Main Groups of Insects

What is an insect?

An arthropod (an animal with a segmented body and external skeleton) with the following characteristics:-

– Six legs (hence the term Hexapoda: ‘hexa’ = six, ‘poda’ = feet)
– Most have wings (or have lost them); some people now regard these as the true insects, forming the largest part of the Hexapoda.
– The basic life cycle is egg, larva and adult.
– In many, the larval and adult ecology is different allowing adaptability in life-style and in exploiting food resources.
– The more advanced insects also have a third or ‘pupal’ stage. The pupa allows for a major re-organisation of the body between larval and adult stages. The body design for feeding and for reproduction can be very different, giving huge flexibility, and contributing to the great success of insects.

The insects are arranged into orders, such as dragonflies and beetles.

These orders logically group into those with shared features, indicating a close relationship:

Apterygota (literally ‘without wings’)
Pterygota (these are true insects, with wings in the adult or wings have been lost)

MAIN GROUPS OF INSECTS IN BRITAIN AND NUMBERS OF SPECIES

NB. Numbers for species totals are provisional. The true world totals must be very much higher in most groups.

Numbers in brackets are where all species are introduced.

Major types of insects

The insects are arranged into orders, such as dragonflies and beetles.

These orders logically group into those with shared features, indicating a close relationship.

Apterygota. The name means without wings (‘a’ = without, ‘ptera’ wings).

These qualify as Hexapoda but they are very different from the true insects. Historically they have been regarded as insects but there is some modern opinion that they are a separate evolutionary line, or lines, from ancestral Hexapods. Thus arguably they are not insects, but for present purposes that distinction is not particularly important.

Pterygota. These are true insects, with wings in the adult (or wings have been lost)

There are two basic types of life history.

Exopterygota (External wing development)

These have the primitive type of life cycle:- egg, larva and adult. The larva develops wing buds on the outside of the thorax (external) and finally moults directly into the adult form.

Over half the insect orders are of this type, mainly small orders such as dragonflies, but the most advanced order, the bugs, has huge numbers of species.

[Some recent opinion is that the mayflies and dragonflies are the most primitive and should be grouped as Palaeoptera (‘palaeo’ = ancient, ‘ptera’ winged). The rest of the Endopterygota are then placed in the subclass Neoptera (‘neo’ = new, ‘petra’ = winged).

Most orders of Neoptera are allied to grasshoppers so are in the division Orthopteroidea, whilst the booklice and bugs are in a separate division, the Hemipteroidea]

Endopterygota (Internal wing development)

An advanced type of life cycle comprises egg, larva, pupa and adult. The addition of a pupal stage allows major reconstruction between the larval stage and the adult, including the development of wing buds into a ready to expand state.

Though there are fewer orders, the most advanced ones such as the beetles, flies, wasps and moths have become hugely successful in terms of adaptation and having evolved a vast numbers of species.

[An alternative name is Holometabola referring to the complete lifecycle.]