Insects are found all over the world. They are small creatures which fly, buzz around on a variable number of legs. They are by far the most numerous of all animal species. About 7,000 new insect species are classified each year. More than 850,000 different kinds of insects are already known to scientists. Roughly eight out of ten of the creatures on earth are insects. Insects range in size from tiny fleas which can be seen only through a microscope, to beetles as big as your palm.
All insects have a similar body plan with three main parts: head, thorax and abdomen. On its head is a single pair of antennae or feelers, a pair of compound eyes, usually three small simple eyes, and a set of mouth parts which work sideways rather than up and down. The middle part or thorax carries three pairs of jointed legs and sometimes wings. The abdomen contains the stomach, reproductive organs and breathing tubes which open to the air through tiny holes called ‘spiracles’. Inside the insects body the tubes, called ‘tracheae’, branch repeatedly and end in the muscles. Normally oxygen passes through the tracheae into the tissue. Larger insects, such as bees and wasps, have air sacs in the muscles which can take in extra air as the muscles expand and contract. This increases the supply of oxygen to the muscles. These tubes work in the same way as man’s wind pipe. An insect breathes just like us, except that it may have hundreds of breathing tubes in its belly to take in the air. Among smaller insects these tubes do not take up much space.
The rate of breathing depends upon the size of the animal. The larger the size, the slower is the rate of breathing. While the big animals like the elephant breathe about 10 times a minute, a mouse has to breathe about 200 times per minute. The oxygen taken in by the insects is needed to oxidize certain food materials for the body. Waste products, including water and carbon dioxide, are eliminated from the body by exhaling.
