Carboniferous
Fossils of air-breathing insects myriapods and arachnids are known from the late Carboniferous, but so far not from the early Carboniferous. These arthropods were both well-developed and numerous. Their large size might have something to do with the fact that the oxygen concentration in the Earth's atmosphere in the Carboniferous was much higher than today. This required less effort for respiration and allowed arthropods to grow larger. The up to  millipede-like Arthropleura was the largest-known land invertebrate of all time.