WildlifeCampus – Game Ranging Course 1
Module # 2 – Component # 7
Introduction to Insects
Objectives
To gain an understanding of the most diverse group in Phylum Arthropoda –
the insects.
Expected Outcomes
?? To be able to detail the characteristics common to all insects.
?? To understand and discuss flight among insect species..
?? To become aquainted with different feeding strategies within the group
?? To become familiar with different life strategies (complete and incomplete
metamorphosis)
?? To gain insight into the widely diversified ecologies among this very large
group
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Phylum: Arthropoda Class: Insecta
Insects are the most abundant and successful of all terrestrial animals,
making up 75% of the species diversity on earth. There are some one
million species that have been described world-wide, with five to ten
thousand new species being described annually. The relationship between
man and insects varies from one of intense conflict to absolute harmony.
The disease - bearing insects and the agricultural pests have had a war waged
on them from the earliest days. However, we have thus far been unable to
eradicate even one single pest species, despite an expenditure estimated in
the tens of billions of dollars !
On the other hand, species such as bees are absolutely essential to the
success of agriculture. Two thirds of all flowering plants are pollinated by
insects.
Southern African insect fauna is exceptionally well represented by 30 orders.
These Orders comprise 580 families and well over 80 000 species.
The highest biodiversity of insects is to be found in the tropical rain forest
regions with more than 750 000 species.
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Insect characteristics
?? All insects have bodies divided into three distinct parts viz. the head, thorax
and abdomen.
?? The head functions in sensory perception and food ingestion.
?? There is a single pair of jointed antennae.
?? There are a large pair of compound eyes and several smaller simple eyes,
which may be arranged in several different ways.
?? The mouthparts consist of mandibles, maxillae and a labium. These are
usually covered with an extension of the head known as the labrum. The
arrangement of mouthparts is very diverse, and it is not feasible to cover
them in this course.
The aspect of the head in relation to the body provides a means of grouping
insects into three different categories. Insects having a head situated at right
angles to the body are known as hypognathous species.
Their mouthparts are situated at the bottom of the head, and allow for chewing
and sucking types of feeding. Those species with the head in line with the long
axis of the body are called protognathous and have their mouthparts located
at the very front edge of the head. This arrangement is usually found in
predatory species.
The last group are known as opisthognathous, and have their mouthparts
facing rearwards between the legs and beneath the body. These insects are
usually sap sucking species.
The Thorax is the “chest” area of the insect and consists of three divisions, the
prothorax, mesothorax and the metathorax. Each of these divisions has a
single pair of legs (making 6 in total), and most species have two pairs of
wings, the first being attached to the mesothorax and the second to the
metathorax. The front legs pull the insect along, while the middle and rear legs
push it forwards. The middle pair of legs is usually the longest to prevent
interference with the others.
The Abdomen usually consists of 9 to 11 segments. The breathing apparatus
are located on these segments, and comprise openings to the atmosphere
known as spiracles. At the rear of the abdomen is a pair of sensory
appendages called cerci.
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Insect Wings and Flight
Most species have wings, even if these do only develop at certain times in the
lifecycle e.g. termites and ants. Other species such as fleas, lice and scale
insects have no wings.
The way in which the wings are carried in relation to the body is thought to
reflect the relative advancement of insects.
The primitive species such as dragonflies carry their wings at right angles to
the body and beat their wings independently of each other.
The more advanced orders such as wasps, for example, have developed the
ability to fold their wings and also beat their wings in synchrony. This is
achieved by a special hook that joins the two wings together in flight. In
this way turbulence problems are not created as is the case with the
dragonflies whose wings are always beating in the draught created by the
other wing pair.
Terrestrial forms such as beetles have developed a hard wing covering called
the Elytra. This is simply a hardened pair of front wings which are used to
protect the insect from dehydration and predation. Only the rear wings are
used in flight.
Flight is achieved through a complex arrangement of muscles and elastic
ligament systems that work antagonistically to each other. This system allows
for wingbeats of up to 1000 per second in some species of midges.
Butterflies and locusts typically beat their wings at 4 - 20 per second and
bees and flies at about 190 per second.
The co-ordination of flight is not achieved by the ganglia (“brain”), but rather
by a system of flight receptors located in the wings, the antennae and
sensory hairs. These receptors provide information as to the speed, pitch,
roll, yaw etc. being experienced by the insect in flight. Direction is maintained
by locating the eye in a constant relation to the sun. Any variation off this path
is corrected by altering the frequency and strength of wingbeats on different
sides of the body.
Some insects such as the flies (O. Diptera) have a system of gyroscopes that
keep them correctly orientated during flight. Their second pair of wings has
degenerated into small club-like appendages known as Halteres. These
send back information to the fly about flight control systems. Flight is
automatically stopped the second the tarsi touch the substrate on which the
insect lands.
Insects are the only “cold blooded” creatures capable of flight, and require a
certain minimum temperature in order to fly successfully. Some species open
their wings and take in heat from the sun before flying, while others beat their
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wings in “neutral gear” to warm up the muscles. This is one reason why bees
rescued out of swimming pools cannot fly away immediately.
Food and Feeding
As previously mentioned, the mouthparts of insects vary widely, but basically
fall into the following categories:
• Chewing mouthparts are supplied with heavy cutting mandibles and
may be used for carnivorous or herbivorous feeding. The larvae of
many insects have these mouthparts. Examples are dragonflies and
beetles.
• Sucking mouthparts are usually seen on butterflies and moths. The
mouthparts consist of highly elongated maxillae which form a tube
through which nectar is sucked.
• Piercing and sucking mouthparts are adapted to piercing through
hard layers of skin or plant material and thereafter for sucking the
contents from the pierced object. These mouthparts are seen in
mosquitoes and sap-sucking insects.
• Sucking and chewing mouthparts are a combination of two different
forms, and are seen on insects such as bees and wasps. The chewing
mouthparts handle wax and pollen, whilst the sucking mouthparts take
care of the nectar and other liquids e.g. sugary sodas.
• Biting mouthparts are seen on insects such as horseflies. The skin of
the victim is lacerated and the blood that seeps to the surface is
sponged up as opposed to the tubular drinking system possessed by
mosquitoes.
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Reproduction
In the majority of species fertilisation is internal. This is accomplished
through the direct transfer of a spermatophore from the male to the female via
copulation.
The spermatophore usually releases its sperm soon after copulation and this
finds its way into the spermatheca of the female. The sperm cells remain here
until egg laying takes place at which time the egg is fertilised. This system
allows for huge numbers of fertile eggs to be laid from a single mating.
The way in which the lifecycle of an insect progresses is very important. All
species undergo what is known as metamorphosis. This is the series of
changes that the insect undergoes before it can become a reproductive
adult.
There are two different types of metamorphosis. The first type is called
incomplete metamorphosis and the second complete
metamorphosis.
The incomplete system is characterised by the eggs hatching into miniatures
of the adult. These juveniles undergo a series of moults or ecdysis, and
eventually develop sexual organs and are able to reproduce.
Complete metamorphosis is characterised by the eggs hatching into larvae
which moult several times and at their last moult turn into pupae. This phase is
called pupation. These pupae then emerge as fully developed adults. The
discarded pupa is called a chrysalis.
Alternative terms. In true scientific fashion, entomologists were not satisfied
with mere English terms and invented a few new ones, therefore:
?? Complete metamorphosis = holometabolous development
?? Incomplete metamorphosis = hemimetabolous development
?? No metamorphosis = ametabolous development
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Differences between Arachnids and Insects
Insects Arachnids
6 legs 8 legs
Head, thorax, abdomen Cephalothorax, abdomen
Mostly winged Never winged
Carnivores and herbivores Mostly carnivores
Complete & incomplete Incomplete metamorphosis
metamorphosis
Insect Communication
Insects are able to communicate in many different and interesting ways. These
methods include visual, tactile, auditory and chemical.
Chemical One of the most commonly used methods is that of
the release of pheromones. These chemicals may
be used as a sex attractant, for trail marking and
territorial marking.
Visual Other species such as the fireflies use visual
stimuli.
Auditory A very common communication method is that of
auditory signalling. Grasshoppers, crickets and
cicadas all use this system.
Tactile This type of communication is observed in social
insects, inside bee hives and termite mounds.
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