Cicindela purpurea © Robert A. Cannings/M.B. Cooke 1983

Order COLEOPTERA (Beetles)

Order Description

From the Greek koleos = sheath; ptera = wings. The forewings of beetles are usually hardened, sheathing cases protecting the hindwings and much of the body.

Beetles are tiny to very large insects (about 0.4 mm to 130 mm long) of variable shape and colour, but mostly strongly sclerotized, compact and more or less flattened so that the lateral sclerites are mostly ventrally placed, The compound eyes are normally conspicuous; ocelli are almost always absent. Antennae usually have 10 to 14 segments (most are 11-segmented) and are variable in form - threadlike, comblike, beaded, sawtoothed, elbowed or enlarged at the end in various ways. Mouthparts are normally of the chewing type; a very few beetles have mouthparts modified into a sucking tube. Adults normally have two pairs of wings, the front ones (elytra - singular, elytron) are usually hard and shell-like, folding over the back and meeting in a straight line to make stout covers for the folding, membranous second pair, which are used for flight. Some species have reduced wings. The legs are normally strongly sclerotized; tarsi usually have 3 to 5 segments. Beetles have both a larval and pupal stage. Larvae have sclerotized head capsules, chewing mouthparts, distinct antennae and, usually, ocelli. There are no labial silk glands. Thoracic legs are the rule; sometimes they have fewer than the usual 5 or 6-segments, or are absent. The abdomen lacks legs, or rarely has one or two pairs of prolegs. The pupa normally is exarate, with the appendages free, but sometimes they are fused to the body (obtect type).

The Coleoptera is the most diverse order of living organisms. The numbers are extraordinary - the more than 350,000 named species represent about 40% of all insects and 30% of all animals. There are at least six times as many beetles as vertebrate species and 90 times more than the number of mammals. The order is usually divided into four suborders (Archostemata, Myxophaga, Adephaga and Polyphaga) and about 150 families. Perhaps the single most important factor in the success of the Coleoptera is the development of the elytra, which protect the folded hind wings, permitting the occupation of enclosed spaces and hidden habitats by adults. The resulting subelytral space, enclosing most of the spiracles, reduces water loss through transpiration.

Beetles live in almost every conceivable terrestrial and freshwater habitat and even in some marginal marine ones. They thrive in plant foliage, flowers, fruits and seeds; in living plant tissue in galls, stems and roots; in soil, leaf litter, under bark and in rotting wood; under stones and in fungi; in dung and carrion; in the nests of vertebrates and social insects; in stored foods; in running and still fresh waters; in saline ponds and estuaries; in sand and mud along water; among debris along the ocean and in caves.

Many species live in fresh water, either in the larva stage or in both larval and adult stages. Many adults, especially in the Hydrophilidae and Elmidae, have ventral patches of fine setae (plastrons) that trap air bubbles for use in breathing. Larvae have various sorts of external gills and siphon-like spiracular tubes.

Most species of beetles probably eat living plant tissue, but many feed largely on decomposing material acted on by fungi and bacteria. Others are fungal feeders. Most phytophagous species belong to the Scarabaeoidea, Chrysomeloidea and Curculionoidea, and the families Buprestidae, Elateridae, Bostrichidae and Anobiidae. Carnivorous beetles are common, and include almost all Adephaga and Polyphaga such as the Hydrophildae (larvae), Histeridae, Staphylinidae, Elateridae, Lampyridae, Cantharidae, Cleridae and Coccinellidae. Most are general predators, but some are more specialized; for example, Coccinellidae feed mostly on aphids and coccoid Hemiptera and the Lampyridae feed mostly on snails.

Beetles are of immense ecological and economic importance. Many are vital in the cycles of decomposition of plant and animal matter. Others are predators of insects and other invertebrates that damage crops and other plants. Many beetles feed on the foliage and roots of plants, causing much damage to crops; they can kill huge tracts of valuable forests in a short time. Others damage our wooden homes, our furniture and many kinds of stored foods and other products.