In ancient and medieval warfare, skirmishers typically carried
bows,
javelins,
slings, and sometimes carried light
shields. Acting as
light infantry with their light arms and minimal armour, they could run ahead of the main battle line, release a volley of arrows, slingshots or javelins, and retreat behind their main battle line before the clash of the opposing main forces. The aims of skirmishing were to disrupt enemy formations by causing casualties before the main battle, and to tempt the opposing
infantry into attacking prematurely, throwing their organization into disarray. Skirmishers could also be effectively used to surround opposing soldiers in the absence of friendly
cavalry.
Once preliminary skirmishing was over, skirmishers participated in the main battle by shooting into the enemy formation, or joined in
melée combat with daggers or short swords. Alternatively, they could act as
ammunition bearers or
stretcher-bearers.
Due to their mobility, skirmishers were also valuable for
reconnaissance, especially in wooded or urban areas. During the
gunpowder era, a skirmish line could discover the extent of the enemy front line.
In classical Greece, skirmishers had low status. For example,
Herodotus, in his account of the
Battle of Plataea of
479 BC, mentions that the
Spartans fielded 35,000 light armed
helots to 5,000
hoplites yet there is no mention of them in his account of the fighting.
http://en.wikipedia.org/wiki/Skirmisher#cite_note-Greek_Warfare_p61-0 Often Greek historians ignored them altogether.
http://en.wikipedia.org/wiki/Skirmisher#cite_note-Greek_Warfare_p61-0 It was far cheaper to equip oneself as light armed as opposed to a fully armed hoplite indeed it was not uncommon for light armed to go into battle equipped with stones.
http://en.wikipedia.org/wiki/Skirmisher#cite_note-1 Hence the low status of skirmishers reflected the low status of the poorer sections of society who made up skirmishers.
http://en.wikipedia.org/wiki/Skirmisher#cite_note-Greek_Warfare_p65-2 Additionally, "hit and run" contradicted the Greek ideal of heroism.
Plato gives the skirmisher a voice to advocate "flight without shame," but only to denounce it as an inversion of decent values.
http://en.wikipedia.org/wiki/Skirmisher#cite_note-3 Nevertheless, skirmishers chalked up significant victories, such as the
Athenian defeat at the hands of the Aetolian javelin men in
426 BC and, in the same war, the Athenian victory of
Sphacteria.
http://en.wikipedia.org/wiki/Skirmisher#cite_note-Greek_Warfare_p65-2
Celts did not, in general, favour
ranged weapons. The exceptions tended not to include the use of skirmishers. The
Britons used the sling extensively, but for siege warfare, not skirmishing.
Among the Gauls likewise, the bow was employed when defending a fixed position.
http://en.wikipedia.org/wiki/Skirmisher#cite_note-5 The Celtic lack of skirmishers cost them dearly during the
Gallic Invasion of Greece of
279 BC, where they found themselves helpless in the face of Aetolian skirmishing tactics.
http://en.wikipedia.org/wiki/Skirmisher#cite_note-6
In the Punic Wars, despite the Roman and Carthaginian armies' different organizations, skimishers had the same role in both: to screen the main armies
actually I love it. Nothing like a good discussion. 
