With Fleetwood languishing in 18th in the English fourth division, Arteta made nine changes from Saturday's win over Everton - opting for a blend of youthful talent and Premier League non-starters.
Making just his second senior appearance for Arsenal, Salmon lacked the physicality needed to hold off Loupalo-Bi before the Fulham loanee jabbed the ball through Meslier's legs - and the 17-year-old defender was hauled off at half-time.
Fleetwood's defenders, meanwhile, gave everything to keep Arsenal at bay, with Rhys Bennett, Conor Haughey and Shaun Rooney all throwing themselves in front of every shot as they weathered wave upon wave of relentless pressure.
The home fans - perhaps expecting Arsenal to open the floodgates at some stage - took delight in their side's ability to hold on for so long , and chants of "champions of England, you're having a laugh" echoed around Highbury Stadium.
Ultimately, it came down to conviction. Ronan Coughlan, Neal, Will Davies, Andy Cannon and Haughey each slammed their penalties emphatically past Meslier, showing a fearlessness that stood in stark contrast to Arsenal's timid overall performance.