Late republic deployment

[McClane]

During the late republic era, specifically Caesars war in Gaul and the subsequent civil war with Pompey did the legion still deploy in the checkerboard pattern of the early republic? if not how were the lines deployed?

[Bryan]

To be honest there is no definite proof they fought in checkerboard patterns during the mid Republic. While Livy and Polybius' account strongly suggest it many believe it was only used during the approach and that the Romans would close the gaps by either moving the posterior century to the left of the prior or by some method of doubling files and/or opening ranks.

Another common belief is that the shift to cohorts as the primary tactical unit meant that centuries inside would have formed in line in close proximity but there might have been gaps between cohorts (leaving massive holes in the legion's frontage.

I think that maniples marched forward to attack in quincunx/checkerboard (movement into formation done from column with a pivoting movement after reaching left limit of battle line). I think that they either kept the gaps or closed them slightly by opening ranks to Polybius' rank spacing, filling the gaps with skirmishers (either light infantry or armed calones). If they needed to provide a tighter frontage they could form maniples into close order (Vegetius' description), with either a maniple of the second line closing the gap in the line or more cohorts were added to the first line.

I think that manipular and cohort legions kept the same spacing between maniples, only changing it so units of the same number weren't staggered front to back but right to left. There is no reason to close the gaps, no reason to change the size of gaps from maniple to cohort.

We should remember that Romans forming in cohorts was not a sudden reform, there is ample evidence of Romans formed in them since Scipio's Spanish campaigns during the 2nd Punic War, down through the 2nd Century BC in Spain and then to Numidia where the reformer, Metellus, had two old campaigners with lots of experience in Spain, Marius and Rutilius Rufus. Besides that the Socii infantry that always served alongside Roman legions has been formed in cohorts centuries before the Romans finally did it (largely for reasons of recruitment).