The old accepted view was that terrestrial arthropods are limited in size by their system of respiration which amounts to exoskeletal pores (spiracles) and internal airways. So terrestrial bugs can't get much bigger because of the body volume (cubic) versus passive respiratory surface (square). (we've got a forced air/blood circulation system so we can get bigger) Consequently it would be fascinating to learn how close Dryococelus australis is to that theoretical limit, or has it developed some sort of active respiratory system? (in which case, maybe it's time to start engineering insect saddles?)