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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 and Ocelli
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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. There are more information in the Dragonfly's Eyes page.   
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.
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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 male moth with feather like antennae, it can smell a female 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.

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. 

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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. For more information on insect's leg, please also check the Katydid's legs page.
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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.

Insect Abdomen

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.
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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.
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.
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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.  

The above information are about adult insects. For information on caterpillars, please click here.

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. 

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Up ] Insect Wings ] Insect's Mouth ] Grasshopper Face ] Dragonfly Eyes ] Katydids Legs ] Caterpillars ]


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Last updated: April 16, 2009.