We’ve seen that arthropods have some powerful tools for success, and they’ve succeeded in many ways — in terms of longevity, number of individuals, number of species, and variety. However, arthropods have not “done it all.” Vertebrates have regularly shut out arthropods in at least one department: size.

Land-dwelling vertebrates have gotten big — really big, in the case of dinosaurs — but even your typical modern vertebrate — a lizard, for example — is heftier than all but the largest terrestrial arthropods.

This evolutionary constraint on size likely has to do with some of the characters that arthropods inherited from their common ancestor. Let’s explore how physics and genetics may keep terrestrial arthropods from evolving large body sizes.