Hymenoptera

The Hymenoptera is one of the larger orders of insect. It includes the bees, wasps, sawflies and ants. There are over 130,000 recognised species, with many more remaining to be described. The name is derived from the Greek words "hymen" meaning 'membrane' and "ptera" meaning 'wings. The hindwings are connected to the forewings by a series of hooks called "hamuli".
Females typically have a special ovipositor for inserting eggs into hosts or otherwise inaccessible places. The ovipositor is often modified into a stinger. The young develop through complete metamorphosis — that is, they have a worm-like larval stage and an inactive pupal stage before they mature (See holometabolism).
Sub-order Symphyta.
These are the sawflies, the most primitive Hymenoptera. Their adults do not have a narrow 'waist' between the thorax and abdomen. The female's ovipositor is adapted to burrowing or sawing into the substrate, usually plant tissue. 
Sub-order Apocrita.
These, the bees, wasps and ants, have a 'waist' between the thorax and abdomen. They are by far the most numerous Hymenoptera, and include the famous groups of colonial insects. In fact, the group with the most species is not colonial; it is the vast number of parasitoid wasps, such as the Ichneumons.
Evolution.
Hymenoptera originated in the Triassic: the oldest fossils belong to the Sawfly family Xyelidae. Social hymenopterans appeared during the Cretaceous.