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A brief guide to the main external features of insects and
the terminology used to describe the various body-parts.
The insect body is divided into three main parts -
head, thorax and abdomen - as shown above.
Head. The head bears the mouthparts, the eyes and a pair of
antennae (singular: antenna). Most adult insects and the immature stages (nymphs)
of Exopterygota have a pair of large, multi-faceted compound eyes (labelled 'eye' in the
diagram above). Many of these insects also have two or more small, single-faceted eyes, called
simple eyes or ocelli (singular: ocellus), usually placed on the top or
front of the head between the compound eyes. In some insects, particularly those belonging to
the Apterygota, and in the immature stages (larvae) of all Endopterygota, compound eyes are
lacking and only ocelli are present, with one or more of these simple eyes on each side of the
head. In a few adult insects and among many larval forms, eyes are completely absent.
Thorax. The thorax consists of three segments - the
prothorax (front), the mesothorax (middle) and the metathorax (back).
Each of these segments bears a pair of jointed legs. Each leg consists of four main
parts - the coxa, the femur, the tibia and the tarsus
(plural: tarsi) - with a small, often inconspicuous segment between the coxa and
femur, called the trochanter. The tarsus usually consists of several small joints,
the last of which generally carries a pair of terminal claws. The last tarsal segment
may be extended between the claws to form a pad-like organ - the arolium (as in (1) below).
Some Diptera (true flies) have two additional pads - the pulvilli (singular:
pulvillus) - lying below the claws on either side of the arolium (as in (2) below),
although in most flies, including the common housefly and its relatives, the arolium is replaced
by a stout central bristle - the empodium (as in (3) below). In many insects, the other
tarsal segments also have ventral pulvillus-like organs, called plantulae. These various
structures - the arolium, pulvilli and plantulae - produce a sticky secretion and act like
'suction-pads', enabling the insect to climb smooth or steep surfaces. This is the secret of the
fly's ability to walk up-side-down on the ceiling!
Segments of the Tarsus (see text above for details)|
|(a = arolium,
c = claw, e = empodium, p = pulvillus)|
In winged insects, the meso- and metathoracic segments each bear a pair of
wings (the forewings and hindwings, respectively), although in many insects one
or both pairs of wings have been lost, or modified into other structures, during the course of
evolution. Special wing modifications include the structures called halteres found in
the Diptera (true flies), and the structures called elytra (singular: elytron)
found in the Coleoptera (beetles) and Dermaptera (earwigs):
- small, club-shaped organs of balance found in Diptera (flies) on each side of the thorax
(metathorax) just behind the forewings and thought to be modified vestigial hindwings.
elytron) - the hard, leathery forewings of Coleoptera (beetles) and Dermaptera (earwigs),
that serve as protective coverings or 'wing-cases' for the membranous hindwings. The hindwings
(if present) are usually folded out of sight under the elytra when at rest. The diagram shows a
green tiger beetle (Cicindella campestris) with the elytra open and hindwings outstretched,
as the beetle appears when in flight.
Abdomen. The abdomen is made up of several, more or less similar
segments and often bears a pair of jointed, terminal appendages called the cerci (singular:
cercus). Typical cerci are illustrated in (a) below, although in some insects they consist
of many more joints than in the example shown.
Abdominal Appendages of Adult Insects|
cerci and tail filament
cerci modified as
forceps e.g. earwigs
e.g. some aphids
In a few groups of insects the paired cerci are accompanied by a
third tail filament, as in (b) above, whilst the cerci of Dermaptera (earwigs) and some
Diplura are modified to form stout pincers or forceps, as in (c). In many insect Orders
the cerci are completely absent or too small to be noticeable, as in (d). The pair of
tube-like abdominal appendages shown in (d) are not cerci, but special glandular
structures called cornicles (or siphunculi); these structures are a
characteristic of many aphid species (Hemiptera).
In the immature stages (larvae) of some Endopterygota, e.g., those
of many butterflies and moths (Lepidoptera), scorpion flies (Mecoptera) and sawflies
(Hymenoptera), the abdomen carries several pairs of appendages called false-legs
or prolegs. These are unjointed, fleshy structures different from, and additional
to, the jointed legs of the thorax. Larvae with prolegs are commonly called caterpillars.
Caterpillar of a Sawfly - side-view showing:
(a) the jointed legs (i - iii) of the thorax, and (b) the fleshy prolegs (i - vii) of
the abdomen. There are always three pairs of thoracic legs (as characteristic of insects
generally), but the pairs of abdominal prolegs may vary in number depending on species.
(click icon for the identification key to insect orders)
Explore the Micro-structure of Bugs
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