agriculture, reported that a larva of the dermestid carpet beetle, Trogoderma inclusum. survived for 3.5 years in a tight tin box. These larvae feed on the dried proteinaceous matter in animal remains, and even if Riley’s larva had started with a tin full of insect specimens, the feat of solitary confinement is remarkable. Trogoderma larvae can even molt to a smaller size under starvation conditions, and then regain size by progressively molting when food returns. Stan Beck found that mature larvae molted retrogressively eight times during a year of starvation, dropping from an initial weight of 9.24 mg to a final, svelte 1.38mg (an 85% weightloss!).
Dramatic variation in reproductive capacity is also observed across the Coleoptera. An abundant plant pest such as the chrysomelid northern corn rootworm, Diabrotica barberi, can colonize cornfields and build populations quickly, since each female lays on average nine clutches of eggs, spaced 6 days apart, totaling 274 eggs over the reproductive period. At the opposite extreme we once again find the diminutive, feather-winged Ptiliidae. In eight species of Bambara ptifiids from Sri Lanka, the males produce spermatozoa that range in length from 220 to 600 |im; the largest size being more than two-thirds the length of the adult male producing them. After mating, these giant sperm pack the female spermatheca, with up to 28 spermatozoa recorded filling this structure. The length of the female sper-mathecae of various Bambara species is consistent within species and varies in proportion to the length of the complementary male sperm,whereas the diameter of the spermathecal duct varies in proportion to the diameter of the sperm. The female also invests heavily in her progeny, maturing one relatively giant egg in her abdomen at a time. The highly complementary male spermatozoa and female spermath-ecae ensure reproductive isolation because of biomechanical incompatibilities associated with any attempted interspecific mattings.
Beetles are among the earliest diversifying groups of the Holometabola. They form a branch on the Tree of Life together with the Neuropteroidea (Megaloptera, Raphidioptera, and Neuroptera). The order Coleoptera is divisible into four major lineages, which are recognized as the suborders Archostemata, Adephaga, Myxophaga, and Polyphaga (Table I). Present-day diversity among the four coleopteran suborders is highly skewed toward the Polyphaga. Taking the numbers of beetle species estimated for Australia, John Lawrence and Everard Britton calculated that Archostemata (9 species) make up 0.03% of the Australian beetle fauna, Adephaga, with 2730 species comprise 9.6%, Myxophaga, with 2 species (0.007%), and, with 25,600 species, Polyphaga, dominates at 90.4% of the fauna. Extrapolating these figures to the estimated world total of 350,000 described beetle species suggests that Polyphaga would account for more than 300,000 species.
Consensus concerning the phylogenetic relationships among all four suborders has yet to be achieved. Recent summaries of morphological data and separate efforts using molecular sequence data reach different conclusions based on the character types and sets of taxa included. Most studies agree that the Archostemata are the sister group to the other three suborders. The position of Myxophaga remains ambiguous, though Beutel and Haas’s comprehensive morphological analysis places them as the sister group to Polyphaga.
The burgeoning discoveries of beetle diversity throughout the course of modern scientific endeavor have begged the question, “Why?” The noted geneticist J. B. S. Haldane, in a lecture on the biological aspects of space exploration, stated that “the Creator, if he exists, has a special preference for beetles, and so we might be more likely to meet them than any other type of animal on a planet that would support life.” No single answer provides the definitive biological explanation for the present-day preponderance of beetle diversity. A number of answers are consistent with the pattern of diversity, with some better supported by the comparative totals of species in the different suborders and the major families.
First, the origin of Coleoptera, relatively early in the Triassic compared with other holometabolous orders, provided ample time for diversification. Having been in existence throughout the breakup of Pangaea, which started in the Jurassic, distinct beetle biotas have evolved in place on the various continental fragments of that supercontinent. A remarkably high percentage of the early coleopteran lineages persist today.
Second, beetle diversification has been explained as the result of a successful body plan incorporating protective elytra and a flexibly articulating prothorax. Although beetles are generally not regarded as fast or agile fliers, representatives of various beetle families have routinely colonized the most remote island systems in the world. In many families, the outward appearance and function of the walking beetle has been maintained, while the metathoracic flight wings have been reduced to nonfunctional straps or vestigial flaps. This brach-ypterous condition eliminates the possibility of winged dispersal by individuals and is associated with increased speciation and ende-mism, most often in ecologically stable, geographically isolated montane, desert, or island habitats.
Third, as representatives of the Holometabola, the larval and adult beetle life stages have been morphologically decoupled via the intervening pupal stage. Larvae may exhibit morphological specializations not observed in the adult stages, and may live in particular microhabitats not primarily occupied by the adults.Fourth, the early diversification of beetles in the Jurassic placed many lineages in prime position to exploit ecological opportunities associated with the Cretaceous diversification of flowering plants. Many of the largest families of Polyphaga (e.g., Buprestidae, Scarabaeidae, Chrysomelidae, Cerambycidae, and Curculionidae) include lineages that are intimately associated with angiosperms. These host plant associations are based on the use of various portions of the particular species or sets of species of flowering plants as larval or adult food. In addition, many other beetle groups use fungi as a food source, and fidelity to fungi of particular types is not atypical. The ability to specialize along with their larval and adult hosts has clearly been associated with extensive speciation across the Coleoptera.