Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Arthropoda
Subphylum Hexapoda
Class Insecta
Order Coleoptera

Common name: beetles

Overview

The Coleoptera, or beetles, are the largest of the insect orders and are extremely diverse in size, shape and colour. The group includes many commonly encountered insects such as ladybirds, weevils and scarab beetles. The most distinctive feature of beetles is their forewings, called the elytra, which are heavily sclerotised (hardened) and form protective covers over the hindwings. The hindwings are membranous and are used for flight; during flight the elytra are opened and raised out of the way to allow the hindwings to unfold and flap. At rest the hindwings are folded protectively under the elytra which meet and may completely cover the abdomen or may be shorter exposing part of the abdomen. Some flightless species lack hindwings and their elytra are generally fused together. All beetles characteristically have mandibulate mouthparts for biting and chewing; antennae present in a variety of forms; compound eyes in a variety of sizes and shape; and legs that vary in form, depending on lifestyle. For example, some beetles have strong slender legs for running, others such as dung beetles have spade-like legs for digging. Adult beetles range in size from minute featherwing beetles (Ptiliidae), adults of which are as small as 0.3 mm long, to large scarab beetles (Scaribaeidae) which can be up to 80 mm in length. The larvae of beetles generally appear grub-like and also come in a variety of shapes and sizes depending on where they live and what they eat. Larvae have a well-defined head capsule, which may be highly sclerotised; short antennae; usually have chewing mouthparts; and legs may be present or absent.

Distribution and diversity

Beetles can be found throughout the world but are most speciose in the tropics. They occur in almost all available terrestrial and freshwater habitats throughout Australia. There are more known species among beetles than among any other group of organisms, with over 350,000 described species making up about 30% of all animals. Estimates of total numbers of species are as high as 3�5 million species. Over 28,000 described species from 117 families are recorded for Australia, and total estimates range from 80,000�100,000 species.

Life cycle

Many male beetles have antennae that are often larger and more elaborate than the females; these are involved in detection of female pheromones. Some males also have horns on the head and/or front of the thorax which they use in combat (�pushing matches�) over breeding sites and females. Many species also perform complex courtship dances prior to copulation. Sperm transfer is direct and eggs are usually laid on or near the larval food source, for example, in the soil or on a host plant. The number of eggs laid depends on the species and may range from 1�2 up to several hundred eggs. After hatching the larvae develop through several instars (usually 3�5) before pupating into adults. Development may take anywhere from a few weeks to several years, depending on the species.

Feeding

Reflecting their diversity, adults and larvae feed on a wide range of plant and animal matter. The majority of species are herbivorous, feeding on nectar, foliage, seeds, bark or timber. Many species feed on rotting wood, carrion, dung, fungi or leaf litter. Some species are predators or ecto- (external) parasitoids of other invertebrates. Some beetles are generalists, eating both plant and animal tissue. Others are highly specialised in their diet. Many species of leaf beetles, longhorn beetles, and weevils are strongly host specific, feeding on only a single species of plant. Although most predatory beetles will capture and eat a wide range of small invertebrates, a few have more specific prey requirements or preferences. The feeding habits of larvae and adults may be the same or can vary. For example, some beetle species are predatory as larvae but feed on plants as adults.

Ecology

Beetles exploit a wide range of habitats. The development of the elytra has enabled them to expand into many habitats such as rock or in sand and gravel, habitats that would otherwise damage the wings of less well protected insects. Most are terrestrial though many families occur largely or wholly in freshwater. Some can even be found living in marginal marine environments. In terrestrial environments, adults and larvae can be found living in soil, leaf litter, under the bark of living and dead trees or in decomposing wood, under stones and logs, in dung, carrion and the fruiting bodies of many types of fungi. Many of these species are well camouflaged, using colour, patterning, structures or behavioural traits to blend in with their surroundings. Some species occur in caves, others live in the nests of vertebrates or social insects such as termites (Isoptera). In aquatic habitats, beetles can be found in temporary pools or mountain creeks to brackish waters and mud flats. Some live in sand, gravel or mud at the edges of creeks, pools and in the high-water debris on the seashore or among the rocks and coral in the intertidal zone. Their extreme diversity, abundance and wide range of biologies make beetles important components of natural ecosystems. Many species are agricultural pests of crops, pasture plants, timber and stored products or domestic pests amongst foodstuffs, furnishings, wooden objects and garden plants. However, other species of beetles are important beneficials, introduced for biological control of weeds, e.g. the weevil, Neochetina eichhorniae that feeds on water hyacinth, Eichhornia crassipes, and insect pests, e.g. ladybird beetles (Coccinellidae) that consume aphids, scale insects (Hemiptera) and thrips (Thysanoptera). Introduced dung beetles (Scaribaeidae) have been used successfully in Australia to reduce cattle dung and thus control dung-breeding flies (Diptera).