For convenience One tends to call almost any little creeping animal an insect, but the best way of recognizing an. insect from other creatures in the large group of Arthropoda is by counting the legs. Insects have six legs, spiders have eight and other creatures have many more. The larval forms are often legless and quite different from the adult. The body of an insect is divided into a number of segments, often made of a hard cuticle, which are linked by a soft membrane. The word insect is derived from the Latin insectum which means “cut into” and refers to this way they are built. Insects have a separate head, thorax and abdomen, whereas spiders and mites (Arachnida) have the head and thorax fused into one, centipedes and millipedes have no separation into these three divisions of the body and – creatures such as woodlice and other sea crustacea have the head and thorax – fused like Arachnida; they also have many pairs of legs.
Well over three quarters of all the known species of animals are Arthropods, and of these the majority’ are insects. The importance and magnitude of insect populations is not generally recognized but they are the most significant natural economic factor in food production throughout the world and, in the balance of nature, a vital part of the food chain, in which one creature eats another for survival. A glance at the chart showing how the world population of animals is divided up, from single-celled animals like the pond amoeba to giraffes and golden eagles, shows startlingly how much of the whole range is taken up by insects and what a very small proportion is taken up by the animals and birds that we know so well and appear to fill the world.
These are two distinct methods by which insects develop and grow from the egg stage, through immature stages, to the adult (often referred to as the imago). Those which undergo the very obvious and varied changes through the caterpillar (or / larva) and the pupa stages (e.g. the butterfly chrysalis) are said to undergo complete metamorphosis. Those insects which have incomplete metamorphosis include grasshoppers and stick insects: from their eggs emerge little legged creatures which are simply miniatures of the adult, though their wings come only when they make the final change to the adult.
The larva develops inside the egg and as soon as it is ready to hatch it cuts its way out either by eating or by employing a specially provided hook or other cutting tool. The skin of an insect is not very elastic and certainly will not allow the growth from an almost microscopic creature to some thing larger than a man’s finger, so it has to be shed periodically. This is so whether it is in caterpillar form or whether it is in the more complicated form of the miniature adult. The cast skin of a large stick insect is a weird, ghostly sight with its transparent colourless appearance, yét all its legs present. The main purpose of the larva is to eat and grow, shedding its skin several times, until the time comes either to pupate or, in the case of incomplete metamorphosis, straight away become an adult. Both changes are made simply by shedding the skin once more, but the skin which appears this time is in a completely different form, recognizable as soon as it has hardened, as the pupa or as the ‘final adult. Inside the pupa there is quite a change going on: much of the contents of the pupal shell breaks down into a fluid and in this the features of the adult start to build up from groups of cells which are called imaginal buds. 
The time taken varies with the species and season, but an average time for a butterfly is about two weeks, and then it is ready to burst out of the chrysalis case. When you look at a diagram of § the inside of the caterpillar, the chrysalis and the adult, you can see the internal similarity and that the very different external shape is very much due to the quite differently formed outer skin (or exoskeleton) that has been developed to suit the different activities of each of the three stages. You can trace the main gut and intestine, the nerve cord and the reproductive system which are common to all the different stages. This makes the whole system of the complete metamorphosis much clearer and less of a complete mystery.
