Beetle Diversity

Although beetles share characters supporting their common evolutionary origin, remarkable variations have evolved on the 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 ceram-bycid 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 shortlived 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, 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 1.38 mg (an 85% weight loss!).

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 ptiliids from Sri Lanka, the males produce spermatozoons that range in length from 220 to 600 |lm; 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 spermatozoons recorded filling this structure. The length of the female spermathecae 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 spermatozoons and female spermathecae ensure reproductive isolation because of biomechanical incompatibilities associated with any attempted interspecific matings.

Beetles are among the earliest diversifying groups of the Holometabola. Together with the orders Megaloptera, Raphidioptera, and Neuroptera, they are classified in the superorder Neuropterodea. 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, Polyphoga, 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. Recent studies

TABLE I Classification of Beetle Suborders, Series, Superfamilies, and Families of the Order Coleoptera

Suborder Archostemata

46. Dryopidae

Cupedoidea

47. Lutrochidae

1. Ommatidae

48. Elmidae

2. Cupedidae

49. Limnichidae

3. Micromalthidae

50. Heteroceridae

Suborder Adephaga

51. Psephenidae

Caraboidea

52. Callirhipidae

4. Gyrinidae

53. Eulichadidae

5. Haliplidae

54. Ptilodactylidae

6. Hygrobiidae

55. Chelonariidae

7. Amphizoidae

56. Cneoglossidae

8. Dytiscidae

Elateroidea

9. Noteridae

57. Artematopidae

10. Trachypachidae

58. Rhinorhipidae

11. Carabidae (incl. Rhysodini, Cicindelini)

59. Brachypsectridae

Suborder Myxophaga

60. Cerophytidae

12. Torridincolidae

61. Eucnemidae

13. Cyathoceridae

62. Throscidae

14. Hydroscaphidae

63. Elateridae

15. Microsporidae

64. Plastoceridae

Suborder Polyphaga

65. Drilidae

Staphyliniformia

66. Omalisidae

Hydrophiloidea

67. Lycidae

16. Hydrophilidae

68. Telegeusidae

17. Sphaeritidae

69. Phengodidae

18. Synteliidae

70. Lampyridae

19. Histeridae

71. Omethidae

Staphylinoidea

72. Cantharidae

20. Hydraenidae

Bostrichiformia

21. Ptiliidae

Derodontoidea

22. Agyrtidae

73. Derodontidae

23. Leiodidae

Bostrichoidea

24. Scydmaenidae

74. Jacobsoniidae

25. Silphidae

75. Nosodendridae

26. Staphylinidae

76. Dermestidae

Sciritiformia

77. Endecatomidae

Scirtoidea

78. Bostrichidae

27. Scirtidae

79. Anobiidae

28. Eucinetidae

Cucujiformia

29. Clambidae

Lymexyloidea

Scarabaeiformia

80. Lymexylidae

Scarabaeoidea

Cleroidea

30. Lucanidae

81. Phloiophilidae

31. Passalidae

82. Trogossitidae

32. Trogidae

83. Chaetosomatidae

33. Glaresidae

84. Cleridae

34. Pleocomidae

85. Acanthocnemidae

35. Diphyllostomatidae

86. Phycosecidae

36. Geotrupidae

87. Melyridae

37. Ochodaeidae

Cucujoidea

38. Ceratocanthidae

88. Protocucujidae

39. Hybosoridae

89. Sphindidae

40. Glaphyridae

90. Nitidulidae

41. Scarabaeidae

91. Monotomidae

Elateriformia

92. Boganiidae

Dascilloidea

93. Helotidae

42. Dascillidae

94. Phloeostichidae

43. Rhipiceridae

95. Silvanidae

Buprestoidea

96. Passandridae

44. Buprestidae

97. Cucujidae

Byrrhoidea

98. Laemophloeidae

45. Byrrhidae

99. Propalticidae

(continues)

(continues)

TABLE I (Continued)

100. Phalacridae

101. Hobartiidae

102. Cavognathidae

103. Cryptophagidae

104. Lamingtoniidae

105. Languriidae

106. Erotylidae

107. Biphyllidae

108. Byturidae

109. Bothrideridae

110. Cerylonidae

111. Discolomidae

112. Endomychidae

113. Alexiidae

114. Coccinellidae

115. Corylophidae

116. Latridiidae Tenebrionoidea

117. Mycetophagidae

118. Archaeocrypticidae

119. Pterogeniidae

120. Ciidae

121. Tetratomidae

122. Melandryidae

123. Mordellidae

124. Rhipiphoridae

125. Colydiidae

126. Monommatidae

127. Zopheridae

128. Perimylopidae

129. Chalcodryidae

130. Trachelostenidae

131. Tenebrionidae

132. Prostomidae

133. Synchroidae

134. Oedemeridae

135. Stenotrachelidae

136. Meloidae

137. Mycteridae

138. Boridae

139. Trictenotomidae

140. Pythidae

141. Pyrochroidae

142. Salpingidae

143. Anthicidae

144. Aderidae

145. Scraptiidae Chrysomeloidea

146. Cerambycidae

147. Chrysomelidae Curculionoidea

148. Nemonychidae

149. Anthribidae

150. Urodontidae

151. Oxycorynidae

152. Aglycyderidae

153. Belidae

154. Attelabidae

155. Caridae

156. Ithyceridae

157. Brentidae

158. Curculionidae

Source: Modified from Lawrence, J. F., and Britton, E. B. (1994). "Australian Beetles." Melbourne University Press, Melbourne, Australia.

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 has 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.

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 brachypterous condition eliminates the possibility of winged dispersal by individuals and is associated with increased speciation and endemism, 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, Ceramby-cidae, 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.

FIGURES 1-2 Fossil beetles. (1) Moravocoleuspermianus (Tshekardocoleidae: Protocoleoptera, Permian). (© Czech Geological Survey.) (2) Notocupes picturatus (Cupedidae: Coleoptera, Triassic).
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