Palaeos		Hexapoda
Arthropoda		Hexapoda

Hexapoda

Mandibulata
|--Myriapoda
`--+--Crustacea
   |
   `--HEXAPODA
      |--+--Protura  
      |  `-- Collembola ( springtails)  
      `--+--Diplura
         `--INSECTA (insects)

Evolution of the insects, from Prothero, 1990, url, .

The most diverse and abundant (or perhaps second most abundant, after nematodes) type of animal life on Earth today, insects, or hexapods (six legged animals) , are small terrestrial arthropod distinguished by their six legs and division of the body into three parts: head, thorax, and abdomen. they are also the only invertebnrates with the power and flight, and this may well be a factor in their success, although some wingless types, such as the tiny Collembola, are astonishingly successful and abundant.

The above diagram presents a simple "tree of life" representation of the evolutionary relationships of the insects, although some may question its Haeckel-like "great chain of being" format. In any case, the insects do show a number of distinct evolutionary stages towards greater complexity (for reasons expressed elsewhere the present writer (MAK) does not agree with the trend to reject all reference to evolutionary advancement to greater complexity). Hence primitive wingless insects give rise to (but also co-exist alongside), paleopterous (dragonfly-like) insects, which quickly (during the mid Carboniferous) evolved into neopterous (folding wing) insects, which in turn during the Permian give rise to holometabolic insects which, as their name indicates, undergo transformation through their life cycle. These last named are the most successful of the whole group

We should distinguish here between hexapods and insects, as these two terms are sometimes confused or synonymised. While colloquially, all hexapods are also insects, in current linnaean and cladistic classifications, insects are a subgroup of hexapods, albeit the largest and most diverse one. Hence we have divided these units according to evolutionary grades. The five grades are:

Hexapoda - six legs, three tagma, spiracles (the latter shared with myriapods)
Insecta - similar to above, but distinguished by further synapomorphies
Pterygota - wings
Neoptera - folding wings
Holometabola (or just Metabola - ref) - change of form during life cycle