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Insects Biology
You
may find the information on this page is boring and hard to remember. However, if
you want to know more about insects, it is the minimum information that you have
to know.
Insects are the animals grouped under class Insecta. Insecta are
invertebrate animals which belong to a large group phylum known as arthropods.
All arthropods have an exoskeleton and jointed limbs. The major arthropod groups
are Arachnida, Crustaceans (shrimps, crabs), Chilopods
(centipedes) and Diplopods (millipedes).
All insects have three pairs of legs (although there are some rare exceptions, e.g. butterfly in NYMPHALIDAE family). Insects do not have internal skeletons. Their inner soft body is protected and supported by their armour-like shell, known as exoskeleton.
The insect body can be divided into three main parts, the head, thorax and abdomen.
Insect Head
On the head there are the eyes, mouth and antennae. It is the insect's main nervous control centre.
- Compound
Eyes
- Most
adult insects have two compound eyes. They usually occupy a
considerable area of the head. Some insects have, beside the compound eyes,
three simple eyes (ocelli) on the top of their head. They are very different from our
human eyes. Each compound eye consists of many single eyes, facets, which are
independent from each other. It is believed that this eyes structure is very
sensitive to moving objects. For those insects greatly depend on their eyesight,
like the dragonflies and praying mantids, they have big and complex compound
eyes. For the insects live underground and in a complete darkness environment,
they may have very small eyes or some have no eyes at all.
- Mouth-parts
- There
are the large variations among the mouths of different insects. The shape is
much depends on the mouths’ function. Some adults insects do not eat, like
some moths, basically they do not have functional mouths. Most moths and
butterflies have
tube-like mouth so that they can reach deep into the flowers and suck the nectar.
Bugs and mosquitoes have different type of tube-like mouths to suck blood
or plants juice. Most other insects’ mouths are for biting. Although there are
so many different shapes, the insect mouths are basically consists of there pairs
of limbs. They are the mandibles,
the maxillas and the labium. Together with the upper lip, known as
labrum, they form the mouth of the insect. For a biting mouth, the function of
mandibles is to chop the food into pieces. Some insects also use mandibles to
cut the wood or dig the soil. The functions of the maxillas and the labium are to hold and taste the food.
For some insects, labium function as the lower lip. For those sucking mouth, the tube is formed by the
modification of either one or both of the three pairs of limbs.
- Antennae
- Antennae
are the major sensing organ for insects. It is used to smell and touch. It look
very different according to their function. For a moth with feather like
antennae, it can small another moth kilometers away. For a cricket or cockroach,
they use their long antennae to touch and feel their way in the dark. Although
the antennae may look like a fine hair, it is segmented and the number of
segments is an important data in classifying insects.
- There are more information about insect head in the Dragonfly's Eyes and Grasshopper's Face pages.
- Legs
- There are great variations on insect legs. Cockroaches have three pair of
strong legs and they run very fast. Grasshoppers have their last pair of legs
modified and they can jump to start a fright quickly. Praying mantids have their
front pair of legs like a weapon to catch prey. Dragonflies do not use their legs
to walk but to capture insects and hold them under their mouths.
Butterflies use their legs to taste the food. Besides, insect legs are also used
for digging and swimming.
- Wings
- There are only four group of animals have flying wings. Compare with the
pterosaur, the birds and the bats, insects were the first group of animals
flying on earth. This is one of the major reasons that insects are now
dominating the earth. Most insects have two pair of wings, but some are
wingless, such as lice, fleas and silverfish, while some have only one pair of
wings, like files. Some insects have four wings but the forewings are modified
not for flying, Like in beetles, the forewings are used to protect the
hind wings and the abdomen. For more information on insect's wings, please also
check the Insect's wings page.
- Abdomen is the largest part the insect. Usually it is soft. Inside is the important organ and storing fat. Most insects have less abdominal segments than the one in above "Typical Insect" picture. They may loss or fusion at the back end during the process of evolution.
- Respiration
- Most insects breath by its respiration system consists of small tubes inside their body called tracheae. Air enters the tubes via holes (spiracles) on the side of their body.
- Circulation
- The insects blood is know as haemolymph, which is the liquid fill the
body cavity. Haemolymph does not circulate in a close veins system like human
blood. It does not carry oxygen neither. It just flood with the internal organs
and supply them with nourishment.
- Digestion
- The digestion process of insects is not much different from other
animals. They eat food from the mouth, then food are process and transfer to
mid-gut and to the rest of intestines. After food nourishment are absorbed,
finally the waste exited via anal orifice.
Insect Thorax
The thorax consists of three segments. Usually they are not obvious. Each segment supports a pair of legs and the last two segment each support a pair of wings. Thorax is the locomotion centre of an insect.
Insect Abdomen
The above information are about adult insects. For information on caterpillars, please click here.
- Reference:
- 1. Animals Without Backbones, Ralph Buchsbaum, Pelican / Penguin Books,1968, pp 302.
- 2. Insects of Australia, George Hangay & Pavel German, Reed New Holland, 2000, pp 16-21.
- 3. Insects of Australia, CSIRO, Division of Entomology, Melbourne University Press, 2nd Edition 1991, pp 3.
