Life Cycle & Structure of Beetles |
Kingdom Animalia Phylum Arthropoda Class Insecta Order Coleoptera Within the order Coleoptera there are four suborders: Archostemata, Myxophaga, Adephaga, and Polyphaga, which between them contain 166 families. With about 350,000 species described, Coleoptera is the largest order in the animal kingdom. It is believed that there are millions more species as yet undiscovered and undescribed. The earliest beetles are found in the fossil record during the Lower Permian, about 265 million years ago. The common name “beetle“ comes from an archaic English word meaning “little biter“. The name Coleptera is attributed to Aristotle; coming from the Greek „koleos,“—sheath or shield, and “ptera,“—wings. The “shields” are the beetles’ eleytra, its wing sheaths. Most insects have two pairs of flight wings; in beetles, the forward pair has evolved into hard wing cases which protect the wings and body. These eleytra are raised during flight, and when closed over the wings and body they meet at the midline of the beetle’s back. Beetles go through complete metamorphosis. Their life cycle consists of four phases: egg, larva, pupa, and adult. Eggs are smooth, soft, and semi-permeable, allowing water and oxygen to penetrate. Their size, shape, and color vary widely among species. The egg hatches into a larva. Typically there are 3-5 larval stages called instars, although some beetles may have as many as 30 instars. The larva’s sole function is to eat and as it develops, it outgrows and molts its exoskeleton, increasing in size with each successive instar. The final larval instar is followed by the pupal stage. Despite appearing to be dormant, the pupal beetle is undergoing a dramatic transformation from an organism optimized for eating to one whose function is reproduction. The adult beetle will emerge to find a mate, and give rise to another generation. Depending upon the species, the lifespan of the adult beetle can vary from a single season to several years. Beetle bodies are made up of three sections: the head, the thorax, and the abdomen, all encased in a hard exoskeleton. The exoskeleton is made up of plates called sclerites, separated by thin elastic sutures. This design combines the protective qualities of armor with flexibility of movement. The legs have five segments with a claw at the end of the last tarsal segment. Diversity of leg lengths and forms reflects the varied habitats and lifestyles. Leg forms are specialized for digging, swimming, running, or leaping. The beetle’s antennae function as organs of smell and contain delicate sensory mechanisms that detect vibration, changes in temperature and humidity, and pheremones. Antennae vary in length, usually shorter than the beetle’s body length, but in some long-horned beetles, up to three times greater than body length. Also extremely varied in form, antennae may be threadlike, sawtoothed, comblike, feather-like, or clubbed. Beetles have hemispheric compound eyes with multiple facets gathering light to form a single image. In species active during daylight hours, the facets are smaller and flatter, while nocturnal species have larger more convex facets. Beetles are capable of perceiving light in the ultraviolet and infrared ranges. Some pond dwelling species have split eyes, somewhat like bifocals, which allow for visual acuity above and below the waterline. The beetle possesses two pairs of “jaws,” one pair of maxillae and one pair of mandibles. The mandibles are the most visible structures, and function in a horizontal scissorlike motion for grasping, cutting, or crushing food or enemies. Two pairs of finger-like appendages, the maxillary and labial palpi, function to move food into the mouth. The basic structures that all beetles share exhibit enormous diversity of form reflective of the diversity of environments they occupy. Evolving to exploit every possible habitat and food source, beetles have become the most successful creatures on earth. Their ability to survive and ultimately capitalize on environmental change virtually guarantees that they will survive on this planet long after we are gone. |