Agriculture Reference
In-Depth Information
Insects
Insects form a major class of organisms within
the phylum Arthropoda. All arthropods have
segmented bodies, with a hard exoskeleton or
body shell and jointed limbs, but insects (class
Insecta) are typified by the presence of three
pairs of true legs, usually two pairs of wings and
a body divided into three distinct regions: head,
thorax and abdomen (Fig. 1) (cf. Acari, p. 75).
EXTERNAL FEATURES
The often tube-like body of an insect is com-
posed of a series of segments: six in the head,
three in the thorax and up to 11 in the abdomen.
Each segment is formed from up to four more
or less horny plates called sclerites - a dorsal
tergum, a ventral sternum and two lateral pleura
(pleura are absent from the insect abdomen).
These plates and the various adjacent body seg-
ments may be fused together rigidly or joined by
soft, flexible membranes that allow for body
movement. The body appendages, such as the
legs, are formed as outgrowths from the pleura.
Where fusion has occurred (particularly in the
head) the segments, or their individual com-
ponents, are not always distinguishable; in the
thorax, the sclerites are themselves often subdi-
vided into smaller plates.
The body of an insect is covered by a protec-
tive three-layered skin (cuticle) formed from
chitin and protein. Depending upon its precise
composition and thickness, the cuticle may be
soft and flexible or hard and rigid; according to
requirements, it may or may not be permeable or
waterproof. Following its deposition, the cuticle
becomes more or less hardened and darkened by
the addition of melanin, during a process called
sclerotization. Pre-adult (immature) insects
usually moult from one growth stage to the next,
sloughing off or bursting out of the 'old' cuticle
and replacing it with a larger one; each moult is
called an ecdysis. Except in certain very primi-
tive forms, the cuticle of an adult insect is not
replaceable. External features of the cuticle (e.g.
details of punctation and sculpturing in adult
beetles) are often characteristic of the species.
Further, the insect body is often adorned with
bristles, hairs, scales, setae or spines, and these
cuticular outgrowths are also of considerable
help in identifying groups or individual species.
Immediately beneath the cuticle lies an almost
continuous single layer of cells (the epidermis); it
is the epidermis that secretes the cuticle. Some
epidermal cells are differentiated into glands
that may secrete compounds to the outside via
ducts that end in characteristic cuticular pores.
Glandular secretions include pheromones, silk,
wax and other products.
The insect head is essentially a capsule which
encloses the brain and bears the usually external
(ectognathous) mouthparts, the eyes and a pair
of sensory antennae. The mouthparts comprise
five basic sections: an 'upper lip' (labrum), the
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