A learner makes a note: a terrestrial insect conserves water while excreting nitrogenous wastes into the gut. The missing scientific idea is:

Malpighian tubules are excretory organs of insects and many terrestrial arthropods. They help remove wastes while conserving water, often producing uric acid. This MCQ belongs to the Arthropoda topic in Animal Diversity-I and is written to test concept understanding rather than memorized wording.