Beetles

Beetles make up the large, extremely diverse order Coleoptera of the class Insecta, and comprise the largest single group of animals on Earth. There are at least 250,000 species of beetles, compared to the 5,000 known species of mammals. The weevil family of beetles alone contains about 50,000 species, and is the largest family in the animal kingdom. Thus, the order Coleoptera, representing about 40% of the known insect species, contributes greatly to making the insects the largest class of the largest phylum—Arthropoda. Arthropods are thought to have first evolved as long as 500 million years ago in Precambrian times, while the most primitive insect fossils date to the rocks of the Middle Devonian period about 350 million years ago. Coleoptera are thought to have evolved in the early Permian period about 225-280 million years ago, and were common even before the age of reptiles.

Beetles are found in virtually all climates and latitudes throughout the world except at very high altitudes or in regions with extreme temperatures, e.g. the Antarctic. Most species of beetles occur in the tropics, but fewer individuals of a particular species are generally found in tropical regions rather than in temperate areas.

The success of the beetles is due to at least three important characteristics. First, they undergo complete metamorphosis (egg, larva, pupa, adult), with larval and adult stages usually living in different places and eating different food. This division greatly expands the number of ecological niches and food available to these insects. Second, the front pair of wings is modified into a hard cover (the elytra) that protects the soft body underneath. Third, most beetles have mouth parts capable of chewing a wide variety of solid foods. Some beetles, however, have mouth parts modified for sipping sap and nectar.

The front pair of wings, modified into horny covers (elytra), hide the rear pair of wings and abdomen, and their inner edges appose each other, creating a straight line down the back of the insect. The elytra form a rigid, closely interlocking sheath that covers the mesothorax and metathorax, and most of the abdomen. (The name Coleoptera is derived from the Greek word koleos, meaning sheath.) The perfect alignment of the edges of the elytra form the characteristic, straight line that seems to split the back of the beetle, and gives these insects their common name (beetle from the German word bheid, meaning to split).

Beetles are found on vegetation, under bark, stones, and other objects, as well as almost anywhere on or in the soil, rotting vegetation, dung, and carrion. They vary widely in size and appearance, and many have noteworthy behavior. Some beetles (e.g., Lampyridae) produce light, while others (Cerambycidae) can stridulate, that is, they can produce sound. Large beetles usually make a loud noise during flight, and some, such as the scarab beetles, have a bizarre physical form.