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.