Coleoptera

(Beetles, Weevils, Fireflies)

James K. Liebherr

Cornell University

Joseph V. McHugh

University of Georgia

Beetle diversity so characterizes Earth that instead of telling future extraterrestrial colleagues we come from the "blue planet," we might better state that we come from the "beetle planet." Beetles comprise 25% of all described animals and plants, single-handedly making them the primary contributor to earth's biodiversity. The 350,000 described beetle species are members of largest order of life on Earth, Coleoptera.

Familiar beetles are known by various names including fireflies, ladybugs, june bugs, and weevils. The vast number of beetle species is reflected by a bewildering array of anatomical and biological diversity in the order. Coleoptera are represented in nearly all biogeographic regions and nonmarine habitats. Most adult beetles can fly; when not in use, however, the delicate flight wings are usually concealed beneath protective shell-like elytra, permitting beetles to utilize diverse resources and engage in a broad range of activities that otherwise would be restricted to either winged or wingless insects. Most beetles are herbivores, fungivores, or predaceous carnivores in the larval and adult stages. Many are considered to be serious pests of our homes, forests, crops, and stored products, whereas some beneficial species are regularly employed as biological control agents. Countless curious youngsters, including Charles Darwin, Alfred Russel Wallace, and Henry Walter Bates, have started their broader studies of biology through beetle collecting, as beetle species often are consistently found in specific sorts of habitats.

The technical name, Coleoptera, was coined by Aristotle to signify the hardened, shieldlike forewings (coleo = shield + ptera = wing). Although several other insect orders possess hardened forewings, beetles are considered to be a mono-phyletic assemblage based on their sum of shared evolutionary derivations that include the following:

1. A holometabolous life cycle, wherein the larval stages are developmentally separated from the adult by the pupal stage.

2. Possession of hardened forewings, called elytra, that abut medially. Flight is powered predominantly by the metathoracic wings, which are folded longitudinally and usually transversely to lie under the elytra when the beetle is walking or at rest. The mesothoracic scutellum is visible as a triangle situated medially between the bases of the two elytral halves.

3. A prothorax that is distinct from, and most often freely articulating with, the following mesothorax. The meso- and metathoracic segments are fused to form the pterothorax.

4. A generally depressed body shape, whereby the legs are situated on the ventral surface of the body. The leg bases, or coxae, are recessed into cavities formed by heavily sclerotized thoracic sclerites.

5. Abdominal sternites that are much more heavily sclerotized than the tergites. These sternites may close tightly against the lateral edges of the elytra, protecting the hind body from the attentions of predators and parasitoids.

6. Antennae usually with 11 or fewer segments.

7. Terminal genitalia that are not visible when in repose; that is, the male aedeagus and the female ovipositor are retracted into the abdominal apex when not in use.

Insects in several other orders may appear superficially similar to beetles. For example, various Hemiptera in the superfamily Pentatomoidea possess an enlarged triangular scutellum and heavily sclerotized forewings. However, these bugs can be distinguished by their beaklike suctorial mouth-parts, whereas beetles retain the more generalized mandibu-late mouthparts seen throughout orders such as Odonata, Orthoptera, and Hymenoptera. In addition, the forewings of Hemiptera always retain an apical membranous portion, whereas beetle forewings are consistently sclerotized throughout their length. Also, Dermaptera, or earwigs, exhibit quadrate forewings, looking much like the foreshortened elytra of staphylinoid, or rove beetles. Earwigs, however, exhibit a radial wing folding mechanism versus the transverse folding system of beetles, retain the presence of abdominal cerci, represented by large tonglike forceps at their abdominal apex, and do not undergo complete metamorphosis incorporating the pupal stage.

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