Introduction - Pests of Ornamental Trees, Shrubs and Flowers - page 2

side of the head. Insect mouthparts are derived from
several modified, paired segmental appendages; they
range from simple biting jaws (mandibles) to complex
structures for piercing, sucking or lapping. Among
phytophagous insects, biting mouthparts are found in
adult and immature grasshoppers, locusts, earwigs,
beetles etc., but may be restricted to the larval stages, as
in butterflies, moths and sawflies. Some insects (e.g.
various dipterous larvae) have rasping mouthparts
which are used to tear plant tissue, the food material
then being ingested in a semi-liquid state. Style-like,
suctorial mouthparts are characteristic of aphids, mirids,
psyllids and other bugs; such insects may introduce
toxic saliva into plants and cause distortion or galling of
tissue. Certain insects (notably some aphids) carry and
transmit virus diseases to host plants.
The thorax has three segments - prothorax,
mesothorax and metathorax - whose relative sizes vary
from one insect group to the next. In crickets,
cockroaches and beetles, for example, the prothorax is
the largest section and is covered on its upper surface
by an expanded dorsal sclerite called the pronotum; in
flies, the mesothorax is greatly enlarged, and the
prothoracic and metathoracic segments are much
reduced. Typically, each thoracic segment bears a pair
of jointed legs. Their form varies considerably but all
legs have the same basic structure. Wings, when
present, arise from the mesothorax and metathorax as a
pair of fore wings and hind wings, respectively. In many
insects the base of each fore wing is covered by a scale-
like lobe, known as the tegula. Basically, each wing is
an expanded membrane-like structure supported by a
series of hardened veins, but considerable modification
has taken place in the various insect groups. In
cockroaches, earwigs and beetles, for instance, the fore
wings have become hardened and thickened protective
flaps, called elytra, and only the hind wings are used for
flying; in true flies, the fore wings retain their
propulsive function but the hind wings have become
greatly reduced in size and are modified into drumstick-
like balancing organs known as halteres. Wing structure
is of importance in the classification of insects, and the
names of many insect orders are based upon it. Wing
venation is also of considerable significance.
The abdomen is normally formed from 10 or 11
segments, but fusion and apparent reduction of the most
anterior or posterior segments are common. Although
present in many larvae, abdominal appendages are
wanting on most segments of adults, their ambulatory
function, as found in various other arthropods, having
been lost. However, appendages on the eighth and ninth
segments remain to form the genitalia, including the
male claspers and female ovipositor. In many groups,

cerci are formed from a pair of appendages on the last
body segment. These are particularly long and noticeable
in primitive insects, but are absent in the most advanced
groups. Abdominal sclerites are limited to a series of
dorsal tergites and a set of ventral sternites; these give
the abdomen a distinctly segmented appearance.
The body cavity of an insect extends into the
appendages and is filled with a more or less colourless,
blood-like fluid called haemolymph. This bathes all the
internal organs and tissues, and is circulated by
muscular action of the body and by a primitive, tube-
like heart which extends mid-dorsally from the head to
near the tip of the abdomen.
The brain is the main co-ordinating centre of the
body. It fills much of the head and is intimately linked
to the antennae, the compound eyes and the mouthparts.
The brain gives rise to a central nerve cord which
extends back mid-ventrally through the various thoracic
and abdominal segments. The nerve cord is swollen at
intervals into a series of ganglia, from which arise
numerous lateral nerves. These ganglia control many
nervous functions (such as movement of the body
appendages) independent of the brain.
The gut or alimentary tract is a long, much modified
tube stretching from the mouth to the anus. It is
subdivided into three sections: a fore gut, with a long
oesophagus and a bulbous crop; a mid-gut, where
digestion of food and absorption of nutriment occurs;
and a hind gut, concerned with water absorption,
excretion and the storage of waste matter prior to its
disposal. A large number of blind-ending, much
convoluted Malpighian tubules arise from the junction
between the mid-gut and the hind gut. These tubules
collect waste products from within the body and pass
them into the gut.
The respiratory system comprises a complex series of
branching tubes (tracheae) and microscopic tubules
(tracheoles) which ramify throughout the body in
contact with the internal organs and tissues. This
tracheal system opens to the outside through
segmentally arranged valve-like breathing holes or
spiracles, present along each side of the body. Air is
forced through the spiracles by contraction and
relaxation of the abdominal body muscles. Spiracles
also occur in nymphs and larvae (they are often very
obvious in butterfly and moth caterpillars) but they are
often much reduced in number. In some groups (e.g.
various fly larvae) the tracheae open via a pair of anterior
spiracles, commonly located on the first body segment
(prothorax), and a pair of posterior spiracles, usually
located on the anal segment; these spiracles are often
borne on raised processes. Morphological details of the
spiracular openings and processes are often used to