All arthropods molt and have an exoskeleton — two factors that, as we’ve just seen, limit the body size of terrestrial animals. Insects, however, got triple-whammied in this department because of the way they breathe. While crustaceans breathe via gills and spiders use gill-like structures called book lungs, insects employ a different system. They get oxygen through tubes called tracheae.
The Crustacean/Spider Model
Water/air passes over gills/book lungs and oxygen diffuses into the blood. The blood carries oxygen throughout the body.
The Insect Model
Tracheae and tracheoles (essentially ductwork) allow air to circulate throughout the body — oxygen diffuses into the tissues near individual cells.