The Meganisoptera, the taxonomic order of remarkably large but now extinct odonates that thrived during the Carboniferous period, included species with wingspans measuring up to 75 cm. Arthropleura, a now extinct genus of Carboniferous period arthropods related to modern millipedes, included members that could reach two and a half meters in length. So why aren’t there any arthropods of such great sizes any longer?