Insect is such a tiny creature that it is difficult to believe that it has a circulatory system and has blood in its body. But interestingly insects have a heart, blood and other organs.
Insects are found everywhere in the world, except in the deep seas. Fossils indicate that they have existed on earth for more than 400 million years. They have been able to adapt quickly and efficiently, to environmental and climatic changes.
The body of an insect is divided into three sections: head, thorax and abdomen. The head has one pair of antennae which serve to convey the senses of touch, taste and smell. There are usually two compound eyes which provide proper vision and two or three simple eyes (ocelli) which detect light or darkness. The mouth may have biting or chewing jaws or piercing and sucking structures. The head also contains a brain which connects with nerve cords in all parts of the body.
The thorax or middle part of the body has three pairs of jointed legs. These legs are equipped with sticky pads or claws at the end. Insects are the only invertebrates with wings. Although most insects have two pairs of wings, some have only one pair or no wings at all.
The abdomen or end-part of the body contains organs for digestion, excretion, respiration and reproduction. There are tiny openings along the length of the abdomen called spiracles. These spiracles open to the tracheae through which an insect breathes. The oxygen diffuses into the blood from the tracheae at a fairly slow rate. This is probably a major reason behind insects staying small throughout their evolution.
In the circulatory system, blood passes into the heart through holes equipped with valves. When the heart contracts, these holes close and blood is driven out through arteries. Insects do not have a system of capillaries and veins as we do.
The abdomen also contains malphigian tubules. These tubules remove wastes from the blood while recycling most of the water to the body and for this reason; insects can live for long without water. A female insect often has an egg-laying tube called ovipositor.
An insect’s body is covered with a tough exo-skeleton which provides protection from injury and controls loss of moisture. Further, the entire body is usually covered with tiny bristles. These bristles are connected to nerves and are very sensitive to contact. This is the reason that an insect can detect even the faintest breeze or movement.
Many insects have special hearing organs located on the abdomen, thorax or legs. Some of these organs are just spaces covered by a thin membrane which responds to vibrations in the air.