Beetles

The mouth parts, which allow beetles to utilize a wide variety of solid foods in their environment, and the elytra, which protect the hindwings, give beetles great survival advantages. Another factor that contributes to the enormous success of beetles is the fact that they undergo complete metamorphosis. Beetles pass through three distinct developmental stages—egg, larva (grub), and pupa—before becoming adults.

Beetles reproduce sexually, although a few species consist of females only and parthenogenesis sometimes occurs. The male reproductive organ is the aedeagus, a hard, tubelike structure that is inserted into the tip of the female's abdomen through the bursa copulatrix during mating. The female stores sperm in a saclike structure called the spermatheca until they are used to fertilize eggs.

The beetle larva hatches from an egg and feeds, growing until its burgeoning body splits the skin (cuticle). The larva crawls out of the old skin and forms a new one, a process called molting. This occurs several times, until the larva is mature.

Beetle larvae are always very different from adults in both form and habits. They usually have only chewing mouth parts even if as adults they develop siphoning or piercing mouth parts. Wings develop internally and are not evident until the pupal stage. Because larvae, pupas, and adults live in different places and eat different foods, they do not compete with each other.

The different larval forms of beetles reflect a wide variety of feeding habits and habitats. The predatory larvae of water beetles (dytiscids) and ground beetles (carabids) are slender or have gradually tapered bodies and long legs adapted to chasing prey, and large, slender mandibles for holding food.

The larva of the tiger beetle (Cicindelidae) lives in the ground, digging a burrow up to 2 ft (0.6 m) deep to avoid high temperatures in the subtropical and tropical environments. The head of the tiger beetle is large and is bent at right angles to the body. When the larva is poised vertically within the burrow waiting for passing prey, its lidlike head acts like a living plug flush with the surface. When a potential meal nears the burrow, the beetle springs out like a jack-in-the-box doing a partial back somersault to catch the prey in its jaws. Two barbed spines on the tiger beetle's back hook into the burrow wall and prevent a strongly struggling victim from pulling the beetle out of its burrow.

The eggs of the European stag beetle (Lucanidae) hatch in the decaying heartwood of old trees, slowly developing into plump larvae, which remain in the tree while they develop into a pupa. A month later, the adult form emerges from the pupa and searches along the forest floor for prey. The large branched jaws of the adult resemble the antlers of a stag. The larvae of ambrosia beetles feed on fungus gardens cultivated by adults in the sapwood of trees.

Following the larval stage, the beetle enters the pupal stage. The pupa develops beneath the skin of the final larval stage, then emerges when the skin splits. The pupa is a soft, pale image of the adult it is to become. The pre-adult appendages are curled or loosely attached to the body and the wings are in flat bags called wing pads. After the pupa sheds its thin skin, the adult emerges, the wings stretch out to full size, and the outer skeleton hardens. The beetle has undergone complete metamorphosis—from egg to larva, to pupa, to adult.