In ancient Rome, who or what was caligae?

Caligae (singular form caliga) are heavy-duty, thick-soled openwork boots, with hobnailed soles. They were worn by the lower ranks of Roman cavalrymen and foot-soldiers, and possibly by some centurions.

Caligae were cooler on the march than enclosed boots, so in warm, Mediterranean climates, this may have been an advantage. But caligae seemed to have been abandoned there by the end of the second century AD, in favour of civilian style ‘closed boots’. By the late 4th century, this seems to have applied throughout the Empire. The emperor Diocletian's Edict on Prices includes set prices for footwear described as caligae, but with no hobnails, made for civilian men, women, and children.

In the early first century AD, the soldiery affectionately nicknamed the two or three-year-old Gaius, the third Roman emperor, ‘Caligula’, meaning ‘little boot’, because he wore a miniature soldier's outfit, complete with small caligae.