Which is the largets group of arthropods?

If you mean by number of species and/or total actual organisms, yes. By the way, if you didn't know, arthropod means "jointed foot", and it is the group of animals with exoskeletons; i.e. insects; arachnids--ticks, Spiders, mites, scorpions, whipscorpions, harvestmen; crustaceans--crabs, lobsters, shrimp, isopods (roly-polies, sow bugs, pill bugs, wood lice); and the 'pedes--millipedes and centipedes.