The Adephaga is two groups with a common origin. They are the Hydradephaga (water beetles) and the Geadephaga (ground beetles. They are mostly predatory. The first fossils of both groups are found in the Triassic period. The ground beetles have formidable chemical defences. They have special pygidial glands which squirt out hot, burning chemicals. Bombardier beetles are famous for squirting their nasty chemicals at attackers.