After about another 24 to 27 days, once the larvae have gone through all of the skin moults, they change into one final stage which is the pupae. Pupae look like white waxy ants that lay with their legs and antennae folded up against their bodies. Some species, such as Lasius niger, do not have a pupa stage but the larvae will spin themselves a cocoon in which it will metamorphosis into the adult ant. To be able to spin a cocoon the larvae must be against a solid object, this could be the ground, wall of a chamber or, in some cases, the larvae will have soil particles placed on it by the to assist them in the spinning of the cocoon. Perhaps the soil granules act as an irritant causing the larvae to spin its cocoon. Sometimes a larvae can be encouraged to spin its cocoon by the worker ant gently pressing its own body against the larvae.
The larvae will eject all its gut contents out and this appears as a small black dot at one end of the cocoon. I have seen my Lasius niger ants with ‘naked’ pupae rather than the usual cocoons in them, and I assume this has happened because the larvae, for whatever reason has not had soil granules placed on it nor has it been surrounded by a hard surface other than the ground it is laying on.
After, approximately, a further 13 to 28 days (about a total of 8 to 12 weeks from egg to adult ant), and again this varies according to species and ambient temperatures, the adult ant emerges looking very pale and soft. A newly born Lasius niger ant is almost white at first but after a few hours it will darken to black and it's exoskeleton will harden.
The particular job that each worker does within the colony depends on its age. New ants tend the queen and brood within the security of the depths of the nest. As the ant gets older so it will change jobs which take it nearer the surface of the ground until, nearer the end of its life, the ant will leave the safety of the nest and forage outside. This is the most economical way of sharing out the chores among the ants.
I once read somewhere that men send their young me to war, whereas ants send their old ladies.
Whether it is a worker, soldier (not seen in native UK species), male or queen depends on various factors:
Worker: Normal development and food intake as a larvae.
Soldier: Increased foot intake as a larvae.
Male: Unfertilised eggs.
Queens: Appears to be food intake related – lots more food than a worker.