I have been reading my lovely Roman legion book and came across the Evocati.
Edit. I got a bit excited.
quote from legions of rome by stephen dando collins:
In times of emergency, retired legionaries could be recalled from their homes to serve behind their old standards, which they appear to have taken into retirement with them. This militia, called the evocati, was controlled by the governor of each province.
Paulinus used evocati to help him defeat boudicca in britian in AD60. Nero sent evocati to egypt in ad66 for an invasion of ethiopia that had to be aborted because of the jewish revolt. Mucianus, the governor of syria, set off for italy in ad 69 to depose emperor vitellius with an army which included 13,000 Evocati from the east. Vitelius fielded evocati from britain and the rhine when his army opposed vespasians army.
Historian tacitus was to say that in ad 59 most of the legionaries discharged that year scattered themselves in the provinces where they had completed their military service. He complained that whole legions were no longer transplanted as in former days with tribunes and centurions and soldiers of every grade to create new colonies so as to forma state by their unity and mutual attachment, with the result he lamented that the latest discharges became a mere crowd rather than a colony.
The evocati were still in existence by around ad 230 in the time of cassius dio, who said that they constitute even now a special corp and carry rods like the centurions, he added I cannot however give their exact number.
I want to see these guys in Rome2. Just to have a hard core of grey haired veterans once again rising to the challenge. I cannot imagine how the legions reacted to these old warhorses.