Insects - Agricultural Entomology - page 7

The many-segmented abdomen is formed
from a series of up to 11 dorsal sclerites called
tergites and ventral sclerites called sternites,
joined by more or less flexible intersegmental
membranes. The anterior and posterior seg-
ments are often fused or much reduced in
size, particularly in adults. The eleventh abdomi-
nal segment, for example, is usually very small
and inconspicuous; it is totally absent in the
higher (most advanced) insects. Some primitive
insects (Protura and Collembola) have fewer
than ten abdominal segments; Collembola never
possess more than six abdominal segments.
Abdominal appendages occur on most segments
of primitive insects (subclass Apterygota) but
are restricted to the hind-most segments of mem-
bers of the subclass Pterygota. Those of the
eighth and ninth segment form the genitalia,
including (when present) the female ovipositor
and the male claspers. Microscopic features of
male and female genitalia are often used by
specialists to identify, or to distinguish between,
species. Some adult and immature insects pos-
sess a pair of cerci, formed from appendages on
the last abdominal segment. Cerci are
particularly well developed in less-advanced in-
sects (e.g. cockroaches, crickets, earwigs, may-
flies) but are usually absent from members of
more-advanced groups. Hump-like or sucker-
like outgrowths from the ventral body wall of
immature insects sometimes form so-called false
legs (prolegs or pseudopods); otherwise, ambu-
latory abdominal appendages, which are com-
monplace in many arthropods, are wanting in
insects.

Fig. 6 Diagram to show the main veins of a general-
ized insect wing.

reaching the wing margin (Fig. 6). The costal
vein and, usually, the anal veins, however, are
unbranched. Cross-veins (e.g. m-cu, which links
the media with the cubitus) may occur. Areas of
the wing membrane delimited by veins are called
cells; these may extend to the wing margin (open
cells) or may be entirely surrounded by veins
(closed cells). Wings have also undergone con-
siderable structural modification. In beetles,
cockroaches and earwigs, for example, the
forewings are no longer used in flight but have
become hardened, leathery flaps known as elytra
(beetles and earwigs) or tegmina (cockroaches,
etc.); these protect the membranous hindwings
which are folded away beneath them when not in
use. The forewings (hemelytra) of certain bugs
(Heteroptera) are hardened but have a membra-
nous tip. In true flies (Diptera). where only the
forewings are used for flying, the hindwings have
become reduced to small, drumstick-like balanc-
ing organs called halteres.

INTERNAL FEATURES

The body cavity of an insect (haemocoel) ex-
tends throughout the head, thorax and abdomen
and also reaches into the appendages. It is filled
with a pale, often colourless, blood-like fluid
called haemolymph. This bathes the internal or-
gans and tissues, and is circulated by a tube-like
heart which runs mid-dorsally from the head to
the tip of the abdomen.

The nervous system consists of a brain, with
close connections to the compound eyes, the an-
tennae and the mouthparts, and a central nerve
cord that extends back mid-dorsally through the
thorax and abdomen. The nerve cord includes a
series of swellings (ganglia) from which arise
various lateral nerves. The brain occupies much
of the head and is the main co-ordinating centre