Why did insects shrink in size over the

past 300 million years?

THE SCIENTIFIC CONSENSUS Involves several  factors.The largest terrestials anthropods arose during the Carboniboniferous and Perian periods, roughly250 million to 350 million years ago. During this tJme, Earth's atmosphere was up to 35 percent oxygen. compared with around 21 percent oxygen today. Insects with larger bodies could more easily and efficiently dellver oxygen to au their tissues because more oxygen was available In the air. These large insects also had  fewer predators. more plentiful food-supplJes and many more ecological niches to occupy than later insectswould have once other types of  animals evolved. Over the past 300 million years, evolutlion began favoring Insects that were more inconspicuous. needed less oxygen. and could live in tiny crevices and similar habitats that were too small for other creatures.

Floyd Shockley. collec1lons manage of entomology, National Museum of Natural History