Insects and their Allies  

Coleoptera: beetles and weevils

Beetles constitute the largest and most diverse order of insects on earth, making up about 30% of all animals. There are over 300 000 species of beetles worldwide and over 28 000 species spread across 117 families in Australia. Beetles come in a variety of shapes and colours and can range from 0.4 to about 80 millimetres in length. Due to the variation within the order it is difficult to give a general description however all beetles characteristically have the following features:

Elytra raised to reveal the membranous hindwings

Elytra raised to reveal the membranous hindwings
  • 2 pairs of wings. Forewings hardened, hind wings membranous
  • Mandibulate mouthparts which are designed for biting and chewing
  • Antennae present in a variety of forms

The forewings of beetles are heavily sclerotised and form protective covers over the hind wings. The forewings are called elytra and Coleoptera means 'sheath wings' i.e. the hard sheath-like elytra over the soft hind wings. The elytra are not used in flight but are lifted out of the way of the hind wings. At rest the elytra meet in a straight line down the middle of the back. The elytra may completely cover the abdomen or may be shorter exposing part of the abdomen. The hind wings are membranous and are used for flight. At rest they are folded protectively under the elytra. Some species do not have hind wings and their elytra are generally fused together.

Coleoptera are the only order of insects that have elytra. This adaptation has enabled them to expand into many habitats such as leaf litter, logs and soil, that would otherwise damage the wings of less well protected insect groups. At first glance beetles may appear to have only 2 body segments because the elytra may cover most of the thorax and abdomen. However if you capture a beetle and turn it over you will be able to see the segments that are hidden by the elytra.

Larval Characteristics: The larvae of beetles also come in a variety of shapes and sizes depending on where they live and what they eat. Larvae generally appear grub-like with a well-defined head capsule, which may be highly sclerotised. They have short antennae and usually have chewing mouthparts. The legs may be present or absent.

Various larval types

Various larval types: Curculionidae, Carabidae, Chrysomelidae, Scarabaeidae

Beetles are often confused with cockroaches (Blattodea) or bugs (Hemiptera) but can be distinguished from the former by their forewings which are modified into elytra and meet in a straight line down the back and from the later by their chewing rather than sucking mouthparts.

Life Cycle
Beetles have a complete life cycle and development may take anywhere from a few weeks to several years. Eggs are usually laid on or near the food source such as in the soil or on a host plant, depending on the species. The number of eggs laid will depend on the species and may range from one or two up to hundreds. After hatching the larvae develop through a series of growth stages known as instars (usually 3 to 5) before pupating into adults.

Beetles are generally herbivores, scavengers or predators, although some adult beetles do not feed at all. The greatest numbers are plant feeders in one form or another, such as nectar feeders (some Buprestidae), foliage eaters (Chrysomelidae), seed-eaters (many Curculionidae) or timber (Cerambycidae) or bark borers (Scolytidae). Others may feed on rotting wood (Lucanidae), carrion (Silphidae), dung (some Scarabaeidae), fungi or leaf litter. Some species are also predators (Carabidae) of other invertebrates. The feeding habits between larvae and adults may be the same or can vary. For example some beetle species are predatory when in the larval stage and plant-feeders when adults.

Beetles can be found in almost all available habitats throughout Australia. Most are terrestrial though many families are largely or wholly freshwater in habit and some can be found living in marginal marine environments. In terrestrial environments beetles can be found living in soil, humus and 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. Some species live solely in caves while others live in the nests of vertebrates or social insects such as termites or in man-made environments such as silos where grains are stored.

In aquatic environments beetles can exploit many habitats ranging from temporary pools or mountain streams to brackish waters and mud flats. Some can be found living in the sand, gravel or mud at the edges of streams and ponds and in the high-water debris on the seashore or among the rocks and coral in the intertidal zone.