Messor Barbarus are a very interesting species, and people who keep them have had mixed results.  They come from the hotter mediterranean countries Southern Spain, Italy, Greece and Tunisia for instance.  They may even extend as far as the middle east.  They thrive in temperature ranges of between 25 and 30 degrees Centigrade, personally, mine are housed at around 27 degrees centigrade.  I specifically bought a 7 watt heating matt to raise the temperature from the 21 to the 27 it rested at, and this seemed to improve their health, you could tell by the amount of activity that increased.

Messors have 3 different worker sizes called Castes.  These are normal workers which look after the eggs and Queen, media workers which forage for food and major workers which I have not yet had a chance to observe.  They range in size from 4mm to about 14mm.  The major workers can be larger than the Queen.  The Queen herself can be up to 13mm long, mine is about that size.

Messor Barbarus are better known (well, commonly known as) Harvester Ants.  This is because they eat seeds.  The media workers will go out from the nest and forage for seeds.  From my own observations, they especially like grass seed.  I had been feeding mine canary mix seed, which was being taken, however, I thought I would try them on some simple grass seed, and this was soon snaffled up.  The media workers take the seeds, and put them inside the entrance to the nest.  The larger medias will then take the seeds and run them down the tubing into the nest proper.  The seeds are stored in their own chambers.  This serves 2 purposes, in the case of bad weather, or in the case of hibernation, the stores can be used.  The seeds are chewed by majors into a paste.  This paste known as "Antbread" is what the ants feed on mainly.  I have also fed my Messors dead flies which have been taken.  The ants are best given water by spraying the foraging area glass with a fine mist.  This is how they would find water in their natural environment.  Also, keep the humidity low within the nest.

From my own experience, the best nest type to keep them in is a plaster one.  I have tried them in a large ant farm, and they did not fair as well as I may have expected.