Arthropods with more than eight legs form several easily distinguished groups, the main ones being the crustaceans, the millipedes and the centipedes.
Millipedes make a natural group: they are vegetarians, and they have two pairs of legs to each segment. Because of their diet, they are easy to culture.
Centipedes make a natural group of venomous carnivores, with one pair of legs on each segment, unlike millipedes, which have two pairs of legs per segment.
Spiders, scorpions and ticks make a natural grouping, based on their number of legs, but in other ways, they are quite distinct in their behaviour and anatomy.
Spiders hunt in a variety of ways: orb weavers use a normal 'spider web', but others cast a net over their prey, while others chase their prey down in the open.
Orb weaver spiders can be kept and observed, so long as they are placed on a frame standing in and over water, where they can make a web, but not escape.
Some species of spiders, and some strains within species, can be social, grouping together. This is a useful characteristic when spiders arrive in a new place.
Australian huntsmen spiders arrived in New Zealand in the recent past, and while they are rarely social in Australia, they are commonly social in New Zealand.
Crustaceans make a natural group that is easy to study: slaters or woodlice can be cultured, and freshwater crustaceans can easily be kept in tanks.
Charles Darwin used a careful analysis of their anatomy to show that barnacles are not shellfish but arthropods: their tentacles are highly modified legs.
Insects make a natural grouping, because they all have six legs, and either have four wings, or can be shown to have evolved from four-winged ancestors.
The changes in insects are called metamorphosis: starting as an egg, an insect larva becomes a pupa which becomes the adult form, called an imago.
In social insects, it is common for all members of a colony to have exactly the same genes, and for one individual to lay all the eggs on behalf of all of them.
In social insects, having identical genomes is important, because the actions of the sterile workers still go to improve the survival of their genome.
The beetles or Coleoptera are a very diverse group of insects, but all of them have elytra, which are modified wings, protecting their flight wings.
Flies have four wings, but fly with just two, the other two (the halteres) being reduced to a very small size and used for balance in flight.
The role of the halteres in balance may be demonstrated by removing the halteres from an adult fly, which will then be unable to fly in the normal way.
Fruit flies can be cultured in the laboratory, and they were commonly used in genetics experiments, because they go through generations very rapidly.
Mosquitoes are a natural division of the flies, based on their reproduction and feeding patterns, where males feed on plants, females on animals.
Mosquitoes have three clear stages of development, with the egg, the larva and the pupa in water: mosquito development may be observed in captivity.
The Hymenoptera (ants, wasps and bees) make a natural grouping: almost all of the Hymenoptera form group nests that have a complex social structure.
Termites, otherwise called the Isoptera, have a complex social structure, featuring a variety of specialized forms or castes within the nest.
Butterflies and moths are a natural grouping, but the butterflies and moths cannot be divided naturally on any differences, as the division is not a natural one.
Butterflies and moths show a variety of adaptations to their environments and predators, including the development of 'fright eye' patterns on their wings.
Caterpillars have specific food preferences, and females will normally lay eggs on food plants suitable for the caterpillars, for obvious evolutionary reasons.
The Lepidoptera, the moths and butterflies, are varied in their size and form, they differ greatly in their food choices, and some migrate over long distances.
Case moths always use local dead plants for their covering, and this can be demonstrated in the laboratory. Some case moths never develop wings.
Some caterpillars and moths are protected by the toxins they eat, and in some cases, they can even pass this protection on to the next generation.
Moths pollinate some white, heavily-perfumed flowers at night, and it seems that the flowers have developed these traits specifically to attract the moths.