Biology Reference
In-Depth Information
Chapter 1
Introduction
Ornamental plants are attacked by a wide range of pests,
most of which are arthropods (phylum Arthropoda).
Arthropods are a major group of invertebrate animals,
characterized by their hard exoskeleton or body shell,
segmented bodies and jointed limbs. Insects and, to a
lesser extent, mites are of greatest significance as pests
of cultivated plants.
Insects
Insects differ from other arthropods in possessing just
three pairs of legs, usually one or two pairs of wings (all
winged invertebrates are insects), and by having the
body divided into three distinct regions: head, thorax
and abdomen.
The outer skin or integument of an insect is known as
the cuticle. It forms a non-cellular, waterproof layer
over the body, and is composed of chitin and protein, the
precise chemical composition and thickness
determining its hardness and rigidity. The cuticle has
three layers (epicuticle, exocuticle and endocuticle) and
is secreted by an inner lining of cells which forms the
hypodermis or basement membrane. When first
produced the cuticle is elastic and flexible, but soon
after deposition it usually undergoes a period of
hardening or sclerotization and becomes darkened by
the addition of a chemical called melanin. The adult
cuticle is not replaceable, except in certain primitive
insects. However, at intervals during the growth of the
immature stages, the 'old' hardened cuticle becomes
too tight and is replaced by a new, initially expandable
one secreted from below. Certain insecticides have been
developed that are capable of disrupting chitin
deposition. Although ineffective against adults, they kill
insects undergoing ecdysis (i.e. those moulting from
one growth stage to the next).
The insect cuticle is often thrown into ridges and
depressions, is frequently sculptured or distinctly
coloured, and may bear a variety of spines and hairs. In
larvae, body hairs often arise from hardened, spot-like
pinacula (often called tubercles) or larger, wart-like
verrucae. In some groups, features of the adult cuticle
(such as colour, sculpturing and texture) are of
considerable value for distinguishing between species.
The basic body segment of an insect is divided into
four sectors (a dorsal tergum, a ventral sternum and two
lateral pleurons) which often form horny, chitinized
plates called sclerites. These may give the body an
armour-like appearance, and are either fused rigidly
together or joined by soft, flexible, chitinized
membranes to allow for body movement. Segmental
appendages, such as legs, are developed as outgrowths
from the pleurons.
The head is composed of six fused body segments,
and carries a pair of sensory antennae, eyes and
mouthparts. The form of an insect antenna varies
considerably, the number of antennal segments (strictly
speaking these are not segments) ranging from one to
more than a hundred. The basal segment is called the
scape and is often distinctly longer than the rest;
the second segment is the pedicel and from this
arises the many-segmented flagellum. In geniculate
antennae, the pedicel acts as the articulating joint
between the elongated scape and the flagellum; such
antennae are characteristic of certain weevils, bees and
wasps. Many insects possess two compound eyes, each
composed of several thousand facets, and three simple
eyes called ocelli, the latter usually forming a triangle at
the top of the head. Compound eyes are large, and
particularly well developed in predatory insects, where
good vision is important. The compound eye provides a
mosaic, rather than a clear picture, but is well able to
detect movement. The ocelli are optically simple and
lack a focusing mechanism. They are concerned mainly
with registering light intensity, enabling the insect to
distinguish between light and shade. Unlike insect
nymphs, insect larvae lack compound eyes but they
often possess several ocelli, arranged in clusters on each
