Insects are any invertebrates or animals without a backbone. They belong to the class Insecta – the largest group of organisms containing more than a million different species. An insect’s body consists of three major sections – the head, the thorax and the abdomen. Each of these sections is made up of segments, which can be fused together. The head has five or six segments, the thorax has three and the abdomen has eleven segments.
All insects have antennae, also called feelers. In some insects they are simple threadlike structures that are longer than the body and in others they are short and swollen. The antennae can also be clubbed, feathery or toothed. Each insect has a mouth opening, which is surrounded by paired mouthparts. The major mouthparts are:
- The labrum, which is the upper lip and an outgrowth of the wall of the had
- Mandibles are jaws – one of each side of the mouth opening. They move sideways against each other.
- Maxillae are a second pair of jaws behind the mandibles. They are used for piercing and holding food or for straining it. There are several parts to the maxillae – the spoonlike structure, a knifelike structure, and a jointed structure, which is called the palpus.
- The hypopharynx is in the center of the mouthparts and looks like a tongue.
Adult insects have six legs – one pair on each segment. Each of the legs also has several segments. The coax connects the legs to the body, the trochanter is a muscular femur and the tibia is long and slender with spurs or spines. The tarsus has five segments and usually has claws at the tip. Insects use their legs for walking and for other purposes. They can flatten the legs and use them like oars when swimming and in some insects, such as grasshoppers, the legs are used for jumping.
Insects also have wings that have hardened stiffenings called veins running through them. They move the wings by changing the shape of the thorax. Flattening the thorax raises the wings and compressing it sideways lowers them. The front and back parts of the wings are connected so that the movements are coordinated.
Insects breathe through tubes called the tracheae. There are openings called spiracles in the sides of the body through which air enters. It then travels through the branches of the tracheae to all parts of the body. The blood of insects may be colorless because it is a combination of blood and lymph. The color of the blood can change depending on the food an insect has eaten. It flows through a tube on the top of the body. The front of this tube is open, so the blood then flows through all the body cavities.
Digestion takes place in the alimentary canal, which is a tube that extends from the mouth to the anus. There are three sections – a foregut, a midgut and a hindgut. In the foregut, the insect can store food. In the midgut, digestion begins as the strong muscles start to break down the food. Wavelike contractions move the food along through the sections and in the hindgut the waste is excreted from the body.