When once the adult stage is reached, no further growth occurs. Frequently the young insect is very different from the adult. (Compare a caterpillar and an adult butterfly). The young insect is, in this case, called a larva.

Before it can assume the adult form it must go through a period of rest during which the necessary changes can occur. This period is the pupal period. The change from larval to adult form is called metamorphosis.

When the young insect is just a small version of the adult (e.g. a grasshopper) it develops gradually into an adult. This young form is called a nymph and its metamorphosis is called partial or incomplete, as opposed to the complete metamorphosis of a butterfly. The life cycle from egg to adult may be very short a housefly can, in warm conditions, develop from egg to adult in little more than a week. On the other hand, mayfly nymphs live for two years or more before the adult emerges for its brief life in the air. Some wood boring moth larvae also live for several years before reaching maturity. Insects are not normally active in the colder months of the year. They may over winter in any of the life stages.

Insects are very successful animals, having colonized land, water and air. Only the sea, with very few insect inhabitants, has proved a serious barrier to their spread. The main factor in their success on land has been the waterproof cuticle that allows them to inhabit dry places. Their small size enables them to exploit numerous habitats which are unsuitable for larger animals. A given area can support large numbers of insects because the individuals are small. These advantages, combined with the power of flight, have helped to establish insects firmly all over the Earth. The first insects appeared as long ago as the Devonian period. By Miocene times all the present day groups were in existence.

Imms divided the insects into 29 orders arranged in two sub classes. The first of these the Apterygota contains small, wingless insects. Most of them live in the soil or decaying vegetation although the household silverfish are included in the sub class. The sub class Pteryota has two divisions: the Hemimetabola (insects with incomplete metamorphosis) and the Holometabola (insects with complete metamorphosis ).

Insectivora - Order of mammals including hedgehog, moles, and shrews. They show many features of the primitive placental mammals such as the full dental formula, in most cases. They do not feed exclusively on insects-worms, slugs, and even vegetable matter do not come amiss to the hedgehog.

Instar - Any stage in an insect lifehistory between two moults.

Instinct - An inborn pattern of activity found in some form or other in almost all animals. Instincts account for much of animal behaviour courtship displays, protective care of the young, migratory drives and reaction to dangers. The pattern of behaviour begins usually after a stimulus to one or more of the sense organs. Such a stimulus is called a releaser. A loud noise and instinctively, animals take evasive action. fleeing or crouching motionless to the ground. Instincts like nest building and web and cocoon spinings are perfect from the start. Caterpillars of different species of moth spin their own types of cocoon once only in their lives but they do it perfectly. Young birds reared in isolation away from parents nevertheless build exactly similar nests even down to the material used. Other instincts, like Man's instinct to walk, take time and practice to become perfect. All members of a species will usually behave in much the same way to a stimulus. Instincts are just as much a part of an animal as the structures which identify its body.

Some instincts are always present in an animal defensive attitudes and defensive colour changes for instance. Other instincts appear only at certain times. Such periodic occurrences of instinct are often due to the action of hormones. Marked changes in animal behaviour can often be brought about by upsetting the hormone balance.

The activity of the hormone does not itself directly cause a piece of behaviour. It makes possible one or a whole series of instinctive actions when certain outside signals are received from the outside world. At breeding periods for instance, a male bird may display to a female of the same species whereas the appearance of another male causes aggressiive behaviour.

Sometimes instinctive drives may he so powerful that they produce behaviour without the usual outside stimulus. In the absence of a mate, captive birds and animals may display to their keepers instead.

Instincts are obviously of great value to animals. They provide a method of survival and a method of successfully breeding and rearing offspring. They have been built up over the ages by the process of natural selection. The individuals performing certain types of behaviour will survive to transmit them to their young.

Insulin - Hormone produced by the Islets of Langerhans in the pancreas. It controls the metabolism of glucose sugar in the body and a deficiency of insulin leads to diabetes mellitus (sugar diabetes). Glucose is the body's fuel: it is burnt in the tissues to provide energy. Excess glucose absorbed from the food is normally stored in the liver and muscles as glycogen. Insulin encourages this storage. As glucose is used up by the tissues, the level in the blood is maintained by the breakdown of glycogen but a deliciency of insulin result in the over-production of glucose by the liver and the consequent increase in the blood sugar level.

The metabolism of fat and protein are also greatly disturbed and abnormally high quantities are broken down into glucose thus enhancing the effects due to the breakdown of glycogen. Insulin lack causes the overproduction of glucose, but it also seems probable that the cells are unable to use carbohydrate in its absence or when reduced amounts of it are present. When insulin is deficient, protein and fat are broken down in place of carbohydrate as an alternative means of supplying energy. In a way not yet understood, insulin acts on the cells enabling them to use glucose for providing energy.

Inter - (=Between). An intercellular space is one between the cells.

Intersex - An individual animal whose features are neither male nor female, but part way between the two, as a result of genetical disturbance or a disturbance of the body's hormones. Unlike the gynandromorph, all the cells of the intersex are the same. (See Hermaphrodite).

Intestine - That part of alimentary canal which is concerned with digestion and absorption of food and with the reabsorption of water from the faeces.

Intra- (=Inside). Intracellular means inside the cells.

Invertebrata - Collective name for all those animals without backbones.The majority of animals are invertebrates the backboned, or vertebrate animals being fishes, amphibians, reptiles, birds and mammals.

 In vitro - Bioslogical  experiments, such as the testing of digestive juices, that are not carried out in a living animal, are said to be performed in vitro. In vivo experiments are those performed with a living organism.

In Vivo - (See III Vitro).

Iris -  Pigmented part of the eye that controls the amount of light falling on the retina.

Irritability - A property of all living things-the ability to react to a change or changes in the environment.

Ischium - Part of pelvic girdle.

Islets of Langerhans - Insulin secreting cells of the pancreas.

Isopoda - Order of Crustacea containing the woodlice the only truly terrestrial crustaceans.

Isoptera - Termites. This order of insects, although primitive and farremoved from the bees, wasps, and ants (See Hymenoptera), contains only social insects that live in highly organized communities. They are common in the tropics and two species extend to Europe. Termites feed mainly upon cellulose and are serious pests of timber. They make nests in trees or underground sometimes making huge mounds on the surface. The soil particles are mixed with saliva and harden into concrete like structures. The termites are unlike the other social insects in that the males and females are equally important. The mated pair, after their 'wedding flight' form a colony together. They live together in a special 'royal chamher' and are fed by the workers.

They queen becomes little more than an egg laying machine. Unlike the other social insects, mating occurs several times during the life of the queen. There are other reproductive forms in the colony and these can produce more workers hut cannot produce kings and queens. The workers and soldiers are of both sexes and carry out the contruction and defence of the nest and the food gathering work. Tunnels from the nest often lead for long distances to food gathering grounds. It is remarkable how closely termites resemble ants in their way of life when one considers how unrelated they are. The name 'white ants reflects this similarity.