<p>A long-standing query about periodical cicadas that emerge every 13 or 17 years was recently answered by a team at USDA Agricultural Research Service: Once periodical cicadas emerge, do they actually feed on vegetation? Their analysis of cicadas' gut contents found plant DNA in mature adult cicadas but none in freshly emerged adults, a strong sign that adult cicadas do feed on plants, since no plant matter in cicadas' guts carries over from their nymph stage.</p>